Note: Descriptions are shown in the official language in which they were submitted.
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
USE OF A HYPERPOLARIZED GAS FOR MRI DETECTION OF REGIONAL
VARIATIONS IN OXYGEN UPTAKE FROM THE LUNGS
Field of the Invention
This invention relates to a method of magnetic
resonance imaging of the human or animal (e.g.
mammalian, reptilian or avian) body by which lung
function and, if desired, morphology may be
investigated. '
Background of the Invention
Lung function is of interest to physicians,
especially when dealing with patients who may have
abnormalities of ventilation or perfusion or other
determinants of gas exchange in the lung. For proper
lung function five conditions must be met:
1. gas (air) must flow into and out of the lungs;
2. the gas must be distributed evenly within the
lungs;
3. gases must be exchanged by diffusion between the
blood and the alveolar space;
4. blood must be pumped through the lungs; and
5. the distribution of the blood in the lungs should
match the distribution of gas in the alveolar space
(i.e. where the gas penetrates to, blood should flow).
All diseases and ailments relating to the lungs and
airways affect one or more of the five conditions above.
It has therefore been known to study lung_
ventilation and perfusion using various diagnostic
techniques. The conventional technique is known as VQ
imaging and involves the use of two different
radiopharmaceuticals, one to study perfusion and the
other to study ventilation.
The perfusion agent is generally a particulate
CA 02327733 2000-10-06
WO 99/53332 PGT/GB99/01095
- 2 -
(e.g. 99mTc-macroaggregated albumin) which is
administered intravenously upstream of the lungs and
lodges in the precapillary arterioles.
Images are recorded with a gamma camera and the
signal intensity may be used to detect local
abnormalities in blood flow.
The ventilation agent is generally a radioactive
gas or aerosol or microparticulate, e.g. 133Xe, 127Xe or
81mKr, or a 99mTc-DTPA aerosol or 99mTc-labelled carbon
particles. The agent is inhaled and an image is
recorded with a gamma camera. Signal intensity and
distribution may be used to detect airway obstructions
or regional abnormalities in ventilation.
Where there is a mismatch between the ventilation
and perfusion images (which are generated at different
times), various different lung malfunctions, diseases or
abnormalities may be diagnosed, e.g. pulmonary embolism,
pleural effusion/atelectasis, pneumonia, tumour/hilar
adenopathy, pulmonary artery obstruction, AVM, CHF, and
intravenous drug use. Heterogenous perfusion patterns
may likewise be used to diagnose various disease states
or disorders, e.g. CHF, lymphangitic carcinomatosis,
non-thrombogenic emboli, vasculitis, chronic
interstitial lung disease, and primary pulmonary
hypertension. Decreased perfusion to one lung may be
used to diagnose pulmonary embolism, pulmonary agenesis,
hypoplastic lung (pulmonary artery stenosis), Swyer-
James syndrome, pneumothorax, massive pleural effusion,
tumour, pulmonary artery sarcoma and shunt procedures
for congenital heart disease.
VQ imaging however involves exposing the patient to
radiation doses from two radiopharmaceuticals in two
temporally separate imaging procedures. Clearance of
the injected particulate agent is relatively slow and
the agent is taken up in other organs besides the lungs.
Moreover, in patients with severe pulmonary
hypertension, the injected particulate causes a risk of
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 3 -
acute right heart failure. For pregnant patients the
radiation dose involved in VQ imaging results in
undesirable levels of radiation exposure for the foetus.
Furthermore, for most diagnostic purposes mentioned
above the resolution of conventional VQ imaging is
unsatisfactory.
There is thus a need for a technique which permits
lung function to be assessed without the drawbacks
associated with VQ imaging.
In magnetic resonance (mr) imaging, radiofrequency
signals from non-zero spin nuclei which have a non-
equilibrium nuclear spin state distribution are detected
and may be manipulated to provide images of the subject
under study. In conventional mr imaging the nuclei
responsible for the detected signals are protons
(usually water protons) and the non-equilibrium spin
state distribution is achieved by placing the subject in
a strong magnetic field (to enhance the population
difference between the proton spin states at
equilibrium) and by exposing the subject to pulses of rf
radiation at the proton Larmor frequency to excite spin
state transitions and create a non-equilibrium spin
state distribution. However the maximum deviation from
equilibrium is that achievable by spin state population
inversion and, since the energy level difference between
ground and excited states is small at the temperatures
and magnetic field strengths accessible, the signal
strength is inherently weak.
An alternative approach that has been developed is
to "hyperpolarize" (i.e. obtain a nuclear spin state
population difference greater than the equilibrium
population difference) an imaging agent containing non-
zero nuclear spin nuclei (e.g. by optical pumping, by
polarization transfer or by subjecting such nuclei ex
vivo to much higher magnetic fields than those used in
the mr imaging apparatus), to administer the
hyperpolarized agent to the subject, and to detect the
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 4 -
mr signals from the hyperpolarized nuclei as they relax
back to equilibrium. In this hyperpolarized mr imaging
technique, described for example in W095/27438, the
hyperpolarized material is conveniently in gaseous form,
e.g. 3He or 129Xe, and it may thereby be administered by
inhalation into the lung and the mr signal detected may
be used to generate a morphological image of the lungs.
Since the relaxation time T1 for 3He in the lungs is
about 10 seconds it is feasible, using fast imaging
techniques, to generate a morphological image of the
lungs from the 3He signal following inhalation of
hyperpolarized 3He gas and at any desired stage of the
breathing cycle, e.g. during breathhold. Since the mr
signal selected is from the 3He atoms and since the
helium is in the gas phase in the lungs, the image
detected is essentially only of the airways into and
within the lungs. By administering the hyperpolarized
agent as a bolus followed or preceded by other gases or
aerosols, e.g. by air, nitrogen or "He, the
hyperpolarized agent can be positioned at any desired
section of the airways or other aerated spaces in the
body, e.g. it may be flushed from the trachiobronchial
tree and the image generated is then essentially only of
the alveolar space.
We have now found that functional imaging of the
lungs may be carried out effectively using mr imaging of
an inhaled hyperpolarized agent by making use of the
variation with time of the relaxation rate T1 of the
hyperpolarized agent in conjunction with imaging of the
regional and temporal distribution of ventilation using
hyperpolarized gases.
Summary of the Invention
Viewed from one aspect therefore, the invention
provides a method of detecting regional variations in
oxygen uptake from the lungs of an air-breathing animal
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 5 -
subject, e.g. a mammalian (human or non-human), avian or
reptilian subject, said method comprising administering
into the lungs of said subject a diagnostically
effective amount of a gaseous hyperpolarized magnetic
resonance imaging agent, detecting the magnetic
resonance signal from said agent in said lungs,
determining the temporal variation in relaxation rate
(e.g. T1 relaxation rate) for said signal for at least
one region of interest within said lungs, and from said
variation generating a qualitative or quantitative value
or image indicative of the oxygen concentration in the
alveolar space in said at least one region of interest,
and if desired the time dependency of such concentration
as a result for example of physiological process, e.g.
oxygen uptake by perfusion.
In a preferred embodiment, the method of the
invention also involves generation of a temporal and/or
spatial image of the distribution of the hyperpolarized
agent in at least part of the lungs of the subject,
preferably in the alveolar space within the lungs.
In a further preferred embodiment, the method also
involves generation of a magnetic resonance image of at
least part of the lungs of the subject following
administration into the subject's vasculature of a
second mr agent, preferably an agent which affects
proton relaxation (with the image generated being a
proton mr image) or more preferably an agent containing
non-proton mr active nuclei (e . g. 19F, 13c, 31P, 170, etc.)
in which case the mr image will be generated from mr
signals from such non-proton mr active nuclei. The mr
active nuclei in the second agent will preferably not be
the same as those in the hyperpolarized agent unless the
image generated using the second agent is generated at a
time when the lungs contain substantially none of the
hyperpolarized agent.
Lung volume may also be estimated from the
integrated 3He mr signal (or by 3He mrs) following
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 6 -
inhalation of the 3He without air, breathhold, and
expiration where the expired volume is measured directly
and the residual hyperpolarization of the retained 3He is
extrapolated from the hyperpolarization value (signal
strength) monitored during breathhold.
In the method of the invention, it is preferred
that for at least part of the mr signal detection period
(preferably at least 1 second, more preferably at least
5 seconds, still more preferably at least 10 seconds,
e.g. 20 sec to 1 minute), there be substantially no flow
of gas into or out of the lungs, e.g. that there should
be a breathhold period, and that the indication of
oxygen uptake be derived from mr signals detected during
at least part of this period. However, in a preferred
embodiment, the method of the invention will also
involve mr signal detection during gas flow into and/or
out of the lungs with or without a period of breathhold.
In this way, spatial or temporal images or other
indications of lung ventilation may be generated from
the detected mr signals.
Because the detected mr signal derives from the
hyperpolarized agent, the signal strength is effectively
independent of the primary field strength of the magnet
in the mr imager. Accordingly low or high field, e.g.
0.05 to 3.5T, machines may be used.
Descriation of the Drawinas
The method of the invention is illustrated by the
attached drawings, in which:
Figures la and lb show 3He mr images showing the
effect of oxygen and flip angle on the images obtained
using a 40 mL bolus of 3He;
Figure 2 shows 3He mr images of the airway;
Figure 3 shows the 3He mr signal strength in the
trachea during inspiration and breathhold where a bolus
of 3He is estimated;
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 7 -
Figure 4 shows a plot of regional FiPO2 against FeCOZ
(see Example 7) ;
Figure 5 showe a plot of F;pO2 versus time (see
Example 7);
Figure 6 shows a plot of Dõ against number of images
(see Example 3);
Figure 7 shows a plot of signal intensity evolution
(see Example 3);
Figure 8 shows a plot of signal against number of
images (see Example 3);
Figure 9 shows a plot of signal intensities as a
function of time (see Example 5);
Figure 10 shows a plot of p02 versus time (see
Example 6);
Figure 11 shows images from a healthy volunteer
after inspiration of a single bolus (see Example 9); and
Figure 12 shows a plot of signal versus time (see
Example 9 ) .
Detailed description of the Invention
The method of the invention involves administration
of a gaseous hyperpolarized mr agent. By a gaseous
agent is meant a gas as such ( e. g. 3He or 129Xe ) or a
particulate agent held in the gas phase, e.g. an aerosol
of powder or droplets. In the latter case, the gaseous
carrier preferably is substantially free of paramagnetic
gases such as oxygen. The hyperpolarized agent will
conveniently have a polarization degree P of 2 to 75%,
e.g. 10 to 50%. The mr active (i.e. non-zero nuclear
spin) nuclei which are hyperpolarized may be any mr
active nuclei which can be hyperpolarized and which can
be presented in a gaseous form (i.e. elemental or
molecular form, e.g. SF6) which is physiologically
tolerable. Examples of appropriate nuclei include
various noble gas, carbon, nitrogen and fluorine
isotopes; however the noble gases, e.g. He and Xe, and
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 8 -
most especially 3He, are the most preferred.
Accordingly, the discussion below will present the
invention in terms of 3He-mr imaging although it does as
indicated above, extend to cover the use of other mr
active nuclei.
During steady state, oxygen transport within the
functional units of the lung, i.e. the alveolocapillary
unit is characterized by a relationship governed by mass
conservation:
The net amount of oxygen entering the
alveolocapillary unit by the airways has to be equal to
the net amount of oxygen leaving the alveolocapillary
unit on the blood side. This may be expressed by the
equation:
V' . ( FIO2 - FEO2 ) = Q . ( CaO2 - CVO2 ) (1)
VI = ventilation
Q = perfusion
F102 = fractional inspiratory concentration of oxygen
FEO2 = fractional expiratory concentration of oxygen
ca02 = oxygen content of arterial blood
c,O2 = oxygen content of mixed venous blood
Rearrangement of equation (1) provides the
following equation for the ventilation-perfusion ratio
V'/Q:
Y-L _ -Q.Q2_Z---Qõ42 (2)
Q F102 - FEO2
Oxygen contents as well as fractional oxygen
concentrations can both be written as functions of
oxygen partial pressure, yielding the following
equation:
~ h-CPa42-~v42)_ + f ( Pa02 - Pv02 ) (3)
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 9 -
Q (Pr02 - Pe02)
Assuming complete equilibration of oxygen partial
pressures across the alveolocapillary membrane, pa02
will be equal to pEO2:
V, c kvFX2IDvQ2l + f (paOZ - Pv02 ) (4)
Q (Pi02 - PE02)
Both k and f depend on a variety of factors, e.g.
on barometric pressure, the solubility constant of
oxygen in plasma, the dissociation curve of oxygenated
haemoglobin, etc., all of which are known.
Until now, quantitative description of these oxygen
transport processes was possible only on a global basis
for the whole organism.
With the present invention one is able to measure
these processes regionally in the lung. The method may
be used to measure regional ventilation, regional
partial pressure of oxygen and its time course, with
high spatial and temporal resolution.
Regional oxygen partial pressure may be measured by
hyperpolarized gas magnetic resonance imaging, e.g.
hyperpolarised 3He gas magnetic resonance imaging.
To this end, ultrafast MRI sequences are preferably
used allowing sequential measurements of the 3He signal,
and its decay, which is dependent both on oxygen and MR
acquisition (see Figures 1 a and b). Signal decay
induced by the MR sequence is corrected for by variation
of the flip angle and/or of the inter-scan delay.
Oxygen concentration inspired into the
alveolocapillary unit is not constant during asingle
inspiration, due to the contribution of deadspace.
Therefore, mean inspiratory concentration may be
calculated based upon determination of deadspace (from
airway imaging by 3He; see Figure 2), and from the
inspiratory concentration administered at the mouth.
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 10 -
Regional ventilation may be measured by
quantitative analysis of temporal changes in
hyperpolarization signal in the trachea, and parallel to
this, in the alveolar space, following inspiration of a
single bolus of hyperpolarized gas. This analysis is
performed on the basis of a mass balance, which allows
the determination of functional residual capacity and
serial deadspace on a global and regional basis. These
signal changes can be measured over several respiratory
cycles by ultrafast pulse sequences (e.g., temporal
resolution <150 ms) and flow flip angles (Fig. 2 and 3).
Knowing intraalveolar oxygen partial pressure and
mean inspiratory oxygen partial pressure, the local V'/Q
ratio can be calculated; the addition of local
ventilation then allows calculation of regional
perfusion. With the assumption that local arterial P02
equals alveolar P02, local oxygen uptake can be derived.
Thus, for the first time, a complete status of regional
oxygen transport in the lung can be obtained.
The preferred MRI sequences for use in the method of
the invention are:
- for oxygen partial pressure determination,
short repetition time gradient-recalled echo sequences
with small flip angle; and
- for determination of ventilation, ultra-short
repetition time (< 2 ms) gradient-recalled echo
sequences with small flip angle, or echo-planar pulse
sequences, or ultra-fast sequences using low flip angle
and free induction decay.
The theory of 3He-MR-based on P02 analysis will now
be discussed briefly:
The decay of longitudinal magnetization, and hence
signal intensity, that occurs with any mr acquisition,
follows a function given by:
S,+l,a (r) = S" * COS'a (5)
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 11 -
where n is the number of image acquisition, r is the
number of radiofrequency impulses (lines) per image
acquired, and a is the flip angle imposed by each
consecutive radiofrequency impulse upon the nuclear spin
polarization of 3He in the acquisition volume.
Simultaneously, signal intensity (Sn) also begins to
decay according to an exponential function, to arrive
(within a given time interval Dt) at Sõ+,:
Sn+l,Dt (t) = Sn * exp{ -Dt/Tl (t) } (6)
The time constant of this decay is determined by
the longitudinal spin relaxation time of 3He, T1, which
is shortened in the presence of paramagnetic molecular
oxygen.
In in vitro experiments, the following relationship
between T1 and oxygen concentration [OZ] in a gas mixture
containing hyperpolarized 3He has already been
established to be:
Tl (02) = k/ [02] , where k = 2.27 amagat*s; (7)
at temperature 37 C
(Tl in seconds; [02] in amagat; 1 amagat = gas
density (2.68675 x 1013 molecules per cm3) )
The combined effects of acquisition and time result
in a decay function of (valid for constant Tl) :
S,,+1(a,t) = Sõ * cosra * exp{-Dt/Tl} (8)
More generally, signal of image n acquired at time
tn (n = 0, 1, ..= nmax) given by
~.S' (tn) = S (cos a) nr exp (- f0 t n [O 2 (t)]dt/k) (8a)
Thus two values (flip angle a and oxygen
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 12 -
concentration [02(t)]) have to be extracted from image
intensities. Therefore make use of imaging with
variation of one parameter, e.g. time interval t between
images, or RF amplitude URF. This can be done either in
two separate imaging experiments ("double acquisition")
or within one experiment with a more intricate sequence
(see attached examples). Thereby both values can be
quantified simultaneously without additional input
parameters.
Hyperpolarized helium-3 (3He) can be produced by
means of direct optical pumping from the metastable
state 1s2s3S1 at lmb with subsequent conversion to
convenient pressures of 1-6 bar. Surkau et al. in Nucl.
Inst. & Meth. A384: 444-450 (1997) describe apparatus
which can be used to produce 3He with a polarization
degree P of at least 50% at a flow of 3.5 x101e
atoms/sec. or 40% at a flow rate of 8x1018 atoms/sec.
The hyperpolarized gas may then be filled into glass
cylinders, e.g. made of glass which has a low iron
content and no coating. These cylinders can be closed
by a stop-cock and transported to the mr imaging site,
preferably within a magnet, eg a 0.3mT magnet. Under
such conditions, the 3He has a relaxation time (T1) of up
to 70 hours.
To perform 3He mr imaging, the hyperpolarized gas is
preferably administered in a bolus into an application
unit through which the subject under study may breath
freely or alternatively ventilation may be supported by
artificial ventilation. For non-human subjects at
least, artificial ventilation apparatus will preferably
be used and the animals will preferably be anaesthetized
ti
and relaxed. For humans, with whom voluntary breathhold
is feasible, free breathing through the ventilation unit
will generally be preferred. In this way, the 3He bolus,
conveniently of 1 to 1000m1, may be administered at a
desired point within the breathing cycle, generally at
or close to the beginning of inspiration. The bolus
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 13 -
size used will depend on the lung size or tidal
respiration volume of the subject and will thus vary
with subject size or species. However a bolus of 2 to
50%, preferably 5 to 25%, of tidal respiration volume
may be suitable.
On inspiration the 3He bolus passes into the airways
within about one second with alveolar filling occurring
rapidly thereafter for healthy/unobstructed tissue. If
inspiration is followed by a period (e.g. of 1 to 60
seconds during which there is substantially no gas flow
into or out of-the lungs, e.g. a period of breathhold),
the 3He-mr signal gradually decays at a relaxation rate
of the order of 10 seconds. The relaxation rate however
is not constant spatially or temporally. Three
significant factors contribute to this: loss of
polarization due to the magnetic field changes required
for mr imaging; loss of polarization due to relaxation
enhancement by gaseous oxygen present in the lungs; and
loss of polarization due to relaxation enhancement by
the tissue/gas boundary. If the same imaging
sequence(s) is used throughout the signal detection
period, then the first and third of these factors are
constant during a period of no gas flow to/from the
lungs; however, 3He filled volumes as well as oxygen
concentration will vary due to physiological processes,
e.g. as oxygen is taken up from the lungs in the
alveolar space. As a result, in a region of interest
where oxygen concentration drops the 3He relaxation time
will increase with time even though absolute signal
intensity will continue to drop.
While relaxation rate enhancement by lung tissue
plays a subordinate role in terms of the overall
contributions to the 3He relaxation rate, it does have a
non-uniform effect as different tissues or abnormalities
have different effects on the relaxation rate. It is
thus preferred not to estimate the oxygen contribution
to the relaxation rate by simple reference to a phantom
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 14 -
undergoing the same field gradient changes as the
subject's lung. Use of a phantom is similarly non-
preferred due to the inhomogeneity in the applied field
across the volume in which the 3He distributes.
Accordingly it is preferred to extract the oxygen
contribution to the relaxation rate by mr signal
detection during at least two different types of signal
generation, e.g. with the different sequences being
interleaved. Thus for example the different sequences
may involve different RF excitation intensities and/or
different sequence intervals (t).
The magnetic field change contribution to the
relaxation is desirably minimized so as to prolong the
period over which a signal with an acceptable signal to
noise ratio can be detected. This is generally achieved
by using small flip angles (e.g. less than 7 , preferably
less than 4 ) in the imaging sequences and in this way mr
signals may be detected for up to 60 seconds following
bolus 3He administration.
For 3He-mr imaging, because of the relatively short
duration of the hyperpolarization and because relaxation
rate change over time is to be studied, it is of course
appropriate to use rapid image generating techniques,
e.g. fast gradient echo techniques or other techniques
with an image acquisition time of less than 2 seconds,
preferably 1 second or less. Such techniques are
mentioned elsewhere in this specification. Images
generated in this way may have a spatial resolution
(i.e. voxel size) of less than 20 mm2, which is far
superior to the scintigraphic ventilation images in
conventional VQ imaging.
The regions of interest studied in the method of
the invention will generally be the alveolar space and
thus it is generally preferable that the 3He bolus be
followed in the same gas intake by air or nitrogen to
flush the 3He from the tracheobronchial tree and into the
alveolar space.
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 15 -
As mentioned above, the method of the invention
may, and probably will, involve generation of
ventilation images, showing spatial and/or temporal
distribution of 3He, thereby.permitting ventilation and
perfusion to be determined in the same imaging procedure
(unlike VQ imaging). On a morphological level, such
ventilation images may identify airway obstructions
simply by identifying regions to which the 'He does not
penetrate, penetrates slowly, or penetrates at lower
than normal concentrations. Obstructions and associated
hypoperfusion, normal perfusion or hyperperfusion can
also be identified by following the time dependence of
the 3He relaxation rate for slowly penetrated alveolar
space as the oxygen concentration in such areas may be
abnormally low or high. Thus while the mr signal
strength may initially be abnormally low, the local
relaxation rate may be or become abnormally high or low.
Thus if local perfusion does not match local
ventillation, oxygen concentration in that part of the
lung will be affected and measurable by the method of
the invention due to the local abnormal relaxation rate.
This would be important in the case of patients with
lung malfunction due to smoking.
As also mentioned above, 3He mr imaging may be
combined with perfusion imaging with or without
administration of a contrast agent, using a second
imaging agent administered into the vasculature, e.g. a
blood pool agent such as a polymeric paramagnetic
chelate, or a superparamagnetic agent or, more
preferably because of its oxygen sensitivity, a 19F
fluorocarbon emulsion. In the former cases, imaging
would be proton mr imaging, in the latter case19F mr
imaging. However, the perfusion data collected in this
way, although equivalent to.the perfusion data collected
in VQ imaging, is not absolutely equivalent to that
generated in the method of the invention since the
second imaging agent distribution merely identifies the
CA 02327733 2004-01-29
29925-5
- 16 -
regions of the lung to which blood flows and not whether
or not oxygen uptake by the blood occurs in such
regions. Accordingly, the perfusion data from the
method of the invention provides a more comprehensive
portrayal of lung function.
The method of the invention may be used as part of
a method of diagnosis of lung malfunction, disease, etc.
or indeed in combination with a method of treatment to
combat, i.e. prevent or cure or ameliorate, a lung
malfunction or disease, etc., e.g. a method involving
surgery or administration of therapeutic agents or a
method of diagnosis of one of the lung malfunctions or
diseases mentioned above. Such methods form further
aspects of the present invention as does the use of 3He
(or other mr active nuclei containing materials) for the
preparation of a hyperpolarized imaging agent for use in
methods of treatment or diagnosis involving performance
of the method of the invention.
The invention will now be illustrated further by
reference to the following non-limiting Examples:
Example 1
The objectives in this Example were to realize
single-breath, single-bolus visualization of
intrapulmonarily administered 3He to analyse nuclear spin
relaxation of 3He in vivo and to determine the regional
oxygen concentration, i.e. [O2], and its time dependent
change by perfusion. A double acquisition technique is
described which also permits estimation of regional gas
transport.
In these examinations, the source of the MR signal
is the large non-equilibrium polarization of 3He. This
polarization is achieved by means of direct optical
pumping from its metastable state 1s2s3S2 at lmb with
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 17 -
subsequent compression to a convenient pressure of 1-6
bar. The apparatus is described by Surkau et al. Nuc.
Instr. & Meth. A384 (1997) 444-450 and is capable of
yielding P > 50% at flow of 3.5 x 1018 atoms/s and 40o at
flow 8 x 1018 atoms/s. The gas is filled into glass
cylinders with long relaxation times. Cylinders for
medical application are made from "Supremax glass" with
low iron content and no coating. They show relaxation
times up to 70 h and can be closed by a stop cock and
disflanged from the filling system. Transport from the
filling site to the MR imaging unit takes place inside a
dedicated 0.3 mT guiding field. To perform 3He-MRI
experiments reproducibly, an application system was
used. Predefined quantities of 3He gas at 1 bar pressure
can be inserted into breath at a predefined position.
Volunteers or patients can breathe freely through the
application unit or ventilation can be supported by a
commercial respiration machine with controlled pressure.
For studies with anesthetized and relaxed animals
ventilation is by a respiration machine.
Relaxation of the non-equilibrium polarization of
inhaled 3He in vivo is mainly caused by NMR excitations
and the presence of oxygen. Relaxation by lung tissue
plays a subordinate role as shown by experiments below.
The time evolution of the polarization P inside a two-
dimensional partition inside ventilated lung spaces can
be described by rate equations. Considering the flip
angle a and the partial oxygen pressure po we define a
time-averaged relaxation rate by NMR via the equation
PRF = -nmx r ln(cosa)/T,,l (12)
(where T,o, = duration of measurement, nmaX = index number
of last image, r = number of NMR excitations per image)
and by oxygen via the equation
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 18 -
r1(o2) = [o2(t))ik
k = 2.27 amagat*s at temperature 37 C referring to 299
Kelvin [see Saam et al. in Phys. Rev. A U (1995)
862-865] . Since [02] changes in vivo by oxygen
consumption, [OZ] is taken as a function of time t. Gas
exchange from neighbouring volumes with polarization P',
e.g. by diffusion, is taken into account by an exchange
rate y, weighted with the polarization difference (P -
P'). Assuming only relaxation by oxygen and wall
contact for P', the time dependence of P is integrated
to:
Pn= Po {r. exp(-forvz(t)dt)exp(-(I'.*Y)tn)(cosa)"x+yexp(- f I'oz(t)dt)exp(-
r~,tn)1
r +rRF ~ J
Experiments have been carried out to investigate
the dependence of P(t) on the given parameters. Signal
intensities were averaged and analysed over regions of
interests (ROIs). Since signal to noise ratios were
always >3, an intensity correction for noise was
performed following the method of Gudbjartsson et al.,
MRM 3A (1995) 910-914. The noise corrected signals Aõ of
the nt'' image (n = 0, 1, ...) are proportional to P,
The data are normalized and linearized by calculating Eõ
= ln (Aõ/Ao) .
Imaging of thick and thin partitions is feasible:
(a) all spins in the lung are equally excited.
This greatly simplifies matters and is to be
preferred in practical applications. In this
case, the effect of gas exchange is rendered
unobservable, i.e. (P - P') = 0 for all times.
Experimentally, it can be achieved either by
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 19 -
use of thick slices in 2D techniques, or by 3D
acquisitions covering the entire inhaled
volume of 3He .
(b) The volume V of the imaged partition is thin
compared to the surrounding volume V' with
which diffusive contact exists within the time
scale of a typical imaging sequence. In this
case y and y' scale according to the ratio of
the volumes involved, hence y' = Y.V/V'. Thus
y' may be neglected if V V'.
The idea of double acquisition imaging is best
illustrated by a simple example.
Consider a set of images with a single thick slice
(i.e. suppressing diffusion effects). If images are
taken in equidistant interscan times (hence, t,, = n.t)
En =- fn' I'OZ(t) dt + N n ln (cos a) [14]
0
Method 1
The second set of images is acquired retaining t,
but doubling a. Assuming p02 and its time development to
be equal in a given ROI during both series, the Eõ values
of corresponding images can be subtracted giving
E" (a) - E"(2a) =n In cos a [15]
N cos2a
If the left hand side of [15] is plotted against n,
ln (cos a/cos 2a) and furthermore a are obtained from
the slope. In a second step, eq. [14] of either dataset
is corrected for flip angle effects, and roZ is extracted
by a fit.
Method 2
The second set of images is acquired with the same
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 20 -
RF amplitude, but with a different T. In this case,
subtraction of corresponding Eõ values results in
elimination of the (cos a) term in eq. [14):
~=(En(i,) -En(t2)) f oi2poa(i)dt - fostpoa(t)dt [16]
Thus, information about the temporal development of
p02 is obtained. By correcting eq. [14) for this
relaxation effect, depolarization by RF excitations can
be computed.
Example 2
Wall relaxation by lung tissue is negligible. The
effect of wall relaxation was measured in a deoxygenized
lung of a dead pig by double acquisition sampling with
varied flip angles (method 1). Immediately after
inducing cardiac arrest, oxygen was washed out by
ventilating with pure nitrogen for about 15 mins.
Subsequently, two series of 11 images each were taken,
with RF amplitudes URF = lOV in the first and URF = 5V in
the second series. Partition thickness was 120 mm in
coronal orientation in order to excite 3He spins in the
entire lung volume. Interscan time t was 7 secs. A ROI
of 415 pixel (6.5 cm2) within the cranial left lung was
examined. A time constant of longitudinal relaxation T1
= 261(4) secs was fitted to the data.
This is in accordance with a possible residual
oxygen concentration of about 10 mb. The value should
thus be understood as a lower limit of wall relaxation
time. Assuming wall relaxation only, lung tissue shows
a cm/hour rate of at least 1/22 cm/hour (assuming
spherical alveoles with radius r = 200 m). This value
is smaller than that of most bare glass surfaces (see
Heil et al., in Phys.Lett. A 201 337 (1995). It means
that non-diseased broncho-alveolar surfaces contain
practically no radicals nor other paramagnetic centers.
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 21 -
Example 3
One anaesthezied pig (27 kg) was normoventilated,
inside a MRI unit (Siemens Vision scanner with B = 1.5
T, equipped with one of two transmit/receive coils
resonant to 3He at 48.44 MHz). After administering a
100 cm3 bolus of 3He, two series of 2D FLASH (TE < 4 ms,
TR 11 ms), images in transversal orientation were taken
during breathhold. Predefined RF excitation intensities
U were 10 and 20 Volts and intervals Z of 1.5s were
used. Partition thickness was 20 mm. Signal
intensities were averaged and analyzed over regions of
interest (ROIs). An intensity correction for noise was
performed following Gudbjartsson et al. MRM ,34: 910-914
(1995). A first postprocessing was performed
calculating En = ln (A,,/Aõ) for both series, where "in"
denotes the natural logarithm function. Following the
dependence
D En[10V] -E,[20V] = n In cosa [17]
~"~ N cos2a
Figure 6 shows a linear graph (N total number of images
taken, n the considered image number). Solving equation
(17) one determines the flip a = 3.4 . Knowing this
value, one can fit the signal intensity evolution with
the image number given in Figure 7. A linear dependency
of the regional partial oxygen pressure proved by other
experiments is assumed: p(t) = p - mt with time t,
coefficient m and pressure po at the beginning of the
measurement. By method 1, [O2] = 0.108(3) amagat and its
change with time by m = 0.0026(5) amagat/s are extracted
(see Figure 8).
Two more theoretical curves indicate the temporal
evolution, if no change of partial oxygen pressure takes
place (m = 0 amagat/s, p = 0.108 amagat) and if no
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 22 -
relaxation by oxygen would be present (m = 0 amagat/s, po
= 0 amagat). Both curves indicate, the significant
change of partial oxygen pressure. The low value for
regional po found seems to be real from comparison with
other analyses which yield the same flip angles for such
excitation intensities.
Example 4
In this example, we present an example of in vivo
oxygen determination, as obtained from double
acquisition with varied interscan time t(method 2). An
anaesthetized pig underwent controlled ventilation with
room air (oxygen concentration 21%). After 3He bolus
injection, a series of 8 images with tl = 7 s was
acquired during inspiratory apnea (z 50 s). After a
short interval to ensure stability of vital parameters,
a second series of 8 images with t2 = 1 s was taken. RF
amplitude was 10 V in both series, partition thickness
was 120 mm in coronal orientation.
The oxygen density p02(t) is determined from the
sequence of normalised logarithmic intensities E1,
Ez...E,,. The procedure is simplified if it is assumed a
priori that the time dependence of p02 be linear
P02 (t) = po - Rt, L18]
where R is the rate of oxygen decrease.
One then computes
yn En(i) -E~(j2) = po - R 2 (til + ti2) [19]
a I30 Comparison with eq. f18] shows that the
experimental quantities yn just equal the searched for
oxygen density
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 23 -
Y. = P02 ( t ~) [201
at mean times tõ = n(T, + T2)/2.
The time course of p02 (tõ) was obtained via eq. [20]
within a ROI in the middle section of the right lung
which comprises 89 pixel and covers an area of 1.39 cm2.
A linear decrease of poZ with time was observed, thus
confirming the assumption a posteriori.
A linear fit to the data yields po = 0.168(5) amagat
and R = 0.0034(2) amagat/s with a x2 of 1.00 p.d.f.
Consistent with physiology, the initial oxygen
concentration is found to be lower in the functional
residual capacity (FRC) of the lung than in inspired
air.
Once the temporal evolution of p02 is determined,
the flip angle a remains the only unknown parameter in
eq. [8a]. Considering the uncertainties of intensities
as statistical and those of p and R as additional
systematic errors, the t= ls series yields a= 3.36
(10) and the t= 7s series a = 3.1(3)
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 24 -
Example 5
Effects of Gas Transport PhPnomena in the lung on the MR
Signal
According to eq. [14], the dynamics of
intrapulmonary 3He polarization are changed significantly
when diffusive and/or convective gas transport is taken
into account. This is necessarily the case when the
imaged partition is thin compared to the total lung
volume. In this example, a 20mm slice of a porcine lung
was imaged in transversal orientation. Images were
taken after cardiac arrest to ensure a time-constant p02
(i.e. m= 0). The inspiratory oxygen concentration was
set to (30 1)0. Two series of nine images each were
acquired with RF amplitudes of 10 and 20 V respectively.
Interscan delays z were alternating 1.2s and 1.8s. A
ROI of 510 pixel, placed in the left lung, was analyzed
in this example.
The procedure in this case is as follows. As long
as the polarization difference P-P' between the imaged
partition and non-imaged surrounding is small, the
effect of gas exchange is considered negligible, hence
P-P'=0 is approximated in the first three images. Thus,
a and po are computed in the same way as in example 3.
Using y=0 and linear fitting of subtracted logarithmic
intensities E,, (n=0,1,2), we obtain a flip angle a-
2.9(1) for 10 V excitation. Subsequently, flip angle
corrected intensities of these first images are fitted
to determine po=0.31(2) amagat. In a third step, the
entire dataset of one acquisition is utilized to fit y
according to eq. [141.
Fig. 9 depicts the signal intensities Aõ(UõF=10 V)
as a function of time. The upper curve refers to a fit
with PRF=0.070s-1 and p02=po=0.31 amagat as input
parameters. The fit yields y=0.056(26)s"1. Also shown
is a curve for identical flip angle and oxygen
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 25 -
concentration, but with y=0. Clearly this curve tends
to increasingly disagree with the data points after
about 5s, whereas only a small discrepancy is detected
for the first three images, justifying the said method
of analysis.
Example 6
Determination of Oxxcren Concentration using a Single
Acquisition
In this example the imaged object was a rubber bag
of volume 0.5 liters. An application of 3He bolus 0.1 1,
flushed by 0.4 1 of air (02 concentration 21%) was
performed.
The imaging was performed on (Siemens Vision
Scanner with B=1.5 T equipped with transmit/receive coil
resonant to 3He at 48.44 MHz) using a 2D Flash sequence,
partition thickness 12 cm, covering entire volume of
bag.
Parameter variation was realized with one single
imaging sequence, permitting quantification of flip
angle and oxygen concentration.
7 images were taken with UõF=5 V, interscan time
2.6s. Thereafter, 6 images were taken with UHF=20 V,
interscan time is.
Flip angle was determined from a fit of intensities
of a ROI of the last 6 images, "guessing" an initial
oxygen concentration. Obtained result was used to
compute [O2] (t) from a fit of intensities of a ROI of the
first 7 images. Accuracy was improved by iterating this
process 2 times.
Results: flip angle a was determined to be 4.40(7)
for 20 V excitation.
Oxygen concentration was determined to 0.186(7)
amagat, consistent with 02 concentration in room air.
Since a phantom was imaged, no decrease of oxygen
CA 02327733 2004-01-29
29925-5
- 26 -
was observed, see Fig. 10.
Example 7
A First Oxygen Determination Routine using Variation of
Low Flip Angle
Hyperpolarized 3Helium (3He) is described as non-
radioactive inhalational contrast agent for magnetic
resonance (MR) tomography of ventilated lung spaces. In
3He-MRI, signal intensity is destroyed irrecoverably by
(1) the presence of paramagnetic oxygen in the
respiratory gas and (2) MR image acquisition itself.:
Regional intrapulmonary [02] as a sum of inspiratory
oxygen concentration (FIO2), distribution of ventilation,
and oxygen uptake is determined in clinical practice
globally over the whole lung. The aim was to use the
effect of oxygen upon 3He to visualise regional
intrapulmonary [02] in MR for the first time on a
regional basis.
Animal and Methods: Eight anesthetized healthy
pigs (28 2 kg) were normoventilated in a 1.5 T MRI unit
fitted with a Helmholtz transmit-receive coil tuned to
48.4 MHz. Hemodynamic parameters and end-tidal [OZ] were
measured continuously.
Interventions included variation of 3He bolus sizes,
of RF amplitudes for MR-image acquisition (lOV and 20V),
of end-tidal [OZ] (0.16, 0.25, 0.35 and 0.45), and
comparison of intrapulmonary [02] before and after
induction of cardiac arrest.
Using a dedicated application unit specifically
designed by our group, see WO 99/25243,
boli of 3He (up to 45% polarized) were
administered at the beginning of inspiratory tidal
volumes. During subsequent inspiratory apnea, serial 3He
images of airways and lungs were acquired using a two-
dimensional FLASH sequence (image acquisition time = 1
CA 02327733 2000-10-06
WO 99/53332 PCT/G899/01095
- 27 -
s; TR = 11 ms/TE = 4.2 ms; 1.5 s inter-image delay).
The decay of MR signal intensities in various
regions of interest within pulmonary cross-sections was
analysed with respect to the different interventions.
RF excitation effects upon signal intensity decay were
separated from oxygen effects by comparison of image
series acquired with two different flip angles <7 .
Results: Single-breath, single-bolus 3He
administration allowed reproducible visualization of
airways and lungs. Bolus volumina between 20 mL and 100
mL could be administered reproducibly (40 mL: 39 4 mL;
100 mL: 100 4 mL; n=25). Images containing regions
with a signal-to-noise ratio > 3 were required for
analysis of the signal decay function; this could be
achieved in up to 10 subsequent images following a
single 3He bolus. T1 of hyperpolarized 3He demonstrated
a similar relationship to ambient [02] as had been found
in vitro. Signal analysis within two consecutive
images, which were acquired at a known Fet02, allowed
determination of polarization loss due to MR acquisition
(for lOV or 20V, respectively). Taking this effect into
account, the analysis of independently acquired image
series yielded estimates for regional [02]. Analysis of
MR signal decay in defined ROIs of two-dimensional 3He
images yielded values for regional intrapulmonary [02]
which correlated closely with end-expiratory [0Z] (r =
0.94; p<0.001, Figure 4) before induction of cardiac
arrest, and with inspiratory oxygen concentration during
absence of perfusion.
Conclusions: This study demonstrates a)
reproducible visualization of small quantities of 3He in
the lungs, b) in vivo confirmation of the oxygen-T1
relationship described by Saam et al. in Phys.Rev. A52,
862 (1995), c) feasibility of non-invasive MR-based
analysis of regional intrapulmonary [02] in a range of
oxygen concentrations which is used in ventilator-
dependent patients, and d), significant correlation of
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 28 -
3He-MR-determined with measured end-expiratory oxygen
concentrations. As hyperpolarized 3He can be distributed
in special glass cells (half-time of hyperpolarization >
80 h), and technical requirements are limited to a
spectroscopy option for the used MR scanner and a
dedicated 3He-coil, early propagation of this method is
expected. The new technique may provide insight into
regional 02 exchange in the lungs. Further human and
animal studies are necessary to demonstrate the spatial
and temporal resolution in the analysis of 02
distribution and exchange under pathological conditions
by this non-invasive new technique.
Figure 5 shows the analysis of the time course of
oxygen concentration in the lung of a male volunteer,
analysed with the double acquisition method with
variation of flip angle as described in the present
example above. Initial oxygen concentration at the
beginning of the breathhold.(0.189) and calculated
oxygen decrease during apnea (0.01/s) can be followed.
8
Example
3He gas was hyperpolarized to approximately_40-50%
by optical pumping. 12 volunteers and 10 pneurologic
patients inhaled such gas from glass cylinders of 300 mL
volume and 3 bar pressure. 3He-MRI was performed during
breathhold using a 3D gradient-recalled-echo imaging
sequence on a Siemens 1.5T clinical scanner, adjusted to
have a transmitter frequency of 48.4 MHz and using a
Helmholtz transmit/receive RF coil. A flip angle less
than 5 was used.
In quantitative studies, faster, repeated 3D images
(TR=5ms, TE=2ms) were acquired at intervals of 0.8, 16,
42 and 55 seconds in normal volunteers. From these 5
images, extraction of both regional flip angle and
regional T1 was possible defining the effects of repeated
RF pulsing and longitudinal relaxation in terms of decay
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 29 -
rate constants, FRF and I'RELAx respectively. For a pulse
train of duration T, consisting of N pulses of flip
angle (p, I'RF is given by:
I'RF T = [cos (j)) ]" (15)
On the other hand, the.contribution of longitudinal
relaxation depends on absolute time, not on the duration
of the RF pulsing. Thus by using a non-linear image
timing sequence, the two effects can be resolved and
both flip angle and T1 determined regionally.
A final study, using an ultrafast 2D sequence,
generated images every 1 second during inspiration,
breathhold and expiration.
Results: All volunteers and 8/10 patients were
able to perform the necessary inhalation. One patient
was claustrophobic and 1 patient could not maintain a
25-second breathhold. The central airways were
consistently visualized. Volunteers demonstrated
homogeneous signal intensity; patients with obstructive
lung disease and/or pneumonia demonstrated
characteristically inhomogeneous signal intensities,
specific for the disorder.
Flip angle calibration confirmed an estimated flip
angle of 1-2 . T1 was derived to be 32t3 seconds in
normal lung. In phantoms, longitudinal relaxation was
negligible compared with RF pulsing over a time period
of 1 minute (this is consistent with predicted T1 values
of tens of hours).
Using the rapid 2D sequence, the inspiratory
process could be seen to have a timecourse of less than
is in normal lung (providing 'instantaneous' uniform
signal). Expiration gave rise to slower signal change.
The signal reducing effect of expiration could be
clearly discriminated from the continuing destruction of
polarization by RF pulsing, allowing estimation of hung
residual volume.
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 30 -
Conclusion: 3He-MRI with inspiration of
hyperpolarized 3He provides a means of imaging lung
ventilation. Lung filling and ventilatory obstruction
can be examined with dynamic MRI. Quantitation,
particularly of regional 3He T1, provides a means of
assessing local physiologic parameters, such as p02. The
simple quantitative approaches described in this Example
slow 3He-MRI of the lung provides a modality capable of
providing regional functional and physiological
information.
Examnle 9
Ultrafast Ventilation Scan
Material and methods: Coronal images of the lung
were acquired at 48.44 MHz using ultrafast gradient-echo
pulse sequence with TR/TE/a=2.Oms/0.7ms/1.5 . A series
of 160 projection images was obtained with 128ms
temporal resolution. Imaging was performed before,
during and after application of a single bolus of
approximately 300m1 3He in five healthy volunteers
(spontaneous breathing). The signal intensities were
corrected for depolarisation by RF excitation on the
basis of equation (5) of this invention. Images from a
healthy volunteer at time Os, 0.13s, 0.26s, 0.65s,
1.17s, 1.95s, 3.77s and 6.37s after inspiration of a
single bolus (285 mL) hyperpolarized Helium-3 are shown
in Figure 11. Figure 12, meanwhile, shows signal-time-
curves in trachea and in parenchyma on the right side of
the lung in the patient of Figure 11. Shaded areas
denote intervals of expiration (determined from the
diaphragm position), interrupted by intervals of
inspiration (not shaded). During the first phase of
inspiration, 3He signal appears in the trachea. It
reappears during the expiratory cycles. After a delayed
signal increase in alveolar space, 3He signal decreases
CA 02327733 2000-10-06
WO 99/53332 PCT/GB99/01095
- 31 -
there due to T1 relaxation, depolarisation by RF pulses,
and due to expiration and inspiration with air.
Results: In these gradient recalled images no
susceptibility artifacts are observed. Distribution of
the 3He boli was observed in the trachea, in mainstem and
distal bronchi down to fourth order, and in alveolar
space. The temporal resolution was 130 ms, spatial
resolution was 2.5mm x 4.4mm. The signal of a single
bolus of 3He was detected in the lung for up to 20s. The
peak signal-to-noise ratio in the lung was 11.7 7.7.
While the time-to-peak of the bolus signal in the
trachea was 260ms, it was significantly longer in lung
parenchyma (910ms).
Conclusion: Individual phases of inspiration,
distribution of 3He within the alveolar space and
expiration can be visualized by ultrafast imaging of a
single bolus of hyperpolarized 3He gas. This method may
allow for regional analysis of lung function with
temporal and spatial resolution superior to conventional
methods.
