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Description  |
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FIELD OF THE INVENTION
This invention relates to improvements in and relating to magnetic
resonance imaging, in particular imaging of phenomena associated with
blood flow variations and abnormalities.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging (MRI) has been used successfully to study blood
flow in vivo. Moreover Villringer et al. Magnetic Resonance in Medicine
6:164-174 (1988), Cacheris et. al. Society of Magnetic Resonance in
Medicine, 7th Annual Meeting, San Francisco, 1988, (SMRM 1988) Works in
Progress, page 149, Belliveau et. al. SMRM 1988, Book of Abstracts, page
222 and Moseley et. al. SMRM 1988, Book of Abstracts, page 43 have
proposed the use of certain paramagnetic lanthanide chelates as magnetic
susceptibility, that is T.sub.2 * shortening, MRI contrast agents for
studies of cerebral blood flow and perfusion.
Unlike many previous imaging procedures, T.sub.2 or T.sub.2 *-weighted MRI
using magnetic susceptibility (MS) contrast agents (hereinafter MS
imaging) enabled blood perfusion deficits, e.g. cerebral ischemias, to be
visualized rapidly as the MR signal intensity was reduced in the regions
of normal perfusion due to the effect of the contrast agent, with ischemic
tissue being revealed by its retention of signal intensity.
Blood perfusion deficits are associated with several serious and often
life-threatening conditions. Rapid identification and location of such
deficits is highly desirable in order that the appropriate corrective
action, be it therapeutic or surgical, may be taken promptly. Thus in the
case of cerebral ischemia, any delay in post ischemic recirculation and
reoxygenation of brain tissue reduces neuronal survivability.
MS imaging therefore represents a major improvement over routine T.sub.2 or
T.sub.2 *-weighted imaging in the absence of MS contrast agents, since in
the routine procedures ischemias or infarcts only become detectable 2 to 3
hours after the event, e.g. a stroke, which gave rise to the perfusion
deficit. However, while determination of the existence and location of a
perfusion deficit is important, it is also desirable to be able to detect
the degree or severity, and if possible the onset and duration of blood
flow abnormalities or variations, in a quantifiable manner. We now propose
that this be done using a modified MS imaging procedure.
SUMMARY OF THE INVENTION
Viewed from one aspect the invention provides a method of detecting blood
flow abnormality or variation in a human or non-human, especially
mammalian, body, said method comprising administering into the systemic
vasculature of a said body a contrast enhancing amount of an intravascular
paramagnetic metal, e.g. transition metal or lanthanide, containing
magnetic resonance imaging contrast agent, subjecting said body to a
magnetic resonance imaging procedure capable of generating from magnetic
resonance signals from said body a series of temporally spaced images of
at least a part of said body into which said agent passes, and detecting
temporal variations in said signals or images whereby to identify regions
of abnormal or modified blood flow in said body and to indicate the degree
of blood flow abnormality or modification therein.
Thus the method of the invention provides a quantitative and temporal
determination of local perfusion variations, e.g. deficits or increases,
which may arise from, for example, stroke, microsurgery or administration
of blood flow modifying pharmaceuticals.
The method of the present invention is preferably carried out using
spin-echo techniques. Alternatively and also preferably the method may be
carried out using a so-called fast or ultra fast imaging technique in
order to enable a series of T.sub.2 * dependent images to be generated
with as short as possible a time interval between successive images. For
this reason, techniques capable of generating images with time intervals
of less than 5 seconds, especially less than 0.5 seconds and more
especially less than 100 milliseconds, are particularly preferred. Thus,
in general, techniques such as spin echo, gradient echo, TurboFLASH, and
most especially the various varieties of echo planar imaging (EPI), are
particularly suitable for use in accordance with the method of the
invention.
In the method of the invention, an indication of the degree of blood flow
abnormality or modification for a given voxel may readily be determined by
comparison of the MR signal intensity for that voxel with a reference
value, e.g. the signal intensity for similar tissue with normal blood
flow. The reference intensity values may be predetermined or may be
selected as the MR signal intensity values for voxels of normal tissue in
the same image. In the case of cerebral ischaemias, signal intensity
values from the normally perfused gray matter and white matter of the
brain may be used to provide reference values for the affected tissue. As
is discussed in further detail below, the location and spatial extent of
the blood flow abnormalities, and the location and spatial extent of the
regions having the most severe blood flow abnormalities detected in this
way according to the method of the invention, correspond closely to the
same extents and locations as determined using conventional non-MRI
techniques such as histopathologic tissue-staining and quantitative
autoradiography.
In one particularly preferred embodiment of the invention, temporally
spaced images are generated following repeated administrations of the MS
contrast agent, e.g. at intervals of no less than 15-30 minutes, whereby
to detect the time of onset and thereafter to monitor the development of
the blood flow abnormality or modification, e.g. to identify the extent
and location of reperfusable tissue and the degree of success of
reperfusion, or to identify tissue for which surgical intervention is
required before reperfusion is possible.
Thus the method of the invention may be used to characterize quantitatively
the regional microcirculation of the brain before and after acute arterial
occlusion and differentiate between regions with normal blood flow,
reduced blood flow and no blood flow. With the different distributions of
MS contrast media in occluded and reperfused cerebral tissues, the method
may also be used to document reperfusion of ischemic tissue. Moreover,
with the use of MS contrast media in the method of the invention,
distinction can be made between central cores of necrosis and the
surrounding penumbrae of salvageable tissue, i.e. between irreversibly and
reversibly injured brain tissue. Using ultrafast imaging techniques in the
method of the present invention, the kinetics of the distribution of the
contrast medium into tissue and of the wash-out of the contrast medium
from the tissue can be followed so as to provide a diagnostic "signature"
which could be used to distinguish between normal, ischemic, infarcted and
reperfused tissue and to characterize the type of ischemic event and to
identify tissues at risk from ischemia.
In one embodiment of the method of the invention, using a fast imaging
procedure, the determination of the location and severity of ischaemia is
effected by determining the time dependence of the MR signal intensity for
the voxels in the seconds following administration of the contrast agent,
and generating an image where voxel image intensity value is dependent on
the time post-administration at which MR signal intensity for that voxel
is lowest. Normal tissue reaches minimum MR signal intensity sooner than
ischaemic tissue, and the resulting image thus enables the spatial extent
and local severity of blood flow abnormality to be visualized.
Alternatively, a similar image may be generated by making the voxel image
intensity value dependent on the time taken before voxel MR signal
intensity reattains a pre-selected control value, e.g. its pre-injection
value or a percentage of that value (for example 80%).
The contrast agent used according to the method of the invention should be
an intravascular contrast agent, that is to say one which is substantially
retained within the systemic vasculature at least until it has passed
through the body region or organ of particular interest. Generally,
therefore, blood pooling, particulate and hydrophilic contrast agents or
contrast agents possessing more than one of these properties are of
particular interest.
Besides its obvious application in terms of identifying and giving an
indication of the severity of cerebral or cardiac ischemias or infarcts,
the method of the present invention has a broad range of possible
diagnostic and evaluative applications of which the following list names
but a few:
Assessment of cerebral perfusion in brain dysfunction associated with acute
severe symptomatic hyponatremia;
Evaluation of new therapies (for example thrombolytic therapies and clot
removal, calcium channel blockers, anti-inflammatory agents, angioplasty,
etc) in the treatment of cerebral vasospasm;
Assessment of cerebral perfusion following induced subarachnoid
haemorrhage;
Assessment of different degrees of ischemia in large tissue masses;
Study of the relationship between blood ammonia, lactate, pH and cerebral
perfusion in cerebral ischemia associated with acute liver failure (this
has implications for the treatment of Alzheimer's disease);
Localisation and assessment of thrombus and plaque;
Evaluation of new therapies for stroke (for example t-PA, aspirin
antiphospholipids/lupus anticoagulants, antiphospholipid antibodies, etc);
Evaluation of risk factors for stroke (for example elevated serum lipids,
etc);
Assessment of the impact of induced brain hypothermia on cerebral perfusion
during neurosurgery for stroke;
Assessment of the effects of ageing on cerebral perfusion including the
study of the etiology of lacunar infarcts;
Assessment of the effects of cocaine, amphetamine and ethanol on cerebral
perfusion in mildly and severely ischemic brain;
Definition of the "therapeutic window" in reversible focal ischemia for
heparin, vasodilators, antihypertensives and calcium antagonists; and
Monitoring of other induced vasodilator effects.
Thus viewed from a further aspect the invention provides a method of
detecting and quantitatively evaluating the severity of ischemias in a
human or non-human, especially mammalian, body, said method comprising
administering into the systemic vasculature of said body a contrast
enhancing amount of an intravascular paramagnetic metal containing
magnetic susceptibility magnetic resonance imaging contrast agent,
subjecting said body to a magnetic resonance imaging procedure capable of
generating from magnetic resonance signals from said body a series of
temporally spaced images of at least a part of said body into which said
agent passes, and detecting temporal variations in said signals or images
whereby to detect ischemic tissue and to provide a quantitative indication
of the degree of blood perfusion deficit therein.
Viewed from a still further aspect, the present invention also provides a
method of monitoring the vasodilatory or vasocontractory effects of a
physiologically active substance administered to a human or non-human
animal body, for example a calcium antagonist, said method comprising
administering said substance into said body, administering into the system
vasculature of said body a contrast enhancing amount of an intravascular
paramagnetic metal containing magnetic susceptibility magnetic resonance
imaging contrast agent, subjecting said body to a magnetic resonance
imaging procedure capable of generating from magnetic resonance signals
from said body a series of temporally spaced images of at least a part of
said body into which said agent passes, and detecting temporal variations
in said signals or images whereby to monitor the vasoconstriction or
vasodilation induced by said substance.
Viewed from a still further aspect, the present invention also provides a
method of monitoring surgically induced blood perfusion variations, either
before or during surgery, said method comprising administering a contrast
enhancing amount of an intravascular paramagnetic metal containing
magnetic susceptibility magnetic resonance imaging contrast agent into the
systemic vasculature of a human or animal body which is undergoing or has
undergone surgery, in particular microsurgery on said vasculature,
subjecting said body to a magnetic resonance imaging procedure capable of
generating from magnetic resonance signals from said body a series of
temporally spaced images of at least a part of said body into which said
agent passes, and detecting temporal variations in said signals or images
whereby to identify regions of surgically induced variations in blood
perfusion.
Viewed from a still further aspect the invention provides the use of a MS
contrast agent for the manufacture of a contrast medium for use in the
methods of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-9 show T.sub.2 -weighted spin-echo images of a cat brain (FIGS.
1-3), hyperintensity contour plots derived therefrom (FIGS. 4-6), and
superimposed versions (FIGS. 7-9), acquired at 108, 160 and 320 minutes
following unilateral MCA occlusion and subsequent reperfusion (see Study
2).
FIGS. 10-11 show, respectively, TTC-stained histopathologic cat brain
sections and superimposed staining contours obtained in Study 2.
FIGS. 12-14 show echo planar images of a cat brain obtained before, during
and after unilateral MCA occlusion and subsequent reperfusion (see Study
3).
FIGS. 15-16B show, repectively, the 20% or greater hypersensitivity area
and a contour map of hypersensitivity obtained during the cat brain
occlusion of Study 3.
FIGS. 17-18 show TTC-stained histopathologic cat brain sections obtained in
Study 3.
FIGS. 19A-22C show brain images and intensity contour maps of a 99Tc-HMPAO
autoradiograph (FIG. 19), TTC-staining of a histopathologic section (FIG.
20), a T.sub.2 -weighted MR image without contrast agent (FIG. 21), and a
T.sub.2 -weighted MR image with DyDTPA/BMA contrast agent (FIG. 22), as
described in Study 4.
FIGS. 23-24 show T.sub.2 -weighted spin-echo images of a cat brain (FIGS.
23A and 24A), hyperintensity contour maps derived therefrom (FIGS. 23B and
24B), and superimposed versions (FIGS. 23C and 24C), acquired at 128 and
280 minutes following unilateral MCA occlusion (see Study 5).
DETAILED DESCRIPTION OF THE INVENTION
The magnetic susceptibility, T.sub.2 -reducing effect, of MS contrast
agents is to a large degree dependant on the magnitude of the magnetic
moment of the magnetic species within the contrast agent--the higher the
magnetic moment the stronger the effect. Indeed the effect is
approximately proportional to the square of the magnetic moment making the
effect of Dy(III) about 1.95 times larger than that of Gd(III). In general
paramagnetic metal species having magnetic moments of .gtoreq.4 BM will be
preferred. The contrast agents particularly preferred for use in the
method of the present invention are those containing paramagnetic
lanthanide ions, especially high spin lanthanides such as ions of Dy, Gd,
Eu, Yb and Ho, in particular Dy(III).
In order that they may be administered at effective but non-toxic doses,
such paramagnetic metals will generally be administered in the form of
ionic or much more preferably non-ionic, complexes, especially chelate
complexes optionally bound to larger carrier molecules which may be
selected to manifest greater residence times in plasma, or to enhance the
blood pooling nature of the contrast agent or to reduce the osmolality of
the contrast medium by increasing the number of paramagnetic centres per
contrast agent molecule (or molecular ion).
A wide range of suitable chelants, polychelants, and macromolecule bound
chelants for paramagnetic metal ions has been proposed in the patent
literature over the last decade and in this respect particular regard may
be had to U.S. Pat. No. 4,647,447 (Gries), U.S. Pat. No. 4687659 (Quay),
U.S. Pat. No. 4639365 (Sherry), EP-A-186947 (Nycomed), EP-A-299795
(Nycomed), WO-A-89/06979 (Nycomed), EP-A-331616 (Schering), EP-A-292689
(Squibb), EP-A-232751 (Squibb), EP-A-230893 (Bracco), EP-A-255471
(Schering), EP-A-277088 (Schering), EP-A-287465 (Guerbet), WO-A-85/05554
(Amersham) and the documents referred to therein, the disclosures of all
of which are incorporated herein by reference.
Particularly suitable chelants for the formation of paramagnetic metal
chelate MS contrast agents for use in the method of the present invention
include the following:
N,N,N',N",N"-diethylenetriaminepentaacetic acid (DTPA),
6-carboxymethyl-3,9-bis(methylcarbamoyl-methyl)-3,6,9-triazaundecanedioic
acid (DTPA-BMA),
6-carboxymethyl-3,9-bis(morpholinocarbonylmethyl)-3,6,9-triazaundecanedioi
c acid (DTPA-BMO), 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic
acid (DOTA), 1,4,7,10-tetraazacyclododecane-N,N',N"-triacetic acid (DO3A),
1-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-N,N',N"-triacetic acid
(HP-DO3A), 1-oxa-4,7,10-triazacyclododecane-N,N',N"-triacetic acid (OTTA),
polylysine-bound DTPA and DTPA derivatives or DO3A and DO3A derivatives or
DOTA and DOTA derivatives (eg. DTPA-polylysine, DO3A-polylysine and
DOTA-polylysine), soluble dextran-bound DTPA and DTPA derivatives having
with a total molecular weight .gtoreq.40KD, preferably in the range
60-100KD (DTPA-dextran).
Particularly suitable paramagnetic metal ions for chelation by such
chelants are ions of metals of atomic numbers 21 to 29,42,44 and 57 to 71,
especially 57 to 71, more especially Cr, V, Mn, Fe, Co, Pr, Nd, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb and Ln, in particular Cr(III), Cr(II), V(II),
Mn(III), Mn(II), Fe(III), Fe(II) and Co(II) and especially Gd(III),
Tb(III), Dy(III), Ho(III), Er(III), Tm(III) and Yb(III) especially
Dy(III), Ho(III) and Er(III).
All paramagnetic ions have both T.sub.1 and T.sub.2 reducing effects on the
surrounding non-zero spin nuclei and as the effect on MR signal intensity
of these two effects is generally opposed in unweighted images, T.sub.1
reduction leads to image intensity increases whereas T.sub.2 reduction
leads to image intensity losses. Thus for the purposes of the present
invention it is particularly preferred to use paramagnetic metals which
have relatively poor T.sub.1 -relaxivity in order to maximize the MR
effect of the contrast agents in T.sub.2, or T.sub.2 weighted MR imaging.
Thus Dy(III) or even Yb(III) would generally be used in preference to
Gd(III).
In order to perform the method of the invention with as high as possible a
safety factor, the ratio between the dose of the contrast agent and its
LD.sub.50, it is particularly preferred to use non-ionic or low osmolality
chelates, i.e. chelates which carry no overall ionic charge, such as Dy
DTPA-BMA for example, or where the complex has an overall ionic charge to
paramagnetic metal centre ratio of 1.5 or less.
Furthermore, to ensure that the contrast agent remains wholly or
essentially within the blood vessels during passage through the body
region of interest, the contrast agent will as mentioned above preferably
be hydrophilic and retained in the vasculature for a sufficiently long
time to permit effective imaging.
Examples of suitable blood-pooling agents include the inert soluble
macromolecule-bound chelates of the type described by Nycomed in
EP-A-186947 and WO-A-89/06979. Binding the chelant to a macromolecule,
e.g. a polysaccharide such as dextran or derivatives thereof, to produce a
soluble macromolecular chelant having a molecular weight above the kidney
threshold, about 40KD, ensures relatively long term retention of the
contrast agent within the systemic vasculature.
Examples of suitable hydrophilic contrast agents include linear, branched
or macrocyclic polyamino-carboxylic acid chelates of paramagnetic metal
ions, especially chelates of chelants in which carboxylic acid groupings
are replaced by hydrophilic derivatives thereof, such as amides, esters or
hydroxamates, or in which the chelant backbone is substituted by
hydrophilic groupings such as for example hydroxyalkyl or alkoxyalkyl
groups. Chelants of this type are disclosed for example in U.S. Pat. No.
4,687,658 (Quay), U.S. Pat. No. 4,687,659 (Quay), EP-A-299795 (Nycomed)
and EP-A-130934 (Schering).
Particular mention however must be made of the Dy(III), Ho(III) and Er(III)
chelates of DTPA-BMA, DTPA-BMO, and DO3A and HP-DO3A.
The dosages of the contrast agent used according to the method of the
present invention will vary according to the precise nature of the
contrast agent used. Preferably however the dosage should be kept as low
as is consistent with still achieving an image intensity reduction in
T.sub.2 -weighted imaging. Thus for Dy(III) based chelates, for example,
dosages of Dy of 0.05 to 0.5 mmol/kg bodyweight, and especially 0.08 to
0.3 mmol/kg, are particularly preferred. In this way not only are
toxicity-related problems minimized but the sensitivity of the imaging
method towards the detection of ischemia of varying degrees of severity is
increased. At higher dosages the signal suppression by the MS contrast
agent may be unduly abrupt and intense, making regions with relatively
minor perfusion deficits appear to have the characteristics of relatively
normal blood flow. For most MS contrast agents the appropriate dosage will
generally lie in the range 0.02 to 3 mmol paramagnetic metal/kg
bodyweight, especially 0.05 to 1.5 mmol/kg, particularly 0.08 to 0.5, more
especially 0.1 to 0.4 mmol/kg. It is well within the skill of the average
practitioner in this field to determine the optimum dosage for any
particular MS contrast agent by relatively routine experiment, either in
vivo or in vitro.
Where the contrast agent is ionic, such as is the case with Dy DTPA, it
will conveniently be used in the form of a salt with a physiologically
acceptable counterion, for example an ammonium, substituted ammonium,
alkali metal or alkaline earth metal cation or an anion deriving from an
inorganic or organic acid. In this regard, meglumine salts are
particularly preferred.
Contrast agents may be formulated with conventional pharmaceutical or
veterinary aids, for example stabilizers, antioxidants, osmolality
adjusting agents, buffers, pH adjusting agents, etc., and may be in a form
suitable for injection or infusion directly or after dispersion in or
dilution with a physiologically acceptable carrier medium, e.g. water for
injections. Thus the contrast agents may be formulated in conventional
administration forms such as powders, solutions, suspensions, dispersions
etc., however solutions, suspensions and dispersions in physiologically
acceptable carrier media will generally be preferred.
The contrast agents may therefore be formulated for administration using
physiologically acceptable carriers or excipients in a manner fully within
the skill of the art. For example, the compounds, optionally with the
addition of pharmaceutically acceptable excipients, may be suspended or
dissolved in an aqueous medium, with the resulting solution or suspension
then being sterilized. Suitable additives include, for example,
physiologically biocompatible buffers (as for example DTPA or
DTPA-bisamide (e.g.
6-carboxymethyl-3,9-bis(methylcarbamoylmethyl)-3,6,9-triazaundecanedioic
acid)) or calcium chelate complexes (as for example calcium DTPA salts,
calcium DTPA-bisamide salts or NaCaDTPA-bisamide) or, optionally,
additions (e.g. 1 to 50 mole percent) of calcium or sodium salts (for
example, calcium chloride, calcium ascorbate, calcium gluconate or calcium
lactate and the like).
Parenterally administrable forms, e.g. intravenous solutions, should of
course be sterile and free from physiologically unacceptable agents, and
should have low osmolality to minimize irritation or other adverse effects
upon administration and thus the contrast medium should preferably be
isotonic or slightly hypertonic. Suitable vehicles include aqueous
vehicles customarily used for administering parenteral solutions such as
Sodium Chloride Injection, Ringer's Injection, Dextrose Injection,
Dextrose and Sodium Chloride Injection, Lactated Ringer's Injection and
other solutions such as are described in Remington's Pharmaceutical
Sciences, 15th ed., Easton: Mack Publishing Co., pp. 1405-1412 and
1461-1487 (1975) and The National Formulary XIV, 14th ed. Washington:
American Pharmaceutical Association (1975). The solutions can contain
preservatives, antimicrobial agents, buffers and antioxidants
conventionally used for parenteral solutions, excipients and other
additives which are compatible with the contrast agents and which will not
interfere with the manufacture, storage or use of products.
In the method of the present invention where the lanthanide has any
significant T.sub.1 -reducing effect, which is especially the case where
the paramagnetic metal is Gd rather than Dy, this T.sub.1 -reducing effect
may also be utilised to increase the degree of certainty with which
perfused regions are identified by generating corresponding T.sub.1
weighted images and determining the signal ratio for each pixel or voxel
between the two types of image. In this way tissue with very limited
perfusion may perhaps be distinguished from tissue in which blood flow has
ceased entirely. Where such a technique is used however it will be
especially desirable to use the low toxicity, low osmolar forms of the
paramagnetic complex in order to operate with as large a safety factor as
possible. Thus for Gd it will generally be preferably to use GdHP-DO3A,
GdDO3A, GdDTPA-BMA or GdDTPA-BMO rather than GdDOTA or GdDTPA salts.
Generally data manipulation forms a major part of the method of the
invention since information regarding the severity of perfusion deficit
may be extracted from the rate at which signal intensity loss takes place
for the region of interest following MS contrast agent administration (the
less obstructed the blood flow the more rapidly the signal is lost) and
the duration and magnitude of signal loss. Clearly comparison with data
obtained for healthy tissue will enable a form of perfusion calibration to
be made. Moreover indications of the blood volume affected may also be
obtained by measurement of the area under the curve for a plot of pixel or
voxel signal intensity loss over time for the duration of the MS contrast
agent induced signal loss. The necessary data manipulation, including
display of zones of reduced or enhanced perfusion optionally superimposed
on a selected background image, e.g. the "native" image obtained in the
absence of the MS contrast agent, can of course be performed by a
computer, generally the same computer as is arranged to operate the MR
imager and generate MR images from the detected MR signals.
The methods of the invention are particularly suited to the early detection
of ischaemias as ischaemic events may in this way be detected
significantly less than 1 hour after occurrence, as opposed to the 2-3
hours or more of conventional T.sub.2 weighted MRI, so making it possible
to take steps to reperfuse the affected tissue at an earlier stage or to
treat it with a cerebroprotective pharmaceutical, and thus raising the
chances of reducing permanent tissue damage and of increasing tissue
survivability.
The method of the invention will now be described further by way of example
with particular reference to certain non-limiting embodiments and to the
accompanying drawings in which FIGS. 1 to 24 are images or diagrams of the
cat brain before, during or after unilateral MCA occlusion.
Study 1
Young adult cats weighing 2.0 to 4.5 kg were anaesthetized with 30 mg/kg
i.v. Nembutal. Polyethylene catheters were placed in the femoral artery
and vein for blood pressure monitoring and drug administration. The right
middle cerebral artery (MCA) was isolated via the transorbital approach
and occluded just proximal to the origin of the lateral striate arteries
with bipolar electrocautery followed by complete surgical transection. The
dural incision and orbit were covered with saline moistened gauze and
absorbable gelatin sponge.
A General Electric CSI (2 Tesla) unit, equipped with Acustar S-150
self-shielded gradient coils (.+-.20 gauss/cm, 15 cm bore size) was used.
MRI was performed with an 8.5 cm inner-diameter low-pass birdcage proton
imaging coil. Successive multislice T.sub.2 -weighted coronal images were
obtained for up to 12 hours following occlusion. Spin-echo T.sub.2
-weighted images (TR 2800, TE 80 and 160, 3 mm slices, 1 mm gap) were
obtained with a field-of-view (FOV) of 80 mm in which two scans were
averaged for each one of the 128 phase-encoding steps resulting in a total
acquisition time of 12 minutes.
In order to evaluate the anatomic region of perfusion deficiency following
MCA occlusion, cats were injected with a non-ionic T2* shortening contrast
agent, DyDTPA-BMA. The DyDTPA-BMA complex was prepared by refluxing an
aqueous suspension containing stoichiometric amounts of dysprosium oxide
and DTPA-BMA. The contrast agent was infused i.v. at doses of 0.25, 0.5 or
1.0 mmol/kg beginning at phase-encoding step #32 and finishing at step #60
(approximately 3 min) of T.sub.2 -weighted image acquisition. DyDTPA-BMA
injections were given at different time points post MCA occlusion in
individual cats. After injection, the magnetic susceptibility effect was
quantified for up to 60 minutes in both ischemic and normal hemispheres by
comparing region-of-interest (ROI) intensity to pre-contrast T.sub.2
-weighted ROI intensities. ROI image analyses were carried out in the
ischemic inferior parietal gyrus, caudate, putamen, and internal capsule,
and compared with the corresponding uninjured contralateral regions. A
signal intensity ratio was calculated as the ROI image intensity ratio of
an abnormal, ischemic region over that of the normal, contralateral side.
Results were expressed as the mean percentage change.+-.Standard Error of
the Mean (X.+-.S.E.M.)
At the conclusion of the MR protocol, 15 ml/kg of a 2% solution of
2,3,5-triphenyl tetrazolium chloride (TTC) was infused transcardially. The
brain was removed from the cranium after 10-20 minutes, immersed in a 2%
TTC solution for another 10-20 minutes, and then stored overnight in 10%
buffered formalin in a light shielded container. The brain was sectioned
coronally (2-3 mm slices) from 24-36 hours later and immediately examined
for histologic evidence of ischemic damage as evidenced by pallor of
TTC-staining.
Using a 0.5 mmol/kg dosage of DyDTPA-BMA, maximum signal intensity losses
of 35% were observed in the gray matter of the normal non-occluded
cerebral hemisphere during the first 15 minutes after injection. Signal
intensity changes in white matter (internal capsule) in both the normal
and ischemic hemispheres were smaller than in gray matter, presumably
because of higher cerebral blood flow to gray matter. The resulting
contrast-enhanced images had superior gray/white matter contrast than
T.sub.2 -weighted spin-echo MR images without contrast. At 45 minutes
after administration of DyDTPA-BMA, signal intensity had recovered to at
least 90% of pre-contrast control values in all cerebral tissues.
Increasing the dosage of DyDTPA-BMA from 0.5 to 1.0 mmol/kg produced only
a minimal difference in immediate post-contrast signal intensity. Long TE
times (160 msec) produced the highest gray/white matter contrast after
DyDTPA-BMA at each of the 3 doses tested. (In general in the method of the
invention using higher TE values leads to a slight loss in signal to noise
ratio but also to increased sensitivity to T.sub.2 -induced proton
dephasing and hence to the MS contrast agent).
Perfusion deficits resulting from occlusion of the MCA were detected as
regions of signal hyperintensity of the occluded ischemic tissue compared
to the normally perfused areas in the contralateral hemisphere. Relative
hyperintensity was found in the occluded basal ganglia as early as 30
minutes post-occlusion for both the 1 mmol/kg and 0.5 mmol/kg dosages.
Signal differences between ischemic and contralateral control tissues were
observed for gray matter in the inferior parietal gyrus (42.+-.14%), and
basal ganglia (26.+-.8%), and to a lesser extent, for the white matter in
the internal capsule (5.+-.4%). By comparison, T.sub.2 -weighted MRI
without contrast failed to demonstrate any significant signal differences
prior to approximately 2-3 hours post MCA occlusion (see Table 1). As
well, DyDTPA-BMA administration allowed detection of small developing
infarcts that were not visible or were ambiguous on T.sub.2 -weighted
images without contrast (see Table 2).
TABLE 1
______________________________________
Effect of DyDTPA-BMA administration on the
time of detection of cerebral ischemic
damage.
Onset of signal hypertensity
Dose (relative to pre-contrast
DyDTPA-BMA T.sub.2 -weighted image)
(mmol/kg) # Cats Tested
Earlier Same Time
Later
______________________________________
0.25 5 1 4 0
0.50 16 12 4 0
1.0 6 4 2 0
______________________________________
TABLE 2
______________________________________
Effect of DyDTPA-BMA administration on the
definition of injury site (signal intensity
ratio of injured tissue to corresponding
contralateral control tissue) compared to
pre-contrast T.sub.2 -weighted image.
Signal intensity ratio
(relative to pre-contrast
Dose T.sub.2 weighted image)
DyDTPA-BMA # injections
(mmol/kg) contrast Better Same Worse
______________________________________
0.25 5 4 1 0
0.50 23 17 6 0
1.0 16 14 2 0
______________________________________
Within 3-5 hours after MCA occlusion, T.sub.2 -weighted images also
demonstrated tissue injury clearly, including increased mass-effect and
hyperintensity (edema) throughout the MCA territory. The distribution of
increased signal intensity correlated well anatomically with regions of
perfusion deficiency demonstrated with DyDTPA-BMA-enhanced MR imaging. A
continuing close anatomic correspondence between areas of perfusion
deficit and edematous regions was seen 9 hours and 11 hours post
occlusion. In subsequent TTC-stained coronal sections, these areas were
found to exh | | |
