Display of three-dimensional MRA images in which arteries can be distinguished from veins

An intravascular MR contrast agent is administered to a living patient. A series of three-dimensional dynamic MR datasets is acquired from the Volume of Interest ("VOI"), beginning after administration of the contrast agent and continuing for a sufficiently long time as to reflect contrast agent enhancement of all arterial and venous blood vessels within the VOI. A three-dimensional MR angiogram of the VOI is acquired after the contrast agent has reached equilibrium. For each voxel within the VOI, enhancement of that voxel as a function of time post administration of the contrast agent is computed. Parameters that distinguish enhancement of voxels relating to the patient's arteries from enhancement of voxels relating to the patient's veins are selected, and the intensity of each voxel in the MR angiogram is scaled in accordance with the selected parameters. A maximum intensity projection reconstruction of the VOI is generated from the MR angiogram in which voxel intensity has been scaled.

BACKGROUND OF THE INVENTION 
The invention relates to magnetic resonance (MR), and more particularly 
relates to MR angiography (MRA). In its most immediate sense, the 
invention relates to three-dimensional MRA in which contrast agent is 
used. 
In MRA, and particularly in three-dimensional MRA, it is advantageous for a 
clinician (usually a radiologist) to be able to distinguish arteries from 
veins. Existing methodology for doing this has proven unsatisfactory. 
In conventional MRA (which does not use contrast agents), venous or 
arterial signals are selectively eliminated during image acquisition by 
presaturating the venous or arterial blood flow and thereby preventing it 
from producing an MR signal. The typical result is an MR angiogram 
depicting only arteries, or only veins. The effective shortening of the T1 
relaxation time in blood which produces a high signal-to-noise ratio in 
contrast-enhanced MRA also causes such conventional presaturation 
techniques to fail. Because of this, to distinguish between arteries and 
veins, contrast-enhanced MRA data is typically acquired in two steps, the 
first being carried out to acquire an enhanced arterial image and the 
second being carried out when both arteries and veins are enhanced. The 
first image displays the arteries, and a subtraction image formed between 
the first image and the second image displays the veins. Each of these 
images must be acquired rapidly; the first image must capture the peak of 
the arterial bolus, and the second must be carried out before the venous 
enhancement diminishes, and in the case of conventional extravascular 
agents, before the surrounding tissue is significantly enhanced. Because 
each of the images must be acquired quickly, the images necessarily have 
low spatial resolution. 
Because the subtraction image only relates to the state of the patient's 
circulatory system at two particular times, such a methodology provides 
only a limited capability of distinguishing between arteries and veins and 
is highly sensitive to the timing and speed of data acquisition relative 
to the injection of the contrast agent. If this timing is miscalculated, 
or if the acquisition lasts too long, the acquisition does not acquire a 
purely arterial phase, but rather a combination of arterial and venous 
phases together. Furthermore, if some veins have not yet enhanced by the 
time of the second acquisition, they will not appear in the subtracted 
image. 
It would be advantageous to provide improved methodology for acquiring 
three-dimensional MRA data from a patient's arteries and veins within a 
volume of interest (VOI) and for displaying an image reconstructed from 
such data in which it would be possible to visually distinguish the 
arteries from the veins. 
It is therefore an object of the invention to provide a method that would 
make it possible to distinguish arteries from veins in a three-dimensional 
MRA image. 
The invention proceeds from the realization that by administering a bolus 
of an MR contrast agent into the patient's circulatory system and by 
tracking the position of the bolus as a function of time, it is possible 
to distinguish arteries from veins. This is because the bolus normally 
passes through the patient's circulatory system along a route that is 
known in advance, and the position of the bolus within the circulatory 
system can therefore in the normal case be correlated with time post 
administration. For this reason, the progress of the contrast agent 
through the patient's circulatory system as a function of time can 
therefore be used to distinguish arteries from veins. Indeed, even if the 
blood does not flow along the expected path (as can be the case when e.g. 
the patient's heart is malformed) the actual flow path can be mapped out 
as a function of time, thereby providing the physician with blood flow 
images that are outside conventional "venous" or "arterial" categories. 
In accordance with the invention, this time-based information is used to 
scale the intensity of voxels in a three-dimensional MR image. Put another 
way, in accordance with the invention, for each voxel within the VOI, the 
enhancement of that voxel as a function of time is determined, and 
individual voxels in a three-dimensional MR image of the VOI are visually 
emphasized and de-emphasized in accordance with this determination and the 
requirements of the physician or technician. 
The preferred embodiment of the invention further proceeds from the 
realization that if an intravascular contrast agent is used, a high 
resolution acquisition of longer duration can be run after the contrast 
agent has reached an equilibrium state in the bloodstream. The time-based 
information is used to scale the displayed intensities of voxels in a 
high-resolution MR angiogram, and the image intensities of individual 
voxels in the high-resolution MR angiogram can be manipulated so as to 
visually distinguish arteries from veins. While this scaling step is 
particularly advantageous because of the higher resolution of the 
diagnostic image, it is not necessary. Even without an intravascular 
contrast agent and a high-resolution MR angiogram, the time-based 
information is sufficient to characterize the path of blood flow and to 
thereby distinguish and depict arteries and veins. 
In accordance with another of its aspects, the invention resides in a) 
administering a bolus of contrast agent to the patient's circulatory 
system in such a manner as to enhance the vascular structure, b) acquiring 
three-dimensional MR image data from the VOI in such a manner as to 
register, as a function of time, movement of the contrast agent through 
the vascular structure, c) scaling voxels in a three-dimensional MR image 
of the VOI in accordance with the individual sections to be selectively 
emphasized, and d) displaying the three-dimensional MR image using image 
values taken from the scaled acquired MR image data. Advantageously, a 
high-resolution three-dimensional MR image of the VOI is acquired after 
the contrast agent has reached equilibrium in the VOI, the scaling step is 
applied to the image data in the high-resolution three-dimensional MR 
image, and the displaying step includes the step of displaying a Maximum 
Intensity Projection ("MIP") of the three-dimensional MR image in which 
the image values have been scaled. 
In accordance with another aspect of the invention, a bolus of contrast 
agent is administered to the patient's circulatory system in such a manner 
as to enhance the patient's blood vessels within the VOI. Then, in 
accordance with this aspect of the invention, a plurality of 
three-dimensional MR datasets (these will be referred to herein as 
"dynamic MR datasets") are acquired from the VOI, beginning with the 
administration of the contrast medium and continuing for a sufficiently 
long time to reflect contrast agent enhancement of all arterial and venous 
blood vessels within the VOI. These dynamic MR datasets, taken as a whole, 
in effect provide time-based information which, as explained above, can be 
used to distinguish arteries from veins (and indeed can be used to 
distinguish between different parts of a single artery or vein). 
Advantageously, and if an intravascular contrast agent is available, a 
three-dimensional MR angiogram of the patient's VOI is acquired after the 
contrast agent has reached equilibrium. This in effect provides a 
high-quality three-dimensional image of the patient's vasculature. Then, 
for each voxel within the VOI, enhancement of that voxel as a function of 
time post administration of the contrast agent is computed based on the 
information derived from the dynamic 3D datasets. After such computation 
has taken place, parameters (advantageously, time-to-peak-enhancement, 
magnitude of peak enhancement, slope of signal enhancement as a function 
of time) that distinguish enhancement of voxels relating to the patient's 
arteries from enhancement of voxels relating to the patient's veins are 
selected and the intensity of each voxel in the three-dimensional MR 
angiogram is scaled in accordance with the selected parameters. Then, 
advantageously in accordance with the preferred embodiment of the 
invention, a maximum intensity projection ("MIP") reconstruction of the 
VOI is generated. Although an MIP reconstruction is presently preferred, 
it is not necessary; it may alternatively be advantageous to use some form 
of 3D surface rendering instead. 
Advantageously, and in accordance with the preferred embodiment of the 
invention, the physician or technologist carries out the scaling step 
interactively while viewing the MIP reconstruction. This permits the 
physician or technologist to so adjust the display as to emphasize only 
the particular structure (vein, artery, or part(s) thereof) of interest. 
Further advantageously, the dynamic MR datasets are acquired rapidly to 
provide high temporal resolution. When so acquired, the dynamic MR 
datasets are of relatively low spatial resolution. If the contrast agent 
is of the intravascular type, a high-resolution three-dimensional MR 
angiogram can be acquired over a longer time. The voxel sizes of the 
dynamic MR datasets and the high-resolution MR angiogram are effectively 
normalized by spatially interpolating the dynamic MR datasets to 
correspond to the spatial resolution of the high resolution angiogram. In 
this manner, a signal enhancement curve is generated for every voxel in 
the high resolution angiogram.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred embodiment herein described is carried out on a conventional 
MR imager (not shown) using a three-dimensional pulse sequence (also not 
shown). The type of imager, and the pulse sequence used, are not part of 
the invention. Furthermore, although the preferred embodiment is 
illustrated using a VOI that includes the heart and cardiac blood vessels, 
this is only because cardiac MRA is of particular interest. The VOI is not 
a part of the invention. 
The preferred embodiment of the invention assumes that a bolus of an MR 
contrast agent is administered to the patient. Although the contrast agent 
may be one of the currently commercially available Gadolinium (Gd)-based 
MR contrast agents, it is particularly advantageous if the contrast agent 
is one of the new intravascular agents currently under development, such 
as MS-325 manufactured by Epix Medical, Inc. Conventional Gd-based MR 
contrast agents diffuse into tissue through the capillaries, but the new 
under-development intravascular agents do not do so; they remain in the 
patient's vascular structure. 
In a first step 10 (see FIG. 5) in accordance with the preferred embodiment 
of the invention, a bolus of the contrast agent is administered (as by 
injection) to the patient's circulatory system. As is known to persons 
skilled in the art, the bolus will travel through the patient's 
circulatory system and will progressively diffuse (i.e. will become less 
and less bolus-like). At some point, the contrast agent will reach 
equilibrium within the patient, i.e. will be evenly distributed in the 
patient's circulatory system within the VOI, where it will uniformly 
enhance the arteries and veins within the VOI. 
After the bolus of contrast agent has been administered, a series of 
three-dimensional MR datasets ("dynamic MR datasets") is acquired from the 
VOI (see step 12 in FIG. 5). In this example, seven such dynamic MR 
datasets are acquired, and each acquisition is two seconds long, but this 
is not required. There may be more or fewer dynamic MR datasets and each 
may be shorter or longer than two seconds. (A person skilled in the art 
can select the number and duration of the dynamic MR datasets in 
accordance with the study being carried out.) Indeed, the dynamic MR 
datasets need not be acquired for identical periods of time and need not 
be evenly spaced apart; this is convenient but unnecessary. The purpose of 
acquiring the dynamic MR datasets is to acquire information about how 
voxels in the VOI are enhanced as a function of time. 
As can be seen from FIG. 1, differences between the dynamic MR datasets 
represent the progress of the contrast agent within the VOI as a function 
of time. Because in accordance with the preferred embodiment each dataset 
is acquired relatively quickly, the dynamic MR datasets are of 
comparatively low resolution, i.e. the acquired MR data is comparatively 
coarse and relates to comparatively large voxels within the VOI. 
In further accordance with the preferred embodiment of the invention, the 
next step 14 (FIG. 5) is to acquire a high-resolution three-dimensional MR 
angiogram of the VOI (FIG. 2) once the contrast agent has reached 
equilibrium. In such an angiogram, all arteries and veins are enhanced and 
arteries cannot be visually distinguished from veins. As will be seen 
below, this angiogram is used as the source of information relating to 
that portion of the patient's anatomy that is within the VOI. Because the 
angiogram is of high resolution, the acquired MR information is 
comparatively fine and relates to comparatively small voxels within the 
VOI. Although acquisition of such a high-resolution angiogram is not 
necessarily required, it is advantageous because it allows the physician 
or technologist to view a higher quality image. 
When, as in the preferred embodiment of the invention, there is a 
difference in the resolutions of the MR datasets and the three-dimensional 
MR angiogram, it is necessary to recast the acquired data so that it is in 
a consistent form. Advantageously, and in accordance with the preferred 
embodiment of the invention, this is done by spatially interpolating the 
MR datasets (step 16 in FIG. 5); such spatial interpolation techniques are 
known to persons skilled in the art. The result of this spatial 
interpolation is to normalize voxel size so that, as explained below and 
illustrated in FIG. 3, the time-based intensity information from the MR 
datasets can easily be exported to the MR angiogram. As is known to 
persons skilled in the art, mapping of temporal information from the low 
spatial resolution dynamic datasets onto a higher resolution MR angiogram 
cannot be properly carried out without two other steps, namely "image 
registration" and "thresholding". "Image registration" refers to 
compensation for patient movement during acquisition of the dynamic MR 
datasets, and "thresholding" refers to the process of excluding certain 
voxels from consideration during later computational steps. (Such 
exclusion is necessary when the detected enhancement of such voxels cannot 
be clearly identified as originating from something other than noise.) 
These steps are not described here; they are not part of the present 
invention. 
In accordance with the preferred embodiment of the invention, for each 
voxel within the VOI, enhancement of that voxel as a function of time post 
administration of the contrast agent is computed (step 18 in FIG. 5). 
Advantageously, this is done using conventional curve-fitting techniques; 
such techniques are known to persons skilled in the art. Exemplary results 
of such effort are shown in FIG. 4. 
The curves shown in FIG. 4 can be distinguished using various mathematical 
parameters. For example, because the bolus of contrast agent passes 
through the arteries before it passes through the veins, the time-to-peak 
enhancement of voxels in the patient's arteries is shorter than the 
time-to-peak enhancement of voxels in the patient's veins. Furthermore, 
because the bolus diffuses as it progressively moves through the patient's 
circulatory system, the peak enhancement of arterial voxels is greater 
than the peak enhancement of venous voxels. Consequently, it is possible 
to distinguish arterial voxels from venous voxels by e.g. sorting them on 
the basis of their time-to-peak enhancement, magnitude of peak 
enhancement, etc. Because time-to-peak and peak enhancement can be 
combined into the single parameter of "slope" of the enhancement curve, 
such slope can also or alternatively be used as a basis for distinguishing 
arterial voxels from venous voxels. Other parameters can also be used 
(alone, or together with other parameters). 
Let it be assumed that a radiologist wishes to view an MR image of a VOI in 
which a portion of patient's artery is visually emphasized. Voxels 
relating to this portion can be specified by identifying those voxels in 
which e.g. the time-to-peak enhancement, the magnitude of peak 
enhancement, etc. meet appropriate criteria. Although voxels so identified 
actually relate to voxels in the dynamic MR datasets, because of the 
above-described interpolation and registration steps they can be mapped to 
corresponding voxels in the MR angiogram. Then, the intensity of the 
voxels in the MR angiogram can be scaled in accordance with the selected 
parameters. For example, the image intensity of the identified voxels can 
be set to maximum. This will cause the selected arterial portion to appear 
bright in the MR angiogram. (See step 20 in FIG. 5.) The degree and manner 
in which voxel intensity depends on enhancement curve parameters can be 
varied to advantageously depict a single anatomical portion of the 
circulatory system. Voxels can be made brighter or dimmer depending on the 
value of some function of the enhancement curve parameters. 
In accordance with the preferred embodiment, a Maximum Intensity Projection 
("MIP") is then generated (step 22 in FIG. 5), using the MR angiogram in 
which the image intensities have been scaled as described above. (MIP 
generation is known to persons skilled in the art.) This produces an image 
in which the vascular structure of interest is visually emphasized. 
Advantageously, the above-referenced scaling step is carried out in an 
interactive manner, i.e. the radiologist modifies the selected parameters 
after viewing the MIP image to make the displayed image more appropriate. 
While one or more preferred embodiments have been described above, the 
scope of the invention is limited only by the following claims: