8 Patrick J. La Rivi`ere
2.2. Cardiac imaging
The heart is without a doubt the most difficult organ in the entire b ody to image.
It contracts and twists relentlessly and rapidly — more than 100 times per minute
in some patients — and sometimes irregularly. Moreover, the structures of great-
est clinical interest, the corona ry arteries, are only a few millimeters in diameter.
Acquiring a snapshot of the beating heart, with motion frozen and the coronary
arteries well resolved, has long been the holy grail of computed tomography imag-
ing. Each new g eneration of multislice scann er and each increase in gantry rotation
speeds has moved us closer to that goal.
9,10
Early approaches to car diac CT imaging with helical scanners concentrated
mainly on obtaining images of the heart frozen at a single, somewhat extended phase
of its cycle — diastole — when motion is minimized. This typically involves using a n
ElectroCardioGram (ECG) signal acquired in synchrony with th e helical imaging
to identify those sectors of the measured data that correspond to the extended
diastolic phase. So long as each of these sectors spans at least a halfscan angular
range (180 degrees plus the fan angle), it is p ossible to reconstruct an image of the
heart a t diastole that is relatively free of motion.
3
In single-slice scanners, of course,
achieving the volume coverage needed to span the heart during a single breathhold
requires some sacrifice in longitu d i n al resolution. Because of their improved volume
coverage, multislice scanners allow these sing le-phase imag e s to be acquired at much
higher longitudinal resol u ti on than do single-slice scanners, providing essentially
isotropic r esolution in each reconstructed volume.
Even more exciting, the sp eed and volume coverage of multislice scanners makes
it possible to perform multi-phase cardiac imaging, in which one attempts to create
snapshot views of the heart at a number of different phases of the cardiac cycle.
The end result is a truly four-dimensional dataset: An animation of the beating
heart. To reconstruct such multi-phase cardiac images, it is generally necessary to
integrate data acquired during several cardiac cycles at a given phase of the heart
cycle.
11,12
Figure 5 depicts a volume rendering of a cardiac study acquired on a
40-slice scanner.
In addition to visualizing the coronary ar teries, cardiac CT is often employed
to produce a calcium score, quantifying the degree of calcification in the coronary
arteries, which is believed to be pre d ictive of the severity of coronary disease and
the likelihood of future coronary events.
9
The use of this technique as a screening
exam is somewhat controversial, however, as some degree of coronary calcification
seems to occur in almost all patients as they age, regardless of whether they are
genuinely at risk of a heart attack. Thus it is unclear whether the benefits of an
invasive procedure in someone with a high calcium score would necessarily outweigh
the risks of such a pro cedure. As far as CT technology, the image-quality demands
of calcium scoring are not quite as high as those for images used primarily for
visualization of the coronaries, since calcium scoring generally involves averaging
CT numbers over some reg i on of interest a nd is thus tolerant of slightly lower
resolution.
我的内容管理
展开
