Page 351 - Biomedical Engineering and Design Handbook Volume 2, Applications

NUCLEAR MEDICINE IMAGING INSTRUMENTATION 329

11.3.3 SPECT Cardiac Devices
With myocardial perfusion studies, patients are injected with a radioactive tracer that distributes
throughout the body, but its distribution in the heart reflects myocardial blood flow. Areas of low
tracer uptake indicate coronary artery disease. Two SPECT studies are performed in the evaluation
of myocardial perfusion, one where the tracer is administered under normal conditions and the other
where it is administered when the patient has had a physical or pharmacologic stress that requires
increased cardiac blood flow. Because the heart is located in the left anterior portion of the thorax,
gamma rays originating in the heart are highly attenuated for views collected from the right lateral
and right posterior portions of the arc. For this reason, SPECT studies of the heart are usually col-
lected using the 180° arc that extends from the left posterior oblique to the right anterior oblique
view. 14 This results in reconstructed images with the best contrast, although distortions are often
somewhat more pronounced than when 360° data are used. 15 Because of the widespread use of
myocardial perfusion imaging, many SPECT systems have been optimized for 180° acquisition by
using two detectors arranged at ~ 90°. This reduces the acquisition time by a factor of 2 over single
detectors and is approximately 30 percent more efficient than triple-detector SPECT systems.
Positioning the detectors at 90° poses some challenges for maintaining close proximity. Most sys-
tems rely on the motion of both the detectors and the SPECT table to accomplish this.
The heart is continually moving during the SPECT acquisition, and this further compromises spa-
tial resolution. Because the heart beats many times per minute, it is impossible to directly acquire a
stop-action SPECT study. However, since the heart motion is periodic, it is possible to obtain this
information by gating the SPECT acquisition. 16 In a gated SPECT acquisition, the cardiac cycle is
subdivided and a set of eight images spanning the ECG R-R interval is acquired for each angular
view. These images are acquired into predetermined time bins based on the patient’s heart rate,
which is monitored by the ECG R wave interfaced to the SPECT system. As added benefits of gat-
ing, the motion of the heart walls can be observed and ventricular volumes and ejection fractions can
be determined. 17
As previously noted, myocardial perfusion SPECT is easily the most common nuclear medicine
procedure and more than 15 million of these are performed each year. It is not surprising then that
special-purpose imaging systems have evolved to serve this need. These instruments can be put into
three categories: (1) general-purpose SPECT systems that can accommodate both myocardial perfu-
sion as well as other SPECT studies, (2) convention-based systems that have been specifically mod-
ified to only perform myocardial perfusion studies, and (3) newly designed devices that are
departures from the scintillation-camera–based systems. The devices in the first category look and
operate like general-purpose SPECT systems. Typically they have two separate scintillation camera
heads that can be configured by the operator to be directly opposed for noncardiac studies or ori-
ented at 90° for myocardial perfusion imaging. The second category features systems with small
field of view scintillation cameras that are fixed in a 90° orientation. Instead of having a horizontal
table where the patient lies, some of the systems are designed to rotate about a reclining chair. At
least one system simplifies the mechanics even further by leaving the detectors stationary and rotat-
ing the patient. Because of the overall reduction in size, these systems can fit into relatively small
rooms and are substantially cheaper than the category (1) systems. However, in terms of the time it
takes to acquire the studies and the methodology, these are very similar. Figure 11.9 shows examples
of cardiac SPECT systems along with an example of perfusion images.
The third group currently has two different devices that having different acquisition strategies that
17
reportedly lead to much faster acquisition times and improved spatial resolution. The first device
is called the CardiArc and is based on the SPRINT II design. The image receptor is a set of NaI(Tl)
bars arranged in an arc. The projections are sampled by a slit-slat combination. Axial slicing is
obtained from a horizontal stack of evenly spaced lead plates (the slats), while the transaxial sam-
pling is obtained by six vertical slits in a lead plate. The combination of the slits and slats approxi-
mates pinhole sampling and by moving the slit plate, angular samples are acquired. The slit plate is
the only moving component of the system. Patients sit in a chair that forms the support for the
CardiArc. Because of the six fold sampling and the high resolution of the pinhole geometry, myocar-
dial perfusion studies can be acquired in less than half the time of a conventional SPECT system.