Page 365 - Biomedical Engineering and Design Handbook Volume 2, Applications

NUCLEAR MEDICINE IMAGING INSTRUMENTATION 343

dedicated whole body PET scanner with a diagnostic multislice CT scanner. PET/CT systems offer
a number of advantages. The quality of the PET images is significantly improved by the CT trans-
mission correction. The CT transmission study is much faster than the radionuclide transmission so
that studies are completed in about half the time of the PET-only systems. This shortened scan time
not only provides for higher patient throughput, it also decreases the artifacts associated with patient
motion. The other obvious advantage to PET/CT systems is the availability of accurately coregistered
images (Fig. 11.17b). All the vendors have convenient viewing software that allows the simultane-
ous review of the PET, CT, and fused coregistered image sets. The viewing physician can localize
an area of concern on any one of the displays and immediately see the corresponding location on
the other views. Several groups have shown that information displayed in this manner provides
gains in sensitivity, specificity, and confidence of interpretation, especially for the less than experi-
enced reader. Although coregistration of PET and CT is possible with data sets acquired on different
systems, the practical implementation is often difficult. Problems include different body orienta-
tions (e.g., arms up on one scan, arms down on another), breath-hold conditions, and access to both
data sets.
There are issues with the PET/CT systems that have to be considered. The cost of a PET/CT sys-
tem is at least 50 percent higher than a PET-only system. A much larger room is required so that the
patient can be fully translated through both devices. The effective radiation dose to the patient from
the CT acquisition adds 500 to 1200 mrem to the study. Also additional technologist training is
required for operating the CT scanner. Other issues related to technologist training and health-care
personnel include the use of contrast agents for the CT portion of the study.

11.4.3 PET MRI

The combination of PET and CT proved to be very successful and has kindled the desire to combine
PET with MRI. There are several challenges posed by performing PET imaging in the vicinity of an
MRI system. The photomultiplier tubes still used in all commercial PET systems are very sensitive
to magnetic fields. There is sufficient light output from LSO that will allow that the PMTs can be
replaced with solid-state avalanche photodiodes. 38 Another problem that needs to be addressed is
attenuation correction. The attenuation values found in CT images can be used to accurately estimate
attenuation values for the 511-keV annihilation photons. However, the information obtained from an
MRI scanner cannot be transformed directly into attenuation coefficients. In some areas like the head
that are fairly homogeneous, this may not be difficult to accomplish, but in the thorax the determi-
nation of lung attenuation will be very challenging. These problems have not stopped the research
and development of PET MRI devices, and prototypes exist for both small animal and human
imaging. Human devices include a brain PET MRI system at the Eberhard-Karls University,
Tuebingen, Germany, and a whole body PET MRI system at the University of Cambridge, England.
An operational small animal PET MRI device has been described by Catana et al. 39

11.5 SMALL ANIMAL IMAGING

Imaging studies with small animals, especially with mice and rats, provide valuable information in
the development of new drugs and treatments for human diseases. There is widespread interest in
small animal imaging and it continues to grow. Investigations with rats and mice are indispensable
for evaluating pathophysiology, radiopharmaceutical development, and genetic research. 40,41 Targeted
research with knock-out and knock-in mice strains is especially important, and all this attention has
stimulated the development of imaging systems from many modalities optimized for these animals.
Because of the high sensitivity associated with the radiotracers and their ability to deliver crucial
information about physiological function, it is natural that both PET and SPECT are major players
in small animal imaging. However, small animal imaging presents many challenges because of the
small volumes and low radioactivity concentrations that have to be accurately imaged.