SPECT/CT Takes Myocardial Perfusion Imaging to the Next Level
There are several critical drawbacks associated with conventional myocardial perfusion SPECT imaging. Attenuation correction is not always ideal, particularly in obese and morbidly obese patients, which can diminish image quality and prevent the nuclear medicine department from obtaining diagnostic results.

SPECT/CT imaging offers a new model. Hybrid scanners such as Siemens Medical Solutions TruePoint Symbia T6 system deliver improved attenuation correction for more accurate results across the entire patient population. Patients who required additional studies under earlier protocols may avoid the additional procedures. The hybrid scan also serves multiple purposes and can provide data about myocardial perfusion, ventricular function and coronary calcium in a single imaging study. In addition, imaging time is decreased. Finally, SPECT/CT remains in it infancy. Researchers continue to explore hybrid techniques to develop new protocols that could deliver improved patient care and enhance utility of the hybrid exam.

These clinical images of a 380-pound woman imaged on a Siemens TruePoint Symbia T6 SPECT•CT scanner at the University of Michigan Health System show the uncorrected images in the top rows and the left polarmap and the corrected SPECT with CT based attenuation correction in the bottom rows and right polarmap.





























The clinical snapshot

Until late in 2004, University of Michigan Health System in Ann Arbor, Mich., relied on a stress-first protocol to assess myocardial blood flow, scarring or infarction, or evaluate and assess the extent of coronary artery disease in most patients. High-risk patients, however, were imaged rest first. That is, the typical first course of action in the nuclear medicine department was a stress myocardial perfusion SPECT study. The rationale for this approach is the high number of normal studies with attenuation corrected SPECT imaging.

The stress-first protocol, however, is not without drawbacks. “The protocol is adequate for many patients, but it does not meet the needs for a significant portion of the patient population,” says James Corbett, MD, director of cardiovascular nuclear medicine. That is, the conventional attenuation correction techniques did not produce diagnostic-quality results for obese and morbidly obese patients.

“We used sealed source attenuation correction, which worked quite well in patients weighing up to 220 to 250 pounds,” recalls Corbett. The sealed source attenuation correction technique is based on the acquisition of a very low-resolution transmission scan. Although the low-resolution scan sufficed for patients weighing less than 220 to 250 pounds, results for more obese patients were not diagnostically viable. That is, University of Michigan Health System, as well as every other site across the country, could not reliably obtain diagnostic myocardial perfusion SPECT results for patients exceeding the 250-pound mark. SPECT results often included lateral artifacts in morbidly obese individuals in addition to exaggerated breast and diaphragmatic artifacts in this patient population related to inadequate attenuation correction that obscured correct diagnoses.

The clinical imaging solution for obese patients without hemodynamically significant coronary heart disease generally entailed an additional imaging SPECT imaging session. “This required a second injection of radiotracer,” notes Corbett. The patient visit was extended by at least one to two hours, and the additional exam procedure slowed workflow for both the nuclear medicine physician and technologist.

In November 2004, the health system acquired its first Siemens TruePoint T6 SPECT•CT scanner. It provided an opportunity to develop a new protocol for myocardial perfusion SPECT imaging that centers on the use of the diagnostic CT for attenuation correction. “The CT-based attenuation correction results are robust — even in patients weighing up to and over 400 pounds. This was never the case with sealed source attenuation correction,” states Corbett.

Now, the nuclear medicine department employs tidal-end expiratory breathhold for CT acquisition for attenuation correction. The new approach ensures absence of misregistration and consequently improves the accuracy of attenuation correction without misregistration artifacts.

Improved image quality is a critical benefit, especially for heavier patients, but the new system delivers additional benefits as well, says Corbett. Image quality improved in all patient populations, although results for patients under the 220- to 250- pound mark are not as dramatic as those of heavier patients. “It’s apparent that the hybrid SPECT/CT exam not only improves the specificity of SPECT results, but also increases sensitivity,” explains Corbett. In addition, the diagnostic CT portion of the exam opens the door to other imaging procedures — particularly coronary calcium imaging.

Another benefit comes in the form of scan time. A conventional gated myocardial perfusion SPECT study takes approximately 16 to 20 minutes, but a combined SPECT/CT approach drops image acquisition time to 12 to 16 minutes. “There is some evidence that we can cut scan time an additional 25 to 50 percent,” adds Corbett. 

The clinical advantages of the SPECT/CT approach are significant. “New SPECT/CT systems such as Siemens TruePoint Symbia T6 are capable of imaging patients weighing up to 500 pounds. In the past, it was not possible to perform a gated SPECT study on patients weighing more than 300 to 350 pounds. We would have no alternative but to switch to a planar study, which does not provide essential information about cardiac function. The stress gated study with SPECT/CT-based attenuation correction, on the other hand, delivers data about myocardial perfusion, ventricular function and coronary calcium,” explains Corbett. Consequently, physicians’ diagnostic confidence is increased.

Patients also benefit from the hybrid approach. Most importantly, physicians can render an accurate diagnosis with a single SPECT/CT study in many patients. Patients avoid the time and additional radiation exposure associated with a second SPECT imaging session.

 
Future directions

“Diagnostic CT coupled with myocardial perfusion SPECT is fairly new,” confirms Corbett. “We are early in the process and constantly learning how to better use hybrid imaging devices and how to better match SPECT acquisitions with CT acquisitions to generate the most reliable imaging data.”

One area of high interest is new reconstruction algorithms. These algorithms could decrease acquisition time to less than 10 minutes, Corbett says. The ultra-short acquisition time will lead to higher patient throughput; however, the real plus of a shorter acquisition time relates to patients with limited abilities to lay supine during the scan. “Some patients, particularly those who are older or sicker, have a fairly limited tolerance for lying supine and motionless during the scan. At our institution, a few patients out of the 15 to 20 scanned daily fall into this category,” explains Corbett. The briefer scan could allow the nuclear medicine department to image and obtain accurate diagnostic results for an even wider patient population. 

Researchers are continuing to explore possibilities associated with new SPECT/CT reconstruction algorithms. “It will take another year or two to determine whether or not new reconstruction algorithms can drop scan times below 10 minutes while still providing accurate diagnostic results,” sums Corbett.


Diagnosing with confidence

Myocardial perfusion SPECT/CT is a fairly new development. The hybrid scans improve conventional myocardial perfusion SPECT results on a number of fronts. For starters, CT-based attenuation correction allows physicians to achieve diagnostic-quality results on a wider patient population. That is, in the conventional protocol, the SPECT scans of obese patients were plagued by artifacts related to poor quality patient density (attenuation) maps. Consequently, physicians were forced to perform a second resting study to secure diagnostic quality results. With the advent of SPECT/CT; however, physicians use CT-based attenuation correction, which eliminates or greatly reduce artifacts in patients weighing up to 400 pounds or more. Image quality, sensitivity and specificity are improved on all myocardial perfusion studies, resulting in increased diagnostic confidence by both nuclear medicine and referring physicians. In addition, scan time for all patients is shortened from approximately 16 to 20 minutes to 12 to 16 minutes, allowing sites to schedule more studies on a daily basis and freeing up patients’ time.

SPECT/CT is a wining combination for myocardial perfusion imaging. It provides physicians with the data needed to provide more accurate results in less time for a larger patient population.


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