Cardiac SPECT Takes It Up a Notch

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Nuclear medicine scans that employ SPECT to detect disruptions in myocardial perfusion and other cardiac events have advanced with refinements to the equipment that have improved techniques and increased physician confidence in the value of these studies.

Jack Ziffer, MD, PhD, chairman of radiology at Baptist Hospital of Miami and director of cardiac imaging for the Baptist Cardiac and Vascular Institute, describes the protocol they adopted 10 years ago to incorporate into routine evaluation of patients with chest pain, possibly of cardiac origin, in whom an electrocardiogram (EKG) did not confirm an acute myocardial infarction (MI).

With a practice focused predominantly in a 500-bed community hospital that serves as a referral center for patients with cardiac disease, he relates that they see a tremendous number of patients in their emergency department with chest pain.

Using their Philips Medical Systems SKYLight SPECT system to evaluate those patients (74 percent of whom experienced resolved chest pain by the time they reached the hospital) resulted in a 16-fold reduction in undiagnosed MIs and a substantial decrease in the "total length of stay" for those hospitalized patients. The system enabled them to streamline the process in the ED, and admit patients who required further treatment.  

Ziffer's department has ordered two hybrid Precedence SPECT/CT systems from Philips, which they anticipate will be installed within the second quarter of 2005. They envisage that the primary benefits of adding these hybrid scanners will include providing enhanced attenuation correction to the SPECT studies, allow calcium scoring on patients to exclude acute coronary syndrome from the differential diagnosis, and for those patients who experience an equivocal scan, they will perform a CT angiogram. Therefore, they expect the addition of the Precedence will enable a comprehensive cardiac imaging workup in one place.


Attenuation correction at the heart of accuracy



In June 2003, the two premier nuclear medicine associations issued a joint statement outlining the clinical significance of attenuation correction: "It is the position of the American Society of Nuclear Cardiology and Society of Nuclear Medicine that incorporation of attenuation correction in addition to ECG-gating with SPECT myocardial perfusion images will improve image quality, interpretive certainty, and diagnostic accuracy. These combined results are anticipated to have a substantial impact on improving effectiveness of care and lowering healthcare costs."

Gary V. Heller, MD, PhD, professor of medicine at the University of Connecticut and director of nuclear cardiology at Hartford Hospital, describes a practice that performs 6,000 cardiac SPECT studies annually with Philips CardioMD scanners, in addition to a laboratory that is involved in multiple research projects designed to move the field of nuclear cardiology forward. He asserts that although attenuation correction has been used for several years, recent advances permit techniques to improve enough so that clinicians are beginning to accept that this is a reasonable solution to a difficult problem. Attenuation correction on the system involves both hardware and software components.  

"It uses a different line source, gadolinium, which scans the patient's body at the same time that the other data are being obtained, with a very low radiation dose," Heller explains. "Using the software, the transmission data and emission data are analyzed to provide one solution."  

Without attenuation correction, many patients require additional procedures such as cardiac catheterization, which Heller describes as a "downtick for nuclear cardiology." In other words, if further evaluation is required due to lack of confidence in the SPECT results, eventually referring physicians will not request nuclear scans.

GE Healthcare approaches the issue of attenuation correction using another methodology via their hybrid Infinia Hawkeye dual-detector SPECT/CT system. This combined SPECT camera and GE single-slice CT scanner registers CT transmission attenuation correction and functional anatomical mapping. It is the same concept as PET/CT that combined anatomy and physiology.

Jesus Bianco, MD, professor of radiology at the University of Wisconsin in Madison, describes their use of the Infinia Hawkeye to accomplish myocardial perfusion with attenuation correction and gating studies, risk stratification for patients with recent acute coronary syndrome and in evaluating patients who have undergone stenting or coronary artery grafting procedures. In those cases, they would use these studies to look for re-stenosis or graft occlusion.

Bianco emphasizes the importance of attenuation correction, "This system has increased my specificity and decreased the number of false positives, especially for post-ischemia of the inferior wall of the heart. That wall is very difficult to assess due to movement of the diaphragm and other attenuation issues.

Dominique Delbeke, MD, PhD, professor and director of nuclear medicine and PET in the department of radiology and radiologic sciences at Vanderbilt University Hospital in Nashville, explains their Infinia Hawkeye system has proven valuable for combined SPECT/CT cardiac perfusion exams.  

The only challenge they face when managing the fused images is in being able to distribute them over PACS. The functional imaging and CT scans are DICOM-compatible as separate and unique data, but the combined and fused images are not in a DICOM format. She believes PACS vendors are developing a solution to this issue.

Besides the concerns over attenuation correction, vendors have worked to develop smaller cardiac SPECT systems, and one provides solid-state gamma cameras.


Smaller and faster


In October 2004, Digirad Corp. launched the mobile Cardius-3, triple head camera system that is designed to fit in a room as small as eight-by-seven feet. The three Solidium, solid-state detectors provide high count imaging in a dedicated cardiac scanner that utilizes cardiocentric techniques so the patient's heart is centered in the field of view.

The Cardius-3 system has been evaluated by Howard C. Lewin, MD, who supervises 15 cardiac labs in southern California as medical director of Cardiac Imaging Associates of Los Angeles.

"Digirad image quality has improved to where now it is indistinguishable from standard SPECT cameras," says Lewin.  "This is an upright camera, and they've made improvements in the design of the chair and how the camera rotates. Patients seem to be more comfortable as they recline in the chair as opposed to lying supine."  

Additionally, Lewin describes the system as capable of capturing images 38 percent faster than a standard system, yielding productivity gains.  This means that a busy practice could either finish their workday earlier or image more patients during their usual hours.

Robert W. Pattillo, MD, FACC, who serves as director of nuclear cardiology for Cardiology Associates of West Redding (Pennsylvania), has a cardiology specialty practice with 17 cardiologists using four nuclear medicine cameras where they accomplish about 486 studies per month, or about 5,000 per year. They use one of the earlier model Digirad single-head cameras; two dual-head units and they have just ordered the Cardius-3 to further develop their capabilities.

He notes that the speed of acquisition provided by the triple-head system reduces motion artifact thereby improving image quality. And he describes the fact that the detectors are solid-state as the primary reason for clarity in the digital images the system produces. Digirad demonstrated the attenuation correction product on the Cardius-3 as a works-in-progress during the American College of Cardiology meeting in March 2005.

In September 2003, Siemens Medical Solutions launched c.cam, a reclining dedicated cardiac gamma camera system for SPECT studies. Featuring an 8-foot-by-8-foot footprint, the system is designed to provide perfusion studies, ejection fraction and wall motion studies via analysis by fully integrated software in either a hospital or cardiology office.

This system uses a reclining chair, which patients have compared to a home recliner that is designed to improve access for patients who face mobility challenges or other serious health conditions.

Keith B. Churchwell, MD, assistant clinical professor of medicine and cardiology at Vanderbilt University Medical Center and a partner in the Page-Campbell Heart Institute, has been evaluating the c.cam system for more than a year. This practice comprises a combination of private practice and a partnership with Vanderbilt and the Heart Institute to read imaging studies from three towns, the furthest being 125 miles away. Utilizing a broadband network through the central Page-Campbell office, they can send an entire imaging packet from multiple sites to the central reading computer in 10 to 15 seconds with no loss of information.

Churchwell has been very pleased in terms of c.cam imaging characteristics, and concludes that imaging studies for cardiac perfusion, left ventricular function and wall motion have proven quite accurate regarding identification of specifics in symptomatic patients and risk stratification for those individuals who are potential candidates for developing significant cardiovascular disease.

He also believes that because this system is less expensive than standard fixed camera systems, that outpatient radiology centers can afford to provide high-quality SPECT imaging.


Future directions


Refinements to the current cameras include the development of smaller systems, and improvements in both quality of images and attenuation correction functionality, but the future seems to lie in release of the hybrid SPECT/CT systems that enable more complete diagnostic studies for coronary artery disease. As Philips releases the Precedence system, Siemens offers the Symbia TruePoint, and GE Healthcare provides the Hawkeye system with a multislice CT/SPECT machine in early development, the field should continue to develop new procedures.

One of the advantages of SPECT over PET (positron emission tomography) is that the radioisotopes used for SPECT have longer half-life characteristics, which enables the monitoring of tissue changes over a period of time. The other benefit is that the isotopes used in SPECT are being designed to be quite specific in their targeting abilities, and this holds promise not only for improved imaging, but also for potential therapeutic applications.


The challenges


Besides the challenge explained by Delbeke regarding DICOM compatibility for fused images, other issues arise from the sheer volume of data generated by the multislice CT components of hybrid systems.

Baptist's Ziffer relates an example. At the end of the last millennium, they had three single-slice CT scanners, which were able to generate three slices per second if they were all running simultaneously. Now, with the addition of the two Precedence machines plus their other imaging modalities, they are able to generate 539 images per second.

"Of course we won't be running all of the machines at the same time, but we could generate, in theory, a terabyte an hour." This theoretical situation pushes them to consider which data should be saved, and which can be discarded. Currently, they are upgrading the short-term cache on their PACS, and trying to anticipate future problems.


Conclusion


Cardiac SPECT has improved patient care as systems are refined to produce better image clarity and attenuation correction that helps to pinpoint myocardial perfusion disruptions and ischemia. Equipment manufacturers have developed hybrid machines that incorporate fast multislice CT scanners with SPECT systems to further benefit management of patients with heart disease.