Arash Kardan MD, Director of Regional Hospitals Nuclear Medicine, University Hospitals
Myocardial perfusion imaging (MPI) refers to the use of radiotracers to image regional myocardial perfusion from coronary artery blood flow to the heart muscle. When effectively used, MPI can provide the clinician with a noninvasive technique that yields not only significant diagnostic information, but powerful prognostic data regarding the functional significance of anatomic coronary artery disease. With this data in hand, MPI can help guide the clinician in therapeutic decision-making by risk stratifying patients with respect to future possibility of adverse outcomes.
The development of MPI has been a road paved by many twists and turns over the years. It has been a dynamic process involving the development of advances in perfusion radio pharmaceuticals, pharmacologic stress agents, hardware technology, and imaging and processing software. It was not very long ago when the application of physiologic functional imaging added to exercise stress in order to increase the sensitivity of coronary artery disease detection was considered an innovation. From early planar imaging of thallium-201 with rectolinear gamma cameras to now fully digital hybrid SPECT and PET systems with CT, Nuclear Cardiology has come a long way from its original roots.
The introduction of single photon emission computed tomography (SPECT) capacity in gamma cameras significantly improved the sensitivity and diagnostic accuracy of the technique over early generation rectolinear planar scanners, allowing for better assessment of ischemic burden and/ orinfarct size in the left ventricle. The development of several technetium-99m based tracers, most notably FDA approval of Tc-99m sestamibi and Tc-99m tetrofosmin, further improved image quality over thallium-201 while reducing total radiation exposure secondary to better inherent imaging characteristics secondary to higher photon energy and significantly lower radioisotope half-lives. A subsequent advancement was the addition of ECG-gating to image acquisition which allowed assessment of left ventricular functional information to further improve interpretation including evaluation of left ventricular size, wall motion, and ejection fraction. Additionally, functional information could aid in the interpretation of perfusion findings improving differentiation of true perfusion abnormalities from artifactual findings related to attenuation and technical factors.
Patrick F. Wojtylak, BS HCL CNMT, Supervisor, Nuclear Medicine Department
Positron emission tomography (PET) has arose as another tomographic imaging technique similar to SPECT, but with important unique imaging advantages related to the hardware design and radiation characteristics of positron emitting isotopes, including intrinsically higher image resolution and the capacity for dynamic image and quantitative information acquisition. Pioneering work demonstrating the advantages of myocardial perfusion imaging capabilities of PET did not result in immediate adoption of the technology, primarily due to the limited availability of equipment and PET radiopharmaceuticals.
With the greater availability of hardware and PET radiopharmaceutical, this has now changed and PET has gained utilization in the diagnostic imaging mainstream. For cardiology applications, PET offers improved image resolution when compared with SPECT, enhancing detection of abnormalities in regional myocardial perfusion and allowing for quantitation of regional myocardial blood flow. This difference has been well established over the years with the developments in iterative image reconstruction methods, time-of-flight PET acquisition, and PET quantitative software packages resulting in increased accuracy and diagnostic confidence. Direct comparisons of myocardial perfusion with PET and SPECT have documented the superiority of PET, especially in large patients and those with equivocal previous SPECT results.
Digital SPECT and PET
Nuclear imaging technology for both single photon emission computed tomography (SPECT) and positron emission tomography (PET) have made significant advancements in the past couple years including the development of digital imaging detectors to improve detector sensitivity and image quality while addressing radiation dose concerns through dose reduction. Other advancements have come in the areas of software to improve image reconstruction quality, offer better clinical qualification, and improved data analytics. Some of the biggest advances in the last few years include the introduction of SPECT cameras using digital cadmium zinc telluride (CZT) fully digital detectors. These replace the old photomultiplier tubes (PMTs) that have been the industry standard for decades. CZT detectors are smaller to decrease the size of SPECT systems. The biggest asset of CZT detectors is the improved image quality with direct digital transfer of photons into electrical signals to create images. The detectors are more sensitive that PMT-based cameras, so it enables exams with half the amount of radiotracers used in exams by conventional cameras. Similarly, new generation digital PET scanner fully digital detectors for improved spatial resolution and decreases in scanning time while reducing radiation doses to patients by decreasing radiopharmaceutical doses required.
CT with Imaging
Originally conceived as an improvement for acquiring body density attenuation maps for attenuation correction of SPECT/PET images, current generation units combine the state-of-the-art technologies of both SPECT/PET and CT into units that have full capabilities for both modalities. Most notably, the fusion of physiologic SPECT/PET images with the anatomic image information from CT. CT is a powerful tool, but will bring in some new complexities such as proper initial patient positioning, proper indexing of scan data sets by the imaging equipment, and minimization of both intrascan and interscan patient motion. Having state-of-the-art CT is vital to SPECT/PET as it adds further CT capabilities for calcium scoring and high resolution angiography if desired. There is great interest currently in assessing the optimal combination of SPECT/PET and CT imaging information that will provide imaging in a one-stop evaluation of any individual patient. Many combinations of the two modalities can be customized to gain the desired clinical data at the lowest feasible radiation.
These combined systems use CT for attenuation correct on the nuclear images, while adding CT anatomical image overlays improves the accuracy of diagnosis by visualizing the coronary anatomy to better pinpoint where blockages are causing perfusion defects.
Nuclear medicine technology has significantly evolved in recent years, but in more recent times this has branched out to not only include scanner advancements, but also advanced informatics, quantification software and analytics. As value-based care changes the way healthcare organizations approach care delivery, there is a greater need for easy-to-use, fast and precise imaging. With increased access to better data, clinicians are looking for ways to make that data actionable. This includes customizing patient treatments and better management of imaging departments.
The outlook of MPI as seems optimistic with constantly emerging developments substantially affecting the future role of how MPI will be used. We are seeing on the horizon new imaging protocols, newtracers, new hardware and faster software. The new isotopes are aimed not only at myocardial perfusion but also at viability imaging, apoptosis and cell death, sympathetic receptor imaging, hypoxia, atherosclerotic plaque stability, and cellular metabolism. The use of MPI in acute chest pain evaluation in the emergency room as a useful and rapid tool for risk stratification in patients with negative serum troponin measurements providing clinicians the ability to accurately risk stratify patients, which in turn creates better outcomes and overall survival for these patients. With all of these changes in sight, the future of MPI will be bright.