SPECT/CT Physical Principles and Attenuation Correction

Abstract

Using nuclear medicine techniques, physiologic activity and processes can be identified in a way that is unique from other modalities. Oftentimes it is helpful to know the exact location of the physiologic uptake that is visualized on a scan. Knowing the exact location can sometimes help to distinguish normal from abnormal physiologic uptake. When an abnormality has been identified, knowing the exact location can then be helpful in treatment planning. The ability to provide precise localization of physiologic data from nuclear medicine studies is now possible with hybrid SPECT/CT systems. Additionally, these systems provide an accurate attenuation correction of the nuclear medicine image data. After reading this article, the technologist will be able to list and describe the inherent problems associated with SPECT image acquisition and reconstruction, briefly explain how data acquired from the CT scanner are used to provide attenuation correction data for SPECT and anatomic information for diagnostic purposes, list and briefly describe the different types of clinical SPECT/CT systems, and discuss the importance of accurate CT and SPECT image registration. For many years, physicians have been faced with the dilemma of identifying a region of increased uptake on a nuclear medicine image and then trying to determine the precise anatomic location of the region. This is of initial importance in differentiating between abnormal uptake and normal physiologic uptake. Once the determination of abnormal uptake has been established, then it is important for physicians to know the exact anatomic location of the region to determine the proper course of therapy. Recent advances in imaging technology have provided physicians with a new and powerful tool to handle this dilemma: integrated SPECT/CT systems. This work was pioneered by Lang and Hasagawa et al. (1), who not only combined hardware components into an integrated system but also developed important algorithms for SPECT attenuation correction using CT images. This technology makes it possible to acquire physiologic and anatomic images in a registered format and fuse them so that precise anatomic localizations of radiopharmaceutical distributions can readily be visualized. An additional benefit of this technologic advance is that the anatomic images can be used to perform high-quality attenuation corrections of the radiopharmaceutical distributions. To properly acquire these registered images and identify potential pitfalls in the process, it is necessary to have a good working knowledge of the physical principles and instrumentation involved in the data acquisition and image reconstruction processes. SPECTData required for constructing images using the techniques of SPECT are acquired by collecting planar images (typically in 64 · 64 or 128 · 128 data-point arrays) at multiple angles around the patient using single-, dual-, or triple-head scintillation cameras typically equipped with parallel-hole collimators, which provide projection views of the radioactivity from tho...

Hybrid Imaging: Instrumentation and Data Processing

“…Similar to in PET/CT, the bilinear method proposed for CT-based AC can be used in combined SPECT/CT systems [18,149]. On the other hand, the most common approach for estimating scatter events in SPECT/CT is the measure of counts in additional energy windows adjacent to the photopeak window.…”

Optimization of attenuation and scatter corrections in sentinel lymph node scintigraphy using SPECT/CT systems

“…Image quality is greatly affected by scattered radiation because injection sites with high levels of radioactivity exist near the lymph node with low levels of radioactivity. Additionally, SPECT/CT systems provide an accurate AC of the nuclear medicine image data [14]. Therefore, we have applied AC and SC to improve the detectability.…”

Attenuation correction of myocardial SPECT by scatter-photopeak window method in normal subjects

“…The SSPAC method only requires dual or triple energy window acquisition: (1) emission by 140keV±10%, (2) scatter by 7% of the lower side of the emission window (3) septal penetration by 7% of the higher side of the emission window. The procedure for AC method was as follows: First the contour maps of the body, lung, heart and liver outlines were generated from photopeak and scatter images.…”

“…Attenuation correction (AC) with X-ray computed tomography (CT) has been sought after for reducing attenuation artifacts [1][2][3][4][5][6][7][8]. The diagnostic accuracy for detection of coronary artery disease will be increased when a myocardial perfusion SPECT (MPS) image is attenuation corrected using patient-specific non-uniform attenuation coefficient maps.…”