Principals of SPECT & SPECT/CT :Principals of SPECT & SPECT/CT Raul Del Pozo Mercedes Gonzalez David Ponton Claudy-Ann Rigaud


Objectives :Discuss the physics of SPECT relative to attenuation scatter and detector response Describe attenuation correction preformed by transmission imaging Discuss the advantages of SPECT/CT systems Describe image quality and artifacts in SPECT/CT imaging Discuss scatter correction methods Describe the general techniques used in automatic reorientation Describe techniques used to acquire gated cardiac SPECT studies List quality control tests performed specifically for SPECT Objectives


SPECT Physics and Image Artifacts :Artifacts in SPECT images arise from the combined effects of photon absorption, Compton scatter, and the degradation in resolution with distance from the collimator SPECT Physics and Image Artifacts


ATTENUATION :As a gamma camera head rotates around the body the attenuation of the projected radiation varies with the amount of tissue and the type of tissue that it had to travel through ATTENUATION


ATTENUATION :The attenuation equation is: Ax = A0e-ux The most frequently noted effects of attenuation in myocardial SPECT are artifacts associated with breast attenuation in women and diaphragmatic attenuation in men Attenuation artifacts are more severe using Tl201 than with Tc99m due to Thallium’s lower energies ATTENUATION


COMPTON SCATTER :When a photon undergoes Compton scattering it changes direction and loses energy If these now lower energy photons are detected in the photo-peak energy window they will be out of place in the image, reduce image contrast and reduce lesion detectability Scatter is more severe using Tl201 than with Tc99m due to Thallium’s multiple energies COMPTON SCATTER


DETECTOR RESPONSE :Most important factors in spatial resolution of an image are: Geometry of the collimator Distance from the camera face As the camera orbits the patient only one spot maintains the same distance from the collimator in all views this point is called the COR or center of rotation DETECTOR RESPONSE


ATTENUATION CORRECTION :The compensation for the effects of radiation attenuation in computed tomography uses mathematical estimates or measured transmission data The most commonly cited complications are attenuation artifacts in myocardial perfusion SPECT ATTENUATION CORRECTION


ATTENUATION CORRECTION :One method of handling attenuation artifacts is simply to reduce their clinical impact through scanning techniques and knowledge of their location and severity when they occur ATTENUATION CORRECTION


Conventional Correction Methods :Conventional Correction Methods The most common correction methods have been a pre-reconstruction method based on work by Sorensen and a post reconstruction method developed by Chang Not used in myocardial perfusion because the thorax is too varied for a constant attenuation coefficient to be effective


Sealed source transmission scan-based correction methods :Sealed source transmission scan-based correction methods Most common sealed source is gadolinium-135 Used as a sealed line source with a collimator on top and passed along the patient either after or at the same as the gamma camera is reading Important to realize that defects in transmission scan can have a detrimental effect to the attenuation corrected images


Sealed Source :Sealed Source Many of the SPECT QC procedures transfer over to the transmission imaging systems You must check the transmission tomogram for artifacts (motion, gating, missing frames) Other problems arise from old line sources decayed beyond being usable and body truncation


SPECT/CT Correction Methods :When CT scan is being used for attenuation correction, the pixel values must be scaled to match attenuation coefficients appropriate for the radionuclide being imaged & the CT and SPECT images must be precisely aligned. SPECT/CT images are acquired sequentially. If the patient moves between the two scans, the spatial mismatch can result in artifactual defects. Many clinics routinely utilize attenuation correction for general SPECT studies – this improves the images and the visibility of focal hot spots. SPECT/CT Correction Methods


Scatter Correction These methods require estimation of the number of scattered photons in each pixel of the image. :Dual window scatter subtraction method : There is a primary energy window (photopeak) and a second window at a lower energy. Projections acquired from the second energy window are multiplied to a single-value scatter fraction, to generate an estimated scatter projection in the primary energy window. The estimated scatter projection in the primary energy window is then subtracted from the projections acquired from the second window to estimate the non-scatter photons in the primary energy window or photopeak window. Scatter Correction These methods require estimation of the number of scattered photons in each pixel of the image.


Scatter Correction :Triple energy window technique: Uses scatter windows on both sides of the photopeak window. The contribution of scattered photons to the photopeak window is then estimated as the average counts in the two scatter windows normalized to the photopeak window. Energy window-based methods can provide only approximate scatter correction, and they may increase image noise. Scatter Correction


Resolution Recovery :Frequency distance principal (FDP) – states that points at a specific source-to-detector distance correspond to specific regions in the frequency space of the sinogram’s Fourier transform. Applying a spatially variant inverse filter to the sinograms performs the resolution recovery. This inverse filter is relatively fast but may amplify noise in the image. Optimum accuracy of SPECT image reconstruction requires correction for Compton Scatter, attenuation, distance-dependent spatial resolution and image noise. Resolution Recovery


Cardiac SPECT Quantification :Cardiac Reorientation – most cardiac images are viewed in a standard format consisting of short-axis (SA), horizontal long-axis (HLA), and vertical long-axis (VLA). Vertical long-axis – a display of a transaxial slice through the middle of the left ventricle. (cut runs through the septum) Horizontal long-axis – cut perpendicular to the vertical axis. Divides the heart into anterior and posterior. Short-axis – also cut perpendicular to the vertical axis. Divides the heart into superior and inferior. Cardiac SPECT Quantification


Automatic Reorientation :Automatic Reorientation Automatic reorientation offers the potential for consistently faster and more accurate image processing and analysis. This process is done by using algorithms that isolate or segment the left ventricle using a combination of: - assumed images - computed projections - comparisons of original projection data along with updated images based on the difference between the calculated and actual projections - rule-based location/size/shape criteria.


Slide 20:Perfusion Quantification All commercially available quantification methods for myocardial perfusion in SPECT are based on the idea that sampled counts in the myocardium of a patient's image can be compared with similar counts sampled from a set of normal. Each short-axis slice, the resulting maximum count values are graphed against the angle a which they were encountered, such a graph called a circumferential profile. Normal boundaries or normal limits for each point in each profile are created based on these statistical values. Analysis is performed and normal limits must be created for males and females because normal differences in body shape cause different normal attenuation and scatter artifacts in the reconstructions.


Slide 21:Abnormal areas or defects seen in the stress images that persist on the rest study are considered fixed Methods vary primarily in the way in which the myocardium is sampled Few studies have compared these various approaches to perfusion quantification, it is not a simple matter to choose one based on its comparative accuracy The manufacturer should U.S. Food and Drug Administration (FDA) approval to market it for clinical use Perfusion Quantification


Slide 22:Polar maps Polar maps, or bull's-eye displays, are another way to view circumferential profiles. The most basal slice of the left ventricle makes up the outermost ring of the polar map. Abnormal regions from the stress study are often assigned a black color, thus creating a black-out map. Additional maps, such as a standard deviation map that shows the number of standard deviation below normal of each point in each circumferential profile. Although they offer a comprehensive view of the quantitation results, polar maps distort the size and shape of the myocardium and any defects. "Volume-weighted" maps are constructed such that the area of each ring is proportional to the volume of the corresponding slice.


Slide 23:Three-Dimensional Cardiac Displays Three-Dimensional graphics techniques can be used to overlay results of perfusion quantification onto a representation of a specific patient's left ventricle. These points can be connected into triangles, which are then color-coded in a way similar to a polar map. They have the advantage of showing the actual size and shape of the left ventricle and the extent and location of any defect in a very realistic manner. The biggest disadvantage of 3D displays is that they require more computer screen space than polar maps.


Cardiac Gating :Cardiac Gating Standard cardiac SPECT images suffer from motion blur, because the heart is always in motion. It is well understood that the intensity of the myocardium in nuclear medicine images is related not only to radiotracer uptake but also to relative myocardial thickness. This effect also is “averaged” into a standard static acquisition and may result in impaired diagnostic accuracy. The electrocardiogram is used to determine the heart rate and the onset of contraction at the QRS complex


Cardiac Gating cont’ :Cardiac Gating cont’ Counts are directed into the first frame during the initial T/N seconds after the QRS complex is detected These four-dimensional data allow 3D analysis of motion and myocardial thickening Although the resulting images are of poor quality and probably not useful for perfusion analysis Each projection set and reconstruction will be reduced in counts by a factor equal to the number of collected frames Gating software should be able to deal with abnormal heartbeats and “reject” or ignore counts from these contractions.


Cardiac Gating cont’ :Cardiac Gating cont’ One commonly seen result of a change in heart rate is late frame drop-off Motion blur is also reintroduced, because for some early projection, the cardiac cycle is divided into eight frames, but for late projections, it is divided into six frames. The raises both processing time and storage space for gated studies Global variables such as left ventricular volumes, mass, and ejection fraction can be calculated Most commercially available programs for quantifying cardiac function are fully 3D approaches, which start by detecting endocardial and epicardial surface points through the cardiac cycle The Quantitative Gated SPECT (QGS) program determines the locations of the surface points through fitting if count activity profiles across the myocardium to asymmetric Gaussian curves


Global Variables :Global Variables Once the left ventricular endocardial and epicardial boundaries have been determined, the number of pixels within the chamber or left ventricular wall can be determined Myocardial mass is calculated by multiplying the myocardial volume by an assumed density, usually 1.05 g/ml


Endocardial wall motion :Endocardial wall motion Endocardial wall motion is if the endocardial surface is detected at each point in cardiac cycle, Regional endocardial wall motion can be assessed by computing how each surface point moves Most analyses of left ventricular wall motion rely heavily on simplified models of motion originally developed for 2D contrast ventriculograms or radionuclide ventriculograms


Myocardial Thickening :Myocardial Thickening Myocardial thickening is known to be a better indicator of myocardial viability than endocardial wall motion SPECT system provide resolution on the order of 1 cm, this thickness/intensity relationship should hold true for the vast majority cases, in which the range of myocardial thickness is 0.5 to 2.0 cm One approach to quantification of wall thickening using this theory samples the myocardial counts at numerous location(which is more than 4,000) for every gated frame. Thickening polar plots or 3D displays can be created to show the resulting "percent thickening" computed around the left ventricle.


Quality Control :Quality Control Quality Assurance Approach taken to ensure that a quality product is provided Quality Control (QC) The specific tests done to make sure you reach quality assurance


Uniformity :Making sure that the images produced by the two heads fall precisely on top of one another Uniformity Uniformity Non-Uniformity


Planar Gamma Cameras QC :Daily basis Energy peaks Uniformity tests Weekly basis Linearity Resolution Make sure to look over the tests with a cautious eye Planar Gamma Cameras QC


SPECT QC :Two Calibrations important two SPECT Uniformity COR ( Center of Rotation) Watch for patient motion SPECT QC


Uniformity Correction :Things you MUST check PMT’s ( Photo multiplier Tubes) Imperfect Collimators * Most Commonly seen in floods or phantom acquisitions Uniformity Correction


Center of Rotation :COR The angle about which the camera rotates COR measurement determines the offset between the center of the camera matrix and the projection of the COR. If the offset is large, it will create a ring If it is small it will blur the image Center of Rotation


Patient motion :One of the most significant causes of artifacts in SPECT Tech's should warn the patient not to move Make sure the patient is comfortable Should monitor the patient during acquisition Images should be checked after acquisition Patient motion


Patient motion (heart) :To evaluate motion of the heart, check for the height of the heart that would indicate motion. Ventricle motion Patient motion (heart)


Acquisition playback :Should be a routine part of the tech’s reconstruction process Tech should watch video at high speeds to check for patient motion Motion is much easier to see in multi-head systems. Acquisition playback


Standards and Procedures :National Electrical Manufacturer Association (NEMA) trade association for manufacturers of electrical products Standards publication for performance measurements of scintillation cameras This document describes the standards by which scintillation cameras may be operated. Standards and Procedures


Slide 40:SUMMARY Artifacts in SPECT images arise from the combined effects of photon absorption, Compton scatter, and the degradation in resolution with distance from the collimator Attenuation correction performed by transmission imaging allows us to correctly gauge the attenuation through the body to correct for it to produce a final picture as close as possible to the actual object being imaged The advantages of SPECT/CT systems are that they improve the images and the visibility of focal hot spots If the patient moves between the to scans the spatial mismatch can result in artifactual defects Dual window scatter subtraction method and triple energy window technique are two types of correction methods Automatic reorientation offers the potential for consistently faster and more accurate image processing and analysis. ECG is used to determine the heart rate in gated cardiac SPECT studies to identify the QRS complex to determine when to acquire data Quality control tests performed for SPECT are: Check for energy peaks Uniformity Patient motion Center of rotation


Bibliography :Christian, Paul E., and Kristen M. Waterstram-Rich. Nuclear Medicine and PET/CT Technology and Techniques. 6th. St. Louis: Mosby Elsevier, 2007. "Automatic reorientation." PubMed. 2008. NCBI. 22 Sep 2008 . Changizi, Vahid. "Scatter Correction for Heart SPECT images using TEW method." Journal of Applied Clinical Medical Physics Vol. 9, 10 March 2008 22 Sep 2008 . Bibliography


Q&A :Q&A Attenuation through the body is the same in all directions TRUE FALSE


Slide 43:2. One of the most important factors in spatial resolution in scintillation images is Geometry of the collimator Distance from the camera face Both A and C How much radiopharmaceutical you injected


Slide 44:3. If a patient moves between the SPECT and CT scans the spatial mismatch can result in artifactual defects TRUE FALSE


Slide 45:4. What are two scatter correction methods used for SPECT? A. dual energy window technique B. photopeak energy window technique C. triple energy window technique D. both and A and C


Slide 46:5. Name three standard formats in which cardiac Images are viewed? short axis vertical long axis horizontal long axis


Slide 47:6. What is the biggest disadvantage to 3-D display? A. they require more computer screen space than polar maps B. not enough magnification C. spacial properties of cathode ray tubes are inaccurate D. none of the above


Slide 48:7. Myocardial thickening is known to be a better indicator of myocardial viability than endocardial wall motion TRUE FALSE


Slide 49:8. What does COR stand for? A. center of rotation B. correction of resolution C. collisions of radiation D. creation of radioisotopes


Slide 50:9. What is a Quality control test that is done on a daily basis? A. Daily flood B. linearity C. constancy D. geometry


Slide 51:10.What is one simple way to prevent patient motion during the scan? A. administer opiates B. make sure the patient is comfortable and informed C. give the patient some gum to chew on D. none of the above


Answers :Answers False C True D SA, VLA, and HLA A True A A B