ASNC IMAGING GUIDELINE Myocardial perfusion planar imaging Peter L. Tilkemeier, MD, Chair,a and Frans J. Th. Wackers, MD, PhDb Although single photon emission computed tomog- patients with mild and moderate CAD, myocardial blood raphy (SPECT) is preferable for myocardial perfusion flow may become abnormal only at high heart rates or at scintigraphy, in a minimal number of circumstances, high double products. At lower heart rates, myocardial planar imaging may be useful or may be the only modality blood flow may be normal and perfusion images will be available. correspondingly normal. In patients with known CAD Purpose. To evaluate regional myocardial perfusion who are being evaluated for extent and severity of and function. Planar imaging is an acceptable method inducible myocardial ischemia, submaximal exercise for myocardial perfusion imaging. The anatomy of the can provide clinically relevant information.2 heart is sufficiently simple that the imaging specialist can comprehend the location and extent of defects from Positioning multiple projections without need of computer recon- struction. Although SPECT imaging is presently The most important part of positioning is the ability considered state-of-the-art for myocardial perfusion to reproduce the same position on initial and delayed (or imaging and preferable, planar imaging still has a role in rest) images. Even slight differences in angulation of the the daily routine of a laboratory. Imaging at the bedside camera, positioning of breasts or other soft tissue, or the of acutely ill patients, or instrumented patients, can only pressure of the camera on the chest wall can produce be performed using planar imaging technique and por- artifacts and inaccuracies in comparing rest and stress table gamma cameras. Planar views can be quickly images. It is vital to bring the camera head as close to or repeated if the patient moves during acquisition. Planar touching the patient’s chest in order to get the highest imaging may be the only way to acquire images in very possible count rates, as opposed to SPECT imaging, obese patients, who are too heavy for the imaging table where some distance is necessary to avoid collisions of a SPECT camera. It may also be the only way to with the patient during rotational acquisition. acquire images in severely claustrophobic patients. The standard imaging positions are supine anterior, Electrocardiography (ECG)-gated planar images supine 45° left anterior oblique (LAO), and a right side- can be obtained using standard software for equilibrium decubitus 90° left lateral (LL). The 90° LL decubitus radionuclide angiography studies. Finally, planar imag- view provides optimal visualization of the inferior wall ing is the basis for good SPECT imaging. The ability to and reduces subdiaphragmatic and breast attenuation obtain high-quality planar images is an essential skill, artifacts. Admittedly, the right-side decubitus position is even for those who routinely use SPECT imaging.1 less stable than the supine position, making it somewhat more difficult to obtain identical repositioning. An alternate LL view is the shallow 70° LAO position. The PROCEDURE latter position is suboptimal at times due to frequent occurrence of artifacts: subdiaphragmatic attenuation of Exercise the inferior wall and breast attenuation of the anterior Adequate exercise is most important if the aim of wall.3 the study is to detect coronary artery disease (CAD). In The LAO view should be chosen in such a way that the right ventricle and left ventricle are well separated by a vertically visualized septum (i.e., ‘‘best septal’’ From Miriam Hospital,a Providence, RI; Yale University School of Medicine,b New Haven, CT. view). One should be aware that in individual patients Approved by the American Society of Nuclear Cardiology Board of the heart may not always be in the same position. Hearts Directors, December 11, 2008. may be rotated clockwise or counterclockwise so that a Unless reaffirmed, retired or amended by the American Society of ‘‘straight’’ 45° LAO will not always display the desired Nuclear Cardiology, this Imaging Guideline shall expire as of March image. It is preferred to search for the ‘‘best septal’’ 2014. Reprint requests: Peter L. Tilkemeier, MD, Chair, Miriam Hospital, view instead of a straight 45° LAO. The angulation of Providence, RI. the detector head for acquisition of anterior and LL 1071-3581/$34.00 views should then be correspondingly modified. The Copyright Ó 2009 by the American Society of Nuclear Cardiology. advantage of this option is that it provides greater doi:10.1007/s12350-009-9057-1 Tilkemeier and Wackers Journal of Nuclear Cardiology Myocardial perfusion planar imaging standardization of the imaged left ventricle, which will ECG gating is used, imaging time per view using simplify quantitative analysis. The disadvantage of this Tc-99m agents should be extended to 8 to 10 minutes. option is the increased complexity, and it carries the For ECG-gated image acquisition with Tl-201, image potential for non-reproducible positioning. acquisition time per view should be at least 10 minutes Female patients should be imaged consistently with per view. Planar images acquired with a 10-inch-diam- the bra off and without camera pressure, which might eter gamma camera should have at least 600,000 counts produce variable tissue displacement. (1 million counts preferred) in the field of view. Alter- Planar imaging may be particularly useful in natively, optimal count density can be defined as at least patients who are unable to maintain the supine posi- 100 counts in the pixel with maximal counts in the left tion for a prolonged period of time. New camera ventricle.5,6 designs allow for imaging in upright or seated posi- In female patients an optional 1-minute image of a tion. As for supine acquisition, images are to be line source marker that delineates the contour of the obtained in multiple views as listed in the table below. breast can be acquired to aid in identifying breast Position of cameras and patients must be exactly attenuation artifacts. reproduced for rest and stress imaging, whether supine Cardiac shields or other masking devices should or upright. not be used. The extracardiac background cannot be determined correctly unless both the cardiac activity and extracardiac activity have been recorded in the raw data. Positioning Patient Detector View position position Alternative Acquisition Shallow Supine/upright 45° ‘‘Best septal’’ Tc-99m Tl-201 LAO Anterior Supine/upright 0° ‘‘Best septal’’ Collimator High resolution Low energy, minus 45° medium Steep Supine/upright 70° ‘‘Best septal’’ resolution LAO plus 25° Field of Full 10-inch field of small Same Left Right decubitus 0° or 90° Not view camera or 1.2-1.5 zoom lateral or upright if upright applicable with large-field-of-view camera Positions of cameras and patients must be exactly repro- Matrix 128 9 128 128 9 128 duced for rest and stress imaging. Window 140 keV 20% centered 78 keV, 30% centered Gating Optional 16 frames/ Same IMAGE ACQUISITION cardiac cycle Imaging 5 minutes (10 minutes 8-10 minutes Gamma camera positions are as shown in the previ- time ECG-gated) (10 minutes ous table. Using either thallium 201 (Tl-201) or tech- (per ECG-gated) netium 99m (Tc-99m)-labeled agents, one may optionally view) use ECG-synchronized gating and acquire in 16-frame, Imaging At least 1 million At least 1 multiple-gated acquisition (equilibrium radionuclide counts million angiography) mode. No beat rejection should be used. In patients with atrial fibrillation, ECG gating should not be performed. ECG-gated acquisition allows for cine review of wall thickening and motion. The multiple frames of the QUANTITATIVE PROCESSING OF PLANAR ECG-gated myocardial perfusion images can be summed IMAGES to produce a single static planar image for conventional visual and quantitative analysis.4 Quantitative processing includes using the computer The total imaging time for static planar Tl-201 to produce standardized raw images for visual evalua- imaging should be 8 to 10 minutes per view, whereas tion. The gray scale should be fully utilized to display the total imaging time for static planar Tc-99m-labeled the heart normalized to maximal left ventricular (LV) agents can be reduced to 5 minutes per view. When count density, and not scaled to visceral activity. Journal of Nuclear Cardiology Tilkemeier and Wackers Myocardial perfusion planar imaging Background subtraction is performed. The background- regions of intense extracardiac activity without causing subtracted images are useful for visual evaluation and significant background error in the background-sub- are used for measurements of myocardial activity. These tracted cardiac image.13 measurements provide quantitative determination of a suspected defect so that consistent standards can be set for defect detection. The measurements are especially Rescaling the Image Gray Scale useful in comparing defects in stress and rest images and When Tl-201 is used as the imaging agent, the heart to detect subtle defect reversibility. Registration of the is usually the organ with the most intense activity. When stress and rest images also can be performed to ensure using myocardial tracers labeled with Tc-99m, activity that the same myocardial region is being sampled. A in the abdominal viscera often exceeds that of the heart. normal database also may be incorporated in the quan- This normally causes the computer to scale image titative program so that segmental tracer activity can be intensities to the extracardiac activity, which will cause compared with the average obtained from the normal erratic and suboptimal visualization of the heart. Any 7-11 database. computer program for quantitative image processing There is no single ‘‘generic’’ description for what should have a convenient mechanism to suppress all good computer programs should offer. There are activity outside the heart if it becomes greater than the common features among several successful programs.