Attenuation Artifacts A major teaching hospital of Harvard Medical School Attenuation Artifacts Thomas H. Hauser, MD, MMSc Director of Nuclear Cardiology Beth Israel Deaconess Medical Center Instructor in Medicine Harvard Medical School Boston, MA

Cases Prone imaging Stress: 99mTc-Sestamibi Rest: 201Tl

Case 1 65 year-old man with a history of HTN who presented with chest pain. He was referred for an exercise stress test with nuclear imaging He exercised for 6.5 minutes of a Bruce protocol Peak HR 143 (92% predicted maximal) Peak BP 194/64 During exercise, he had chest pain but no ECG changes

Case 1

Case 2 82 year-old woman with a history of CAD, s/p multi-vessel PCI, HTN, dyslipidemia who presented with chest pain. She was referred for dipyridamole stress with nuclear imaging. Appropriate hemodynamic response with a fall in BP and an increase in HR. She had no symptoms or ECG changes.

Case 2

Challenge of Fixed Defects Fixed defects can represent either myocardial infarction or an artifact due to soft tissue attenuation Difficult to distinguish between them using standard filtered backprojection images alone Soft tissue attenuation is very common Major limitation in the specificity of SPECT imaging for the detection of CAD

Attenuation

Low Photon Counts

People are not Uniform

People are not Uniform

Outline Typical patterns of attenuation artifacts Supine/Prone Imaging Gated Imaging Attenuation Correction

Outline Typical patterns of attenuation artifacts Supine/Prone Imaging Gated Imaging Attenuation Correction

Attenuation Artifact Patterns Inferior (“Diaphragmatic”) Attenuation Related to weight/abdominal girth Inferior wall Worse near the base Anterior (Breast) Attenuation Anterior wall Usually sparing the apex Arm Attenuation Arms down imaging Anteroseptal and inferolateral walls

Inferior Attenuation

Anterior Attenuation

Anterior Attenuation

Arm Attenuation

Arm Attenuation

Characteristics of Attenuation Artifacts Tend to be of mild intensity, but can be moderate Usually follow one of these typical patterns Usually evidence of attenuation on the projection images or the attenuation map

Outline Typical patterns of attenuation artifacts Supine/Prone Imaging Gated Imaging Attenuation Correction

Supine/Prone Imaging                               

Positional Imaging Supine Imaging Prone Imaging Inferior attenuation increased Anterior attenuation decreased Prone Imaging Anterior attenuation increased Inferior attenuation decreased

Supine/Prone Imaging True perfusion defects are independent of position Attenuation artifacts often change depending on patient position If a defect appears or disappears with a change in position, then it is an artifact Segall et al. J Nucl Med 1989;30:1738-9.

Supine Prone Imaging Pros Cons Cheap Easy Little data Relatively poor performance

Outline Typical patterns of attenuation artifacts Supine/Prone Imaging Gated Imaging Attenuation Correction

Gated Imaging Divides the cardiac cycle into phases Data collected during each phase is pooled to form a single image Images from each phase are put together to compose a series of images called a cine Further information can then be obtained from this data by applying computer algorithms.

Gated Imaging

Gated Images The number of gates depends on the desired temporal resolution and image quality Always a trade-off between them Finite number of counts 8, 16, 32, 64 Traditional vs. List mode List mode not frequently used Fixed vs. Variable RR interval

Gated Imaging

Gated Imaging Although the display used at BIDMC shows four slices, the gated cine images are 3D. Any set of slices can be selected Many systems show the 3D images

Quantification 3D images allow for accurate quantification of volumes in each phase of the cardiac cycle Calculated by using computerized edge detection to determine the endocardial border Usually displayed as a time-volume curve LVEF = 1-(ESV/EDV)

Gated Imaging

Correlation of SPECT and MR EDV Ioannidis et al, J Am Coll Cardiol 2002;39:2059–68

Correlation of SPECT and MR EF Ioannidis et al, J Am Coll Cardiol 2002;39:2059–68

Differences between SPECT and MR EF Ioannidis et al, J Am Coll Cardiol 2002;39:2059–68

Image Quality To get accurate quantification, the computer must be able to accurately detect the endocardium Regular rhythm Motion or other artifacts that significantly affect the perfusion images Severe defects (real or attenuation) No counts, no border Small hearts

Arrhythmia

Arrhythmia If the R wave occurs prior to the expected time Later phases are empty for the prior beat Timing of systole is different for next beat If the R wave occurs after the expected time Little effect on the prior beat Timing of systole is different for the next beat Either causes image blurring To preserve image quality, RR intervals that deviate from the expected are rejected

Arrhythmia Rejection

Arrhythmia Many software packages generate a histogram of RR intervals Helpful to determine presence and severity of arrhythmia If there is frequent arrhythmia rejection, then acquisition time can be overly prolonged Use non-gated imaging with severe arrhythmia

Atrial Fibrillation

Gating Error due to AF

Severe Defect

Small Heart

Small Heart

Small Heart

Gated Imaging and Attenuation Gated images provide functional data about regional systolic function Translation Wall thickening

Inferior Attenuation

Inferior Attenuation

Patients WITHOUT CAD Smanio et al, J Am Coll Cardiol 1997;30:1687–92

Patients WITH CAD Smanio et al, J Am Coll Cardiol 1997;30:1687–92

Change in Interpretation Smanio et al, J Am Coll Cardiol 1997;30:1687–92

Outline Typical patterns of attenuation artifacts Supine/Prone Imaging Gated Imaging Attenuation Correction

Attenuation Map

Attenuation Map

Algorithmic Reconstruction

Truncation

Attenuation Correction

Attenuation Correction

Attenuation Correction

Attenuation Correction: Sensitivity for Detection of >50% Stenosis

Attenuation Correction: Reader Confidence

Attenuation Correction

Attenuation Correction Links et al evaluated 66 patients using information from both attenuation corrected images and gated images Combination of both provided the highest diagnostic accuracy Links et al. J Nucl Cardiol 2002;9:183–7

Attenuation Correction Links et al. J Nucl Cardiol 2002;9:183–7

Attenuation Correction Links et al. J Nucl Cardiol 2002;9:183–7

Attenuation Correction O’Connor et al evaluated the performance of all available SPECT systems with attenuation correction. Highly variable results depending on the system Inability to reproduce normal phantom images in the presence of attenuation Inability to consistently depict inferior or anterior defects Significant artifacts in the presence of adjacent hot spots O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

Attenuation Correction O’Connor et al. J Nucl Cardiol 2002;9:361–76

ASNC/SNM Statement “It is the position of ASNC and the SNM 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 the effectiveness of care and lowering health care costs.” Heller et al. J Nucl Cardiol. 2004;11:229

ASNC/SNM Statement High-quality transmission scans and sufficient transmission counts with low cross-talk from the emission radionuclide are essential to reduce the propagation of noise and error into the corrected emission images. Quality-control procedures for image registration should be used for projection data acquired by use of sequential transmission-emission imaging protocols (eg, computed tomography–SPECT systems). Motion correction, scatter correction, and resolution recovery should be used with attenuation correction. Attenuation correction should be employed concurrently with ECG-gated SPECT imaging. Technologists must have adequate training in the acquisition and processing of attenuation-corrected studies. Physicians must have adequate training in the interpretation of attenuation-corrected images. Physicians should view and interpret both uncorrected and corrected images. Heller et al. J Nucl Cardiol. 2004;11:229

An Integrative Approach to Recognizing Attenuation Artifacts Inspect the raw data for evidence of attenuation Projection images: visualize attenuation Attenuation map: attenuating structures Recognize the typical patterns of attenuation artifacts If available, compare supine/prone images Examine attenuation corrected images Examine the gated images

Case 1 65 year-old man with a history of HTN who presented with chest pain. He was referred for an exercise stress test with nuclear imaging He exercised for 6.5 minutes of a Bruce protocol Peak HR 143 (92% predicted maximal) Peak BP 194/64 During exercise, he had chest pain but no ECG changes

Case 1: Projection Data

Case 1: Attenuation Map

Case 1: Filtered Backprojection

Case 1: Attenuation Correction

Case 1: Gated Images

Case 1: Diaphragmatic Attenuation Mild intensity Fixed Inferior wall Graded appearance Resolves with attenuation correction Normal wall motion

Case 2 82 year-old woman with a history of CAD, s/p multi-vessel PCI, HTN, dyslipidemia who presented with chest pain. She was referred for dipyridamole stress with nuclear imaging. Appropriate hemodynamic response with a fall in BP and an increase in HR. She had no symptoms or ECG changes.

Case 2: Projection Data

Case 2: Attenuation Map

Case 2: Filtered Backprojection

Case 2: Attenuation Correction

Case 2: Gated Images

Case 2: Breast Attenuation Moderate intensity Fixed Anterior wall, with relative sparing of the apex Resolves with attenuation correction Normal wall motion

Case 3

Inferior Ischemia

Case 4

Multivessel Disease

Case 5

Case 5: Attenuation Correction

Case 5: Gated Images

Case 5 Inferior Infarction Mild defect Distribution typical for CAD Persists after attenuation correction Distal inferior hypokinesis

Case 6

Case 6: Attenuation Correction

Case 6: Gated Images

Case 6 Anterior Infarction Severe defect Distribution typical for attenuation Persists with attenuation correction Anterior hypokinesis