Attenuation Artifacts

Attenuation Artifacts

Thomas H. Hauser, MD, MMSc

Director of Nuclear CardiologyBeth Israel Deaconess Medical Center

Instructor in MedicineHarvard Medical School

Boston, MA

A major teaching hospital of Harvard Medical School

Harvard Medical School

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Cases

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


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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


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Case 1


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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.


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Case 2


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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


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Attenuation


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Low Photon Counts


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People are not Uniform


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People are not Uniform


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Outline

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


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Outline



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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


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Inferior Attenuation


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Anterior Attenuation


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Anterior Attenuation


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Arm Attenuation


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Arm Attenuation


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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


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Outline



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Supine/Prone Imaging


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Positional Imaging

• Supine Imaging– Inferior attenuation increased

– Anterior attenuation decreased

• Prone Imaging– Anterior attenuation increased

– Inferior attenuation decreased


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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

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Spec Sens Acc

Supine

Supine/Prone

Segall et al. J Nucl Med 1989;30:1738-9.


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Supine Prone Imaging

• Pros– Cheap

– Easy

• Cons– Little data

– Relatively poor performance


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Outline



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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.


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Gated Imaging


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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


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Gated Imaging


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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


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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)


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Gated Imaging


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Correlation of SPECT and MR EDV

Ioannidis et al, J Am Coll Cardiol 2002;39:2059–68


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Correlation of SPECT and MR EF



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Differences between SPECT and MR EF



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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


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Arrhythmia


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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


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Arrhythmia Rejection


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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


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Atrial Fibrillation


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Gating Error due to AF


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Severe Defect


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Small Heart


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Small Heart


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Small Heart


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Gated Imaging and Attenuation

• Gated images provide functional data about regional systolic function– Translation

– Wall thickening


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Patients WITHOUT CAD

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


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Patients WITH CAD



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Change in Interpretation



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Outline



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Attenuation Map


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Attenuation Map


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Algorithmic Reconstruction


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Truncation


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Attenuation Correction


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Attenuation Correction: Sensitivity for Detection of >50% Stenosis


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Attenuation Correction: Reader Confidence


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• 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


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• 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


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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


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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


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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


THH 10/04

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


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Case 1: Projection Data


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Case 1: Attenuation Map


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Case 1: Filtered Backprojection


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Case 1: Attenuation Correction


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Case 1: Gated Images


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Case 1: Diaphragmatic Attenuation

• Mild intensity• Fixed• Inferior wall• Graded appearance• Resolves with attenuation correction• Normal wall motion


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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.


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Case 2: Projection Data


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Case 2: Attenuation Map


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Case 2: Filtered Backprojection


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Case 2: Breast Attenuation

• Moderate intensity• Fixed• Anterior wall, with relative sparing of the apex• Resolves with attenuation correction• Normal wall motion


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Case 3


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Inferior Ischemia


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Case 4


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Multivessel Disease


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Case 5


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Case 5

Inferior Infarction

• Mild defect• Distribution typical for CAD• Persists after attenuation correction• Distal inferior hypokinesis


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Case 6


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Case 6

Anterior Infarction

• Severe defect• Distribution typical for attenuation• Persists with attenuation correction• Anterior hypokinesis

Documents

Attenuation Artifacts