Joint Program in Nuclear Medicine

Attenuation Artifact in Cardiac Imaging

Gaurav M. Patel, MD
J. Anthony Parker, MD PhD

September 9, 2003

Presentation

A 52-year old man with history of coronary artery disease presented with acute onset of atypical chest pain.

Imaging Findings

Non-attenuation-corrected images show a large, severe, fixed inferior wall defect extending into the apex (shown by arrows), and a medium, moderate severity, partially reversible lateral wall defect (shown by arrowheads). From top to bottom, the rows are short-axis stress, short-axis rest, vertical-long-axis stress, vertical-long-axis rest, horizontal-long-axis stress, and horizontal-long-axis rest.

Attenuation-corrected images show resolution of both the fixed inferior wall defect and the partially reversible lateral wall defect. Gated SPECT images (not shown) demonstrate normal wall motion and thickening, with a calculated ejection fraction of 60%.

Diagnosis

No Fixed or Reversible Defects

Discussion

Attenuation

Attenuation is the decrease in intensity of a photon signal along its path to the detector. During nuclear cardiac imaging, non-uniform attenuation occurs as photons pass through tissues of varying densities, such as the sub-diaphragmatic tissues, chest wall, spine, and breasts. This results in an attenuation artifact whose extent varies with location of soft tissue, overall patient body size, and depth of target organ (heart).

Attenuation artifact leads to a loss of diagnostic accuracy as artifacts may be confused with true perfusion abnormalities, resulting in an increase in false-positives. There may also be cases of under-interpretation of true perfusion abnormalities as the effects of attenuation may be overestimated by the observer.

Clues for Interpretation

One way to minimize the overall negative effects of artifacts is for the interpreter to recognize them. Some ways of doing so:

Viewing the planar image or cine loop to look for anatomic causes of artifact
- Breast shadow
- Superiorly positioned diaphragm
- Arm position
Use of quantitative analysis – severe defects are more likely to be real
Breast markers (used for planar imaging)
- IV tubing filled with radioactive liquid to outline breast position
Changing patient position
- Repeat imaging with prone positioning if inferior wall defect is seen on supine imaging
Functional data
- Fixed perfusion defects which demonstrate normal contractile defect is unlikely to be myocardial scar
- However, reversible defects are difficult to discern from artifacts because both show normal function

Attenuation Correction

Attenuation correction is a technique of using quantitative methods to correct images for the effects of attenuation. While normal nuclear medicine imaging usually involves an emission image obtained from radiotracer within the patient’s body, attenuation correction uses additional transmission data of the patient’s soft tissue distribution to create a ‘map’ of the body’s attenuation effects.

For the Vantage™ system, a gadolinium-153 line source, collimated with a narrow slit aperture running its length, is positioned 180 degrees oposite each of the detectors of a dual detector camera. The line source moves across the field of view, providing transmission data simultaneously with collection of emission data by means of a sliding electronic window that is synchronized to the motion of the line source. The resulting transmission map is used to ‘correct’ the emission data.

Attenuation artifacts usually result in fixed defects as the relative position of the soft tissues with respect to the heart remains constant between rest and stress imaging. However, reversible defects can result from attenuation if there is a difference in patient position between the rest and stress studies, as was likely in this case.

Conclusion

The joint position statement from the American Society of Nuclear Cardiology and Society of Nuclear Medicine states, “…it is our recommendation that the adjunctive technique of attenuation correction has become a method for which the weight of evidence and opinion favor its usefulness…”

References

1. Hendel RC, Corbett JR, Cullom SJ, DePuey EG, Garcia EV, Bateman TM. The value and practice of attenuation correction for myocardial perfusion SPECT imaging: A joint position statement from the American Society of Nuclear Cardiology and Society of Nuclear Medicine. J Nucl Cardiol. 2002 Jan-Feb;9(1):135-43.

2. Hendel RC, Berman DS, Follansbee W, Heller GV, Cullom SJ. A multicenter clinical trial to evaluate the efficacy of correction for photon attenuation and scatter in SPECT myocardial perfusion imaging. Circulation 1999; 99:2742-9.

3. Kjaer A, Cortsen A, Rahbek B, Hasseldam H, Hesse B. Attenuation and scatter correction in myocardial SPET: improved diagnostic accuracy in patients with suspected coronary artery disease. European Journal of Nuclear Medicine. 2002 Nov; 29(11)1438-1442.

4. Slart RHJ, Que TH, van Veldhuisen DJ, Poot L, Blanksma PK, Piers DA, Jager PL. Effect of attenuation correction on the interpretation of 99mTc-sestamibi myocardial perfusion scintigraphy: the impact of 1 year’s experience. European Journal of Nuclear Medicine. 2003 Aug.

5. Hendel RC, Bello S, Berman DS. Nonuniform Photon Attenuation in SPECT Myocardial Perfusion Imaging using Vantage: A Tutorial for Clinical Use. ADAC Laboratories. 1997.

6. http://www.medical.philips.com/main/products/nuclearmedicine/products/vantage_pro.html

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J. Anthony Parker, MD PhD, Tony_Parker@CareGroup.Harvard.edu