Cardiac Case Study

Inferior wall attenuation and correction with CT-based Attenuation

Correction

Linda Campbell, BS

GE Applications Scientist

Patient History

Imaging Procedure:

A 68 year old male was referred for myocardial perfusion imaging three months status post revascularization of the right coronary artery for evaluation of patency. Some atypical chest pain was noted.

The one day Sestamibi protocol was used, and rest imaging was performed first. SPECT imaging was done with 60 stops over 180 degrees. In addition, CT images were acquired. Acquisitions were done on a GE Hawkeye.

Maximal heart rate achieved on treadmill: 90%

Target heart rate (85% of maximum heartrate): 129

 

  Type of acquisition Time per stop Agent Dose

Rest Ungated SPECT; CT 25 seconds 99m Tc Sestamibi

15 mCi

Stress Gated SPECT; CT 25 seconds 99m Tc Sestamibi

30 mCi

Imaging Analysis

As part of the acquisition quality control for this data, fused SPECT-CT images were checked for proper registration of attenuation maps and emission data.

SPECT-CT transaxial fusion SPECT-CT sagittal fusion

Further quality control checks showed the acquisition to be of good quality:

• The stress and rest selective linograms showed no significant motion in the vertical direction and showed no significant extra-cardiac activity that would interfere with the reconstruction.

• The stress and rest sinograms showed no significant motion in the horizontal direction

• Myocardial count density was adequate. The stress maximum pixel value was 201; rest maximal pixel value was 65. The guideline for an adequate count study for 60-64 projection images is to have a maximum pixel value of at least 50 counts within a tight circular/ elliptical ROI around the epicardial edge of the myocardium on an anterior view of the heart.

• There were no gating errors

Question 1 – Acquisition quality control

Which one of the following acquisition factors has the least effect on slice image quality?

A. Horizontal motionB. Myocardial count densityC. Gating errorsD. Vertical motionE. Intense extra-cardiac activity adjacent to the heart

The correct answer is C - Gating errors

Thickening is the parameter most prone to error in the presence of arrhythmias. Slice appearance is maintained in the presence of arrhythmias except in very severe cases. LV ejection fraction (EF) is a robust parameter that is little affected by arrhythmias. Quantitative perfusion measures were also little affected by arrhythmias except in the case of atrial fibrillation.1

As has been well documented in the literature, vertical and horizontal motion and myocardial count density are critical for slice image quality. Intense visceral activity close to the heart can also affect slice image quality as it results in changed slice count densities.

The rotating projection images show soft tissue attenuation in the waist area which can cause increased attenuation of the inferior wall.

Stress projection images Rest projection images

Soft tissue attenuation may create artifacts that mimic true perfusion abnormalities and can lead to false positive interpretation.

Click Click

Question 2 – Attenuation correction

What are the advantages of doing attenuation correction on cardiac studies?

A. Provide images that are more uniform and easier to interpret

B. Decrease acquisition time

C. Increase myocardial count density

D. Increase sensitivity and overall diagnostic accuracy

E. Both A and D

The correct answer is E – Both A and D

The diagnostic accuracy of SPECT imaging is compromised by soft tissue attenuation artifacts. Though education, experience, and the application of gated SPECT imaging have greatly helped, there is a need for improved diagnostic accuracy.

Attenuation correction can provide images that are more uniform and easier to interpret, and increase sensitivity and overall diagnostic accuracy.

Attenuation correction SPECT techniques represent a significant advance in myocardial perfusion imaging and hold great promise for improved assessment of cardiac patients.2

Question 3 – CT-based attenuation correction

What are the further advantages of doing CT-based attenuation correction on cardiac studies?

A. CT-based AC produces high quality transmission maps

B. CT-based AC is consistent with time

C. CT-based AC is consistent over a full range of body types

D. All of the above

The correct answer is D – all of the above

There are significant differences in the ability of different commercial AC systems to reduce artifacts due to attenuation, hot liver / bowel activity and type of orbit. Systems that can generate high quality attenuation maps yielded the best results. A high quality attenuation map appears to be an important determinant of image quality.3

Stress attenuation map Rest attenuation map

Stress and rest images were reconstructed by filtered back projection (FBP) and iterative reconstruction with attenuation correction (IR-AC). Count deficiencies in the inferior walls of both the stress and rest FBP slices have been noticeably improved in the IR-AC slices, correcting for inferior wall attenuation.

Stress SA slices

Stress VLA slices

Rest SA slices

Rest VLA slices

FBP

IR-AC

FBP

IR-AC

EKG Results

Non-specific ECG changes were noted with no ST depression at maximal exertion. Test terminated at maximal heart rate and SOB. No chest pain was noted.

Findings

FBP images were interpreted as inferior wall scar with possible ischemic component. This could also be a diaphragmatic attenuation artifact. With CT-based AC the inferior wall is shown to be viable with no ischemia. The validity of the revascularization of the RCA is confirmed.

Findings and Images Courtesy of VA Medical Center, Baltimore, Maryland

For more information on GE Nuclear Cardiology products and the unique Hawkeye CT based cardiac AC, visit: GE Hawkeye Cardiac Attenuation Correction

References

1. Nichols K, Yao S, Kamran M, Faber TL, Cooke, CD, and DePuey EG. Clinical impact of arrhythmias on gated SPECT cardiac myocardial perfusion and function assessment. J Nucl Cardiol 2001; 8:19-30.

2. Hendel RC, Corbett JR, Cullom SJ, DePuey EG, Garcia EV, and Bateman TM. The Value and Practice of Attenuation Correction for Myocardial Perfusion SPECT Imaging: A Joint Position Statement form the American Society of Nuclear Cardiology and the Society of Nuclear Medicine. J Nucl Med 2002; 43:273-280

3. O’Connor MK, Kemp B, Anstett F, Christian P, Ficaro EP, Frey E, Jacobs M, Kritzman JN, Pooley RA, and Wilk M . A multicenter evaluation of commercial attenuation compensation techniques in cardiac SPECT using phantom models. J Nucl Cardiol 2002; 9:361-76.