Submitted by:

Gavin L. Noble

Clinical presentation

A 61 year-old woman with dextrocardia was referred for dipyridamole Tc-99m sestamibi stress myocardial perfusion imaging prior to bilateral knee replacement. Inquiries elicited symptoms of shortness of breath and atypical, sharp chest pain not related to exertion. Cardiac risk factors included hypertension and a history of smoking. The patient was 64 inches tall and weighed 225 pounds (BMI 38.7), and was scheduled for a two-day protocol (stress/rest) with 37 mCi (1369 MBq) per injection. Baseline ECG was abnormal, demonstrating q waves in the inferior leads and tall r waves in V1 and V2 suggestive of prior inferior-posterior infarction. The patient remained asymptomatic and the ECG did not demonstrate ischemia during dipyridamole infusion.

The imaging technologist was not aware of the history of dextrocardia, and acquisition was performed with the usual 45 RAO to 45 LPO orbit. Stress SPECT images revealed the heart to be in the right chest, and significant liver uptake in the right upper abdomen which was essentially contiguous with a very bright inferior wall. Additionally, there was a fixed defect in the anterior wall suspicious for attenuation artifact. The study was considered un-interpretable. (Figure 1) The patient was scheduled for dipyridamole Rubidium-82 (Bracco Diagnostics, Princeton, NJ) PET myocardial perfusion imaging the next day.

The patient was again asymptomatic and without ECG changes during dipyridamole infusion. PET images (56 mCi [2072 MBq] per dose, 2 minutes delay before image acquisition, 5 minute acquisition) demonstrate essentially no liver uptake of rubidium, and were interpreted as normal. (Figure 2) Gated images demonstrated normal wall motion, with a calculated ejection fraction of 73%. (Figure 3) Non-contrast CT was used for attenuation correction, but also identified the left and right ventricle, right aortic arch originating from the left ventricle, small right lung, and the pulmonary artery originating from the right ventricle. There was good registration between CT and PET images, and quantitative markers of image quality were very good. (Figure 4)

Figure 1. SPECT images demonstrated significant artifact due to closely underlying liver activity, as well as likely anterior attenuation. The images were un-interpretable.    Figure 2. PET perfusion images demonstrating normal perfusion. There is a small area of bowel uptake, no liver uptake, and essentially no interference of sub-diaphragmatic activity with myocardial counts.   Figure 3. Gated images (.avi file) Figure 4. CT and PET overlay demonstrating dextrocardia and good quality PET and CT images. Quality control indicators confirm adequate counts for transmission and emission for both stress and rest, minimal motion and good registration. Emission images demonstrate essentially no liver activity. (Processing and display software: ImagenMD, CVIT, Kansas City, MO)

Discussion
This case demonstrates Rubidium PET myocardial perfusion imaging in a patient with dextroversion, a relatively uncommon form of dextrocardia with an estimated occurrence of 1:29,000. [1] Dextrocardia is a group of congenital disorders with malposition of the heart into the right thoracic cavity, including situs inversus totalis (mirror image), the scimitar syndrome, and dextroversion (rightward rotation rather than mirror image placement). [1] Coronary artery disease is presumed to occur with similar frequency as in the general population.

Malposition of the heart in dextrocardia can complicate non-invasive evaluations of CAD. The heart can be in very close approximation to the liver, further from the detector if the usual orbits are used, and subject to substantial soft tissue attenuation.

Our patient likely had dextroversion and a right aortic arch, but otherwise situs solitus (normally placed vicera). This orientation places the heart and liver in extremely close proximity and positioned the left ventricle just right of midline. The enhanced photon activity in the liver on SPECT imaging caused a bright spot in the inferior wall due to the effects of scatter and inclusion of liver activity. She also appeared to have significant soft tissue attenuation affecting the anterior wall. Rubidium PET images did not suffer from these problems. Lower liver and gut activity with rubidium, better spatial resolution with PET (improving heart liver separation), and the higher energy PET photons (511keV vs. 140 keV, less soft-tissue attenuation) combine to improve image quality. This is an extreme example of the impact of sub-diaphragmatic activity and soft tissue on SPECT imaging, but serves to illustrate the potential utility of PET imaging in overcoming these obstacles which are frequently encountered in SPECT imaging.

Many strategies have been employed to deal with artifact and attenuation on SPECT imaging, although they all have some limitations. Exercise may have improved the images by augmenting liver clearance, but this was not possible in our patient. [2] Thallium-201 would be expected to have lower liver uptake than sestamibi, but may have had more attenuation. [3] Prone imaging would be expected to separate the heart and liver, decreasing interference and diaphragmatic attenuation [4], but may have had unpredictable results in this patient, and would not have dealt with anterior attenuation. Delayed images may have demonstrated decreased liver activity as the radiotracer passed into the gall bladder and small bowel, but lower counts due to decay of the Tc-99m may have led to more attenuation. Attenuation correction hardware/software with scanning gadolinium line sources was used, and did not improve the assessment of the inferior wall. Right sided orbits (from 45 RPO to 45 LAO) may decrease the distance from the heart to the detector, thereby improving spatial resolution and defect contrast. [5] This was done on rest imaging, but did not improve the images in our patient, perhaps due to the midline position of the left ventricle.

The CT (4 slice) in our protocol is used for attenuation correction, a necessary part of PET imaging. Additionally, this can be used to evaluate the relative orientation of the left and right ventricles and their connections with the great vessels. Contrasted could be given if further information is needed, such as pulmonary vein return. This case demonstrates PET perfusion in a patient with dextrocardia. Prior cases have been presented with Thallium and Tc-99m sestamibi, but these have generally focused on situs inversus. [6-8] Many reports of revascularization in cases of dextrocardia and situs inversus are also available. [9,10] Many of these cases do not present the same liver/heart relationship present in our case (ipsilateral), although many patients with congenital heart disease might have significant heart:liver interaction on SPECT imaging. Congenital heart disease survival has improved to the point that many of these patients are now, or will soon be, at risk for acquired coronary artery disease. PET/CT may be the ideal non-invasive test to evaluate these patients, but will take careful consideration until normal variants can be established.

Teaching Points:

1. PET may help overcome problems with sub-diaphragmatic interference and soft-tissue attenuation in patients who have, or are likely to have, suboptimal SPECT images.
2. PET/CT may help evaluate patients with congenital heart disease for acquired CAD as this population ages.

Discussion as Bullet Points

• Dextrocardia: Congential disorders with the heart in the right thorax
• Includes: Situs Inversus, Dextroversion, Scimitar syndrome
• Our patient has Dextroversion: Malrotation rather than mirror image malposition
• The liver and heart are on the same side creating significant artifact.
• Many strategies are available to deal with artifact and attenuation
  1. Exercise: Improved liver clearance
  2. Thallium: somewhat lower liver activity, but possibly more attenuation
  3. Prone imaging: can address inferior attenuation
  4. Delayed imaging: liver clearance to bowel, but fewer counts available
  5. Right sided orbit (45 RPO to 45 LAO): improved special and resolution contrast by decreasing distance to detector
• CT is used primarily for attenuation correction, but can also give anatomic detail
• Patients with congenital heart disease are surviving longer and are at risk for acquired coronary disease, PET/CT may be an ideal imaging method in this complex population.
Teaching Points:
1. PET may help overcome problems with sub-diaphragmatic interference and soft-tissue attenuation in patients who have, or are likely to have, suboptimal SPECT images.
2. PET/CT may help evaluate patients with congenital heart disease for acquired CAD as this population ages.

References

1. Amplatz K, Moller JH. Radiology of Congenital Heart Disease. Boston: Mosby, 1993:950-954.

2. Thomas GS, Prill NV, Majmundar H, et al. Treadmill exercise during adenosine infusion is safe, results in fewer adverse reactions, and improves myocardial image perfusion quality. J Nucl Card. 2000;7(5):439-446.

3. Taillefer R. In: Iskandrian AE and Verani MS, eds. Nuclear Cardiac Imaging, 3rd edition. New York: Oxford University Press, 2003:51-73.

4. Kiat H, Van Train KF, Friedman JD. Et al. Quantitative stress-redistribution thallium-201 SPECT using prone imaging: methodologic development and validation. J Nucl Med. 1992;33(8):1509-15.

5. DePuey EG. Image Artifacts. In: Iskandrian AE and Verani MS, eds. Nuclear Cardiac Imaging, 3rd edition. New York: Oxford University Press, 2003:91-105.

6. Turgut B, Kitapci MT,Temiz NH, et al. Thallium-201 myocardial SPECT in a patient with mirror-image dextrocardia and left bundle branch block. Annals of Nuclear Medicine. 2003;17(6):503-506.

7. Slart R, de Boer J, Jager PL, Piers DA. Added value of attenuation-corrected myocardial perfusion scintigraphy in a patient with dextrocardia. Clinical Nuclear Medicine. 2002;27(12):901-902.

8. Dunseath R, Williams W, Patton D. A scintigraphic demonstration of dextrocardia and complete abdominal situs inversus. Clinical Nuclear Medicine. 1990;15(7):501-503.

9. Wester J, Ernst J, Mast EG et al. Coronary angioplasty in a patient with situs inversus totalis and a single coronary artery. Catheterization and Cardiovascular Diagnosis. 1994;31:304-308.

10. Absensur H, Ramires JA, Dallon LA, Jatene A. Right mammary-coronary anastomosis in a patient with situs inversus. Chest 1988;94:886-887.