Case Author(s): Brigid Gordon, M.D. and Barry A. Siegel, M.D. , 1/25/97 . Rating: #D3, #Q3

Diagnosis: Right-to-left intracardiac shunt.

Brief history:

4-month-old boy with congenital heart disease (Shone's Complex), status post coarctation repair and pulmonary banding. The patient has a VSD by echocardiogram. Evaluate for intracardiac shunt.

Images:

Posterior whole-body image obtained after injection of Tc-99m MAA.

View main image(pe) in a separate image viewer

View second image(pe). Posterior view of the chest and abdomen.

Full history/Diagnosis is available below

Diagnosis: Right-to-left intracardiac shunt.

Full history:

4-month-old boy with congenital heart disease, status post coarctation repair and pulmonary banding. A recent echocardiogram demonstrated a large ventricular septal defect. This study was performed to quantify the right-to-left intracardiac shunt.

Radiopharmaceutical:

0.2 mCi Tc-99m MAA injected via a right foot vein.

Findings:

Image 1 is a posterior whole-body image, which shows activity in both kidneys and the brain parenchyma, consistent with a right-to-left shunt. Image 2 again demonstrates these findings. The fractional perfusion to the right lung was 27% and to the left lung 73% of total pulmonary perfusion (graph not provided).

To quantify the right-to-left shunt, regions of interest were drawn around the entire body and then, separately, around the lungs to determine the counts in these regions. The counts distributed in the systemic circulatory bed is determined by subtracting the counts in the pulmonary region from the counts in the whole-body region. Based on this study, the right-to-left intracardiac shunt was determined to be 36% (cardiac catheterization with oximetry determined the right-to-left intracardiac shunt to be 33%). "Normal" values determined scintigraphically are 1-10%. This apparent "normal" right-to-left shunt reflects the presence of small particles in the Tc-99m MAA preparation (which are accumulated by phagocytic cells of the reticuloendothelial system, as well as free Tc-99m pertechnetate or other Tc-99m oxides in the radiopharmaceutical preparation or released by breakdown of Tc-99m MAA particles. The finding of activity in the parenchyma of the brain is a definitive sign of a right-to-left shunt rather than excessive free Tc-99m.

Discussion:

Tc-99m macroaggregated albumin (MAA) particles have a size range of 10-60 microns, and thus are normally trapped in the pulmonary arteriolar and capillary bed after intravenous administration. If there is a considerable right-to-left shunt, the particles are shunted into the systemic circulation as demonstrated in this patient.

Gates et al.(using posterior scintiphotographs) and Lin et al. (using a whole-body profile device) employed this method to quantify right-to-left shunts. One theoretical difficulty is that activity in the lungs and in the remainder of the body may be detected with different sensitivities caused by differences in attenuation. Correction of activity in the pulmonary region of interest for chest wall and mediastinal activity is another methodological difficulty with this technique.

Reference: Pediatric Nuclear Medicine. Treves, S. Second edition. Springer-Verlag, NY, 1995.

Followup:

See above.

Major teaching point(s):

Intracardiac shunt quantification.

Differential Diagnosis List

Shunt vs. radiopharmaceutical problem.

ACR Codes and Keywords:

References and General Discussion of Perfusion (only) Scintigraphy (Anatomic field:Lung, Mediastinum, and Pleura, Category:Normal, Technique, Congenital Anomaly)

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Case number: pe005

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