Case Author(s): Zhiyun Yang, M.D. and Tom R. Miller, M.D. Ph.D. , 04/16/05 . Rating: #D3, #Q3

Diagnosis: Right-to-left shunt

Brief history:

4-month-old boy presents with hypoxia.


Images of pulmanary perfusion scintigraphy

View main image(pe) in a separate image viewer

View second image(pe). Pulmanary perfusion scintigraphy - spot views

View third image(xr). Chest radiograph

View fourth image(pe). Quntitative activity of whole body and lungs (lower data are lungs vs the rest of the imaged body)

Full history/Diagnosis is available below

Diagnosis: Right-to-left shunt

Full history:

This 4-month-old boy has complex cardiac abnormalities including a large ventriculoseptal defect and pulmonary arterial hypoplasia. He has undergone Blalock-Taussig shunt placement. The patient is now evaluated becasue of hypoxia.


0.37 mCi Tc-99 MAA i.v.


The comparison chest radiograph performed on demonstrates mild right perihilar infiltrate.

The perfusion images show diffusely decreased activity in the left lung, with slightly more prominent activity in the left lung base.

The right lung receives 65% and the left lung receives 35% of total pulmonary perfusion.

Analysis of the global pulmonary activity as compared to the systemic activity demonstrates approximately 60% of activity in the lung and approximately 40% in the remainder of the visualized portions of the body. The findings are consistent with a marked right-to-left shunt.


The 99m-Tc MAA particles are trapped in either the pulmonary or systemic capillaries (capillary sixe is 7um). When there is a right-to-left shunt some of the MAA escapes from the pulmonary circulation and lodges in the systemic capillaries. By estimating the counts in the lungs and in the systemic circulation, it is possible to estimate the size of the right-to-left shunt.

The presence of extrapulmonary activity on perfusion scintigraphy indicates the presence of free Tc-99m pertechnetate or an intra- or extracardiac right-to-left shunt. In a right-to-left shunt, the radiopharmaceutical will accumulate in the systemic capillary beds, primarily the kidneys, brain, and spleen. In contrast, free Tc-99m pertechnate will accumulate most prominently in the stomach, thyroid, salivary glands, and kidneys. An image of the head is the most definitive way to distinguish between a right-to-left shunt and free Tc-99m pertechnetate because intracerebral activity is present with a shunt but not when there is free Tc-99m pertechnetate.

Normal patients should have more than 90% of radioactivity in the lungs. Pulmonary accumulation of Tc-99m MAA drops as a function of time and become less than 90% after 30 min because of breakdown of the MAA. Timing is thus importnat in evaluating a shunt.

Absolute quantification, including the pulmonary to systemic perfusion ratio (Qp/Qs) is possible, but was not completely done in the case because the entire body was not imaged.


1. Hosono M, Machida K, Honda N. Quantitative lung perfusion scintigraphy and detection of intrapulmonary shunt in liver cirrhosis. Annals of Nuclear Medicine. 2002; Vol. 16, No. 8, 577581.

2. Gang L, Wei-jen S, Chou C, et a. Tc-99m MAA Total-Body Imaging to Detect Intrapulmonary Right-to-Left Shunts and to Evaluate the Therapeutic Effect in Pulmonary Arteriovenous Shunts. Clinical Nuclear Medicine. 1996; 21(3):197-202.

3. Verttukattil JJ, et al. Radionuclide investigation of congenital heart disease. Heart 2000;84:467-468.

4. MIR Teaching file case vq026.

5. Ziessman HA, Rehm P. Case review: nuclear medicine. Mosby, Inc. 2002.

Differential Diagnosis List

Right-to-left shunt, free Tc-99m pertechnetate. Less common - recent administration of tracer from another nuclear medicine study.

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: pe008

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