Case Author(s): Gregg Schubach, MD and Keith Fischer, MD , 1/26/96 . Rating: #D2, #Q3

Diagnosis: Free pertechnetate

Brief history:

43-year old woman with pulmonary hypertension.

Images:

Perfusion images

View main image(vq) in a separate image viewer

View second image(vq). Ventilation images

View third image(mm). Lateral image of the head after VQ scan

Full history/Diagnosis is available below

Diagnosis: Free pertechnetate

Full history:

43-year old woman with known pulmonary hypertension. The ventilation-perfusion scintigraphy was performed to assess for possible chronic, multiple pulmonary emboli as the etiology for the patientıs pulmonary hypertension.

Radiopharmaceutical:

Tc-99m macroaggregated albumin

Findings:

The anterior perfusion image (incorrectly labeled posterior) demonstrates activity within the thyroid gland. The ventilation images are normal. The lateral view of the head obtained immediately following the perfusion images demonstrates activity within the salivary glands and scalp.

Discussion:

The thyroid uptake following the administration of Tc-99m macroaggregated albumin (MAA) was an unexpected finding. Investigating the circumstances around this particular examination revealed that the perfusion scintigraphy was delayed approximately 2 hours after the radiopharmaceutical was drawn into the syringe. Presumably, oxygen within the syringe oxidized the reduced Tc-99m, some of which may have dissociated from the MAA during the delay. The differential diagnosis for thyroid uptake in this setting would include a severe case of Gravesı disease (trapping any minute amount of free TcO4-). However, the patient has no such history. The differential diagnosis for extrapulmonary activity with pulmonary perfusion imaging includes radiopharmaceutical problems, delayed imaging after particle breakdown/Tc-99m dissociation, acute respiratory distress syndrome, right-to-left shunting, and superior vena cava obstruction. Right-to-left shunting may occur at the great vessel level (patent ductus arteriosus), cardiac level (ASD or VSD), or pulmonary level (pulmonary arteriovenous malformation). Right-to-left shunting is not the diagnosis in this case as activity in the brain is absent.

Several technical problems commonly affect perfusion studies. "Hot clots" are produced by drawing blood into the syringe containing Tc-99m MAA, forming radioactive clots, and then reinjecting the suspension into the patient. This technical artifact may produce multiple, predominantly centrally located radioactive "clumps" and perfusion defects that mimic pulmonary emboli. This problem may be prevented by gently shaking the syringe to resuspend the particles prior to injection and by administering the radiopharmaceutical without drawing blood into the syringe. These "hot clots" may also be produced by injecting Tc-99m MAA through a vein with thrombophlebitis. Interpretation of the perfusion study with this artifact is difficult (increased likelihood of false- positive) and a repeat perfusion study is recommended 18-24 hours later.

Another common technical problem is the administration of too few Tc-99m MAA particles resulting in quantum mottle. Fewer than 60,000 particles fails to give a homogeneous distribution of radioactivity in the pulmonary vascular bed. With this technical artifact, the perfusion study appears grossly mottled with multiple small perfusion defects bilaterally and a generalized overall decrease in pulmonary perfusion. The most common cause of quantum mottle secondary to too few particles is infiltration of the radiopharmaceutical. Imaging over the injection site confirms infiltration. Repeat perfusion imaging with a second dose may be performed immediately. Perfusion scintigraphy is typically performed with 500,000 particles of MAA, unless the particles are reduced for specific indications (e.g., right-to-left shunt, severe pulmonary hypertension, pediatric patients).

references: 1) Sandler, et al. Diagnostic Nuclear Medicine, 3rd edition, 1996. Williams and Wilkins 2) Datz. Nuclear Medicine: a teaching file, 1992. Mosby Yearbook 3) Datz FL. Gamuts in Nuclear Medicine, 3rd edition, 1995. Mosby Yearbook

ACR Codes and Keywords:

References and General Discussion of Ventilation Perfusion Scintigraphy (Anatomic field:Lung, Mediastinum, and Pleura, Category:Other(Artifact))

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Case number: vq016

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