Cardiac Amyloid Imaging: Blood Pool or Myocardial Uptake?
Planar technetium-99m pyrophosphate (⁹⁹ᵐTc-PYP) images show activity over the heart. Before interpreting this as a positive study, answer one question: is the tracer in the myocardium or still circulating within the cardiac chambers? Residual blood pool activity can mimic myocardial uptake and produce misleading planar visual grades or heart-to-contralateral-chest ratios. Tomographic localisation is essential before drawing a diagnostic conclusion. [1]
Recognise the location
SPECT separates activity within the cardiac cavities from activity in the ventricular walls. SPECT/CT adds anatomical localisation when available. Review multiple planes: a single image or apparent shape is insufficient. [2]
| Feature | Blood pool activity | Myocardial uptake |
|---|---|---|
| Location | Within the cardiac chambers | Along the myocardial walls, including the septum |
| Appearance | Often amorphous or cavity-centred | Follows the ventricular wall contour |
| Delayed imaging | May diminish as circulating tracer clears | Remains localised to the myocardium |
| Interpretive value | Does not establish myocardial tracer binding | Requires grading and clinical correlation |
These are localisation clues, not standalone diagnostic criteria. Blood pool and myocardial activity may coexist. [2]
Three checks before concluding
1. Confirm myocardial localisation. Review reconstructed SPECT images, preferably with CT correlation. Check that apparent uptake is not within a chamber or overlapping bone.
2. Check acquisition timing. Persistent blood pool can obscure interpretation. If substantial blood pool remains on one-hour PYP imaging, ASNC recommends three-hour imaging. Delayed acquisition still requires tomographic assessment.
3. Localise before grading or measuring. Interpret myocardial uptake relative to rib activity only after confirming its location. An elevated heart-to-contralateral-chest ratio cannot independently establish ATTR cardiac amyloidosis or override SPECT findings. [1]
What should the report say
When adequate imaging confidently shows blood pool only: “Cardiac-region activity localises to the intracardiac blood pool, without definite myocardial uptake on SPECT/CT.”
When persistent blood pool prevents reliable assessment: “Persistent blood pool activity limits assessment of myocardial uptake. The examination is indeterminate for ATTR cardiac amyloidosis.”
These statements distinguish an interpretable absence of myocardial uptake from an unresolved examination.
Keep the diagnostic boundary clear
A nonbiopsy diagnosis of transthyretin cardiac amyloidosis requires an appropriate clinical and echocardiographic/CMR context, qualifying myocardial uptake, and exclusion of a monoclonal process using serum free light chains, serum immunofixation, and urine immunofixation. A monoclonal abnormality requires further evaluation; scintigraphy alone cannot establish the amyloid type. [1,3]
Absent myocardial uptake does not exclude AL amyloidosis or every case of ATTR amyloidosis. Persistent clinical suspicion warrants further assessment. [2,3]
- Confirm where the tracer is before deciding what the scan means.
- ASNC. Cardiac Amyloidosis Practice Points Update: ⁹⁹ᵐTc-Pyrophosphate Imaging. Revised 2022.
- ⁹⁹ᵐTc Bone-Avid Tracer Cardiac Scintigraphy: Role in Noninvasive Diagnosis of Transthyretin Cardiac Amyloidosis. Radiology, 2023.
- ACC. 2023 Expert Consensus on Cardiac Amyloidosis: Key Points.