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Mattana et al. Vessel Plus 2022;6:13 https://dx.doi.org/10.20517/2574-1209.2021.87 Page 5 of 11

Most of the studies in the literature describe a good accuracy for the 1 h acquisition when a chest
[7]
SPECT/CT is performed , because the planar images alone could not allow differentiating the blood pool,
[17]
ribs uptake, or possible metastatic focal lesion on the heart area. In a single-center study, Bokhari et al.
identified a very high diagnostic accuracy (area under the curve of 0.992, P > 0.0001) for visual Grade ≥ 2
and a H/CL ratio ≥ 1.5 on 1 h images confirming that planar imaging could correctly distinguish ATTR-CA
from AL-CA.

As reported above, guidelines recommend a 1 h time interval between injection and image acquisition. A
3 h post-injection imaging is considered optional unless there is evidence of excessive blood pool activity.

On 3 h images, a visual evaluation can be performed according to the Perugini visual score, comparing the
heart tracer uptake with the bone tracer uptake: Grade 2 and 3 is consistent with ATTR-CA if a monoclonal
plasma cell dyscrasia has been excluded, while Grades 0 and 1 are more suggestive for AL-CA.

The high diagnostic performance for ATTR-CA using Tc-PYP bone scintigraphy is derived from centers
99m
with expertise in this field and from a highly selected patient population. To reach the same diagnostic
performance, it is crucial to follow the recommended diagnostic criteria. Unfortunately, Harb SC
[21]
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described that at least 30% of responding hospitals perform Tc-PYP imaging to screen for CA, often times
without light-chain measurement to exclude AL-CA and only 21% of institutions performed SPECT/CT
imaging in addition to planar images despite the expert recommendation.

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Tc-DPD
This tracer is widely used in Europe and showed high sensitivity and specificity in the diagnosis of CA. The
acquisition protocol is more standardized among centers than those of Tc-PYP and the diagnostic criteria
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reckon on a visual score analysis at 3 h: Grades 2 and 3 are considered positive for the presence of ATTR-
CA when the presence of AL serum protein has been excluded. In the case of Grade 1, a SPECT or a
SPECT/CT should be performed to exclude blood pool from myocardial wall uptake. In 2005,
[9]
Perugini et al. suggested using an early acquisition at 5 min for the semi-quantitative analysis of heart
retention, whole body retention, and heart/whole body ratio in addition to the Perugini Score on late
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acquisition at 3 h. Other authors suggested to use only early phase at 10 min after injection of Tc-DPD or
99m Tc-HMDP instead of the 3 h acquisition with a heart/mediastinum ratio > 1.28 [24,25] considered suggestive
for ATTR-CA. These two studies were conducted on a small population, therefore the 3 h acquisition
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protocol remains the most used acquisition protocol for Tc-DPD tracer in nuclear medicine centers.

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Subsequent studies reported that mild uptake of Tc-DPD (Grade 1) may be noted in patients with other
[12]
subtypes of CA (i.e., AL, amyloid A amyloidosis, and apolipoprotein A1). Rapezzi et al. evaluated the ratio
of heart-to-whole-body (H/WB) retention of Tc-DPD on the 3 h images in patients with TTR mutation
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and demonstrated that individuals with increased left ventricular myocardial wall thickness (1.2 cm) had
much higher H/WB retention ratio compared to individuals with normal left ventricular myocardial wall
thickness.

EXTRACARDIAC UPTAKE
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The presence of extracardiac uptake (ECU) has been better studied for Tc-DPD than Tc-PYP because
the whole-body acquisition protocol, widely used with Tc-DPD, allowed more consistent data regarding
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ECU. For Tc-PYP, the ECU has been reported in lung, but, considering that the most used acquisition
protocol for this tracer includes only the chest, we can suppose that these data could be underestimated.
Sperry et al. suggested that Tc-PYP cannot be used to image extracardiac uptake in comparison to
99m
[26]