DPD Quantification in Cardiac Amyloidosis

Objectives To assess whether single-photon emission computed tomography (SPECT/CT) quantification of bone scintigraphy would improve diagnostic accuracy and offer a means of quantifying amyloid burden. Background Transthyretin-related cardiac amyloidosis is common and can be diagnosed noninvasively using bone scintigraphy; interpretation, however, relies on planar images. SPECT/CT imaging offers 3-dimensional visualization. Methods This was a single-center, retrospective analysis of 99mTc-3,3-diphosphono-1,2-propanodicarboxylic acid (DPD) scans reported using the Perugini grading system (0 = negative; 1 to 3 = increasingly positive). Conventional planar quantification techniques (heart/contralateral lung, and heart/whole-body retention ratios) were performed. Heart, adjacent vertebra, paraspinal muscle and liver peak standardized uptake values (SUVpeak) were recorded from SPECT/CT acquisitions. An SUV retention index was also calculated: (cardiac SUVpeak/vertebral SUVpeak) × paraspinal muscle SUVpeak. In a subgroup of patients, SPECT/CT quantification was compared with myocardial extracellular volume quantification by CT imaging (ECVCT). Results A total of 100 DPD scans were analyzed (patient age 84 ± 9 years; 52% male): 40 were Perugini grade 0, 12 were grade 1, 41 were grade 2, and 7 were grade 3. Cardiac SUVpeak increased from grade 0 to grade 2; however, it plateaued between grades 2 and 3 (p < 0.001). Paraspinal muscle SUVpeak increased with grade (p < 0.001), whereas vertebral SUVpeak decreased (p < 0.001). The composite parameter of SUV retention index overcame the plateauing of the cardiac SUVpeak and increased across all grades (p < 0.001). Cardiac SUVpeak correlated well (r2 = 0.73; p < 0.001) with ECVCT. Both the cardiac SUVpeak and SUV retention index had excellent diagnostic accuracy (area under the curve [AUC]: 0.999). The heart to contralateral lung ratio performed the best of the planar quantification techniques (AUC: 0.987). Conclusions SPECT/CT quantification in DPD scintigraphy is possible and outperforms planar quantification techniques. Differentiation of Perugini grade 2 or 3 is confounded by soft tissue uptake, which can be overcome by a composite SUV retention index. This index can help in the diagnosis of cardiac amyloidosis and may offer a means of monitoring response to therapy.

A myloidosis is a multisystem condition characterized by the extracellular deposition of abnormally folded protein fibrils, which result in progressive organ dysfunction (1). Primary light chain (AL) and transthyretin-related amyloidosis (ATTR) commonly affect the heart; the latter can either be associated with a TTR gene mutation (variant ATTR) or not (wild-type ATTR).
Previously believed to be rare, more recent research has identified cardiac ATTR (ATTR-CA) in a significant proportion (14% to 16%) of elderly patients with aortic stenosis (2,3), as well as in 13% of the population with heart failure with preserved ejection fraction (4). DPD scintigraphy is currently reported by using the Perugini grading system, which is a visual score of the delayed (3-h) planar image, graded from 0 (negative) to 3 (strongly positive) (6). This grading system offers little prognostic significance (7). Difficulty can also ensue in differentiating very subtle cardiac uptake (i.e., a Perugini grade 1) from a negative scan with some residual blood pool activity, despite the additional use of single-photon emission computed tomography (SPECT) imaging. The clinical importance of this distinction remains to be determined; however, it may well prove relevant given the new armamentarium of amyloid-specific therapies in development (8)(9)(10). A semi-quantitative technique was also proposed by Perugini et al. (6) that involved using the early and late planar images to calculate heart and whole-body retention, as well as a heart/whole-body ratio.
PYP scintigraphy is an alternative radiotracer used in the United States for the detection of ATTR-CA, with imaging currently recommended at 1-h postinjection with both SPECT and planar acquisitions and optional 3-h SPECT or planar imaging (11). It is generally reported using both a visual grading system and a semi-quantitative heart to contralateral lung (H/CL) ratio from the planar images, with ratios $1.5 at 1 h classified as ATTR-positive (11,12). Furthermore, a H/CL ratio $1.6 in patients with ATTR-CA seems to predict a worse outcome (13).
By comparison, SPECT allows the threedimensional visualization of radioactivity within the body, which can be used to display a standardized uptake value (SUV), a semi-quantitative representation of the concentration of radiopharmaceutical in the respective tissues. SPECT quantification has been used in dementia imaging (14) and tumor dosimetry in radioimmunotherapy (15).
Cardiac amyloid deposition increases myocardial extracellular volume (ECV) greater than any other nonischemic cardiomyopathy (16), due to the extracellular deposition of the amyloid fibrils. These increases are detectable using computed tomography imaging (17), which has been validated against both cardiovascular magnetic resonance (18) and invasive biopsy (19).   EXTRACELLULAR VOLUME QUANTIFICATION BY CT.
All CT scans were performed on a Somatom FORCE scanner (Siemens Healthineers, Erlangen, Germany).
The technique for ECV quantification has been described elsewhere (20); additional pre-contrast and 3-min post-contrast datasets were acquired. These datasets were averaged, subtracted to provide a partition coefficient, and then co-registered with the CT coronary angiogram images. The patient's hematocrit measurement (usually taken on the same day) was inputted, and the myocardial ECV was calculated as follows:

RESULTS
Analysis was performed on the DPD scans of 100 patients (mean age 84 AE 9 years; 52% male), of which forty were grade 0, twelve were grade 1, forty-one were grade 2, and seven were grade 3. All patients were identified retrospectively and selected for their   Figure 2). Pairwise comparison revealed a significant difference between all grades except 1 and 3 (p ¼ 0.460) and 2 and 3 (p ¼ 1.000).
Paraspinal muscle peak SUV increased from grade 0 to 3 (p < 0.001 for trend), whereas vertebral peak SUV did the opposite (p < 0.001 for trend). There was no difference in hepatic peak SUV between grades (p ¼ 0.870 for trend).
The composite parameter of SUV retention index helped overcome the plateauing of the cardiac peak SUV between grades 2 and 3, increasing across all grades (p < 0.001 for trend) (Table 1, Figure 2). Pairwise comparison showed a significant difference between all grades except 2 and 3 (p ¼ 1.000).
There was no significant difference between the 2 patients with cardiac AL and a grade 1 DPD scan and those with ATTR in the same group in terms of cardiac peak SUV (p ¼ 0.260) or SUV retention index  Twenty-nine patients were DPD Perugini grade 0, five were grade 1, and ten were grade 2. Cardiac peak SUV increased across the grades as seen in the overall cohort (p < 0.001 for trend). Myocardial ECV CT increased from 31 AE 3% (grade 0) to 34 AE 4% (grade 1) to 44 AE 5% (grade 2) (p < 0.001 for trend). There was good correlation between the increases in cardiac peak SUV and ECV CT (r 2 ¼ 0.73; p < 0.001) (Figure 4).

DISCUSSION
In terms of diagnostic accuracy, SPECT/CT quantification of DPD scintigraphy using either cardiac peak In terms of quantifying cardiac amyloid burden, it is apparent that there is a huge spectrum of disease that is not captured in the Perugini grading system (perhaps unsurprising, given there are only 3 grades of positivity used to describe a complex, multisystem disorder) (6). An example of this is the variation in cardiac peak SUV seen between patients with a grade 2 DPD scan, which can range from 5 to 21 ( Figure 5).
This grading system alone is unlikely to prove sufficiently sensitive to facilitate the monitoring of response (or lack thereof) to amyloid therapies in the future. Unfortunately, what is also apparent is that   (22) and also carries prognostic significance in both ATTR-CA (23) and cardiac AL amyloid (24).
This would suggest that cardiac peak SUV (or, in turn, the derived SUV retention index) may be used to monitor amyloid regression and be associated with prognosis. Further validation is clearly warranted, particularly outside of the elderly population with aortic stenosis studied here, in which the variation in ECV in those without cardiac amyloid is likely to be much less, and therefore the correlation may well prove even better with cardiac peak SUV. Finally, whether these parameters will also track blood biomarkers (e.g., N-terminal pro-B-type natriuretic peptide, troponin T) or predict heart failure hospitalizations is beyond the scope of the current paper but merits investigation. Kingdom. E-mail: leon.menezes@nhs.net.