Background Diagnosis of idiopathic inflammatory myopathies (IIMs) is challenging and so far no pathognomonic signs exist by imaging. Few radionuclide imaging techniques have been tested for this purpose, mainly ^99mTc-pyrophosphate planar imaging and ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (^18F-FDG-PET/CT). However, ^99mTc-PYP uptake has been assessed visually and at best graded semi-quantitatively.

Objectives We aimed for quantitative ^99mTc-pyrophosphate single photon emission computed tomography/computed tomography ^(99mTc-PYP-SPECT/CT) as well as ^18F-FDG-PET/CT imaging in a group of newly onset IIM patients.

Methods Thirteen patients (mean age 62 years) with newly diagnosed, untreated IIM underwent ^99mTc-PYP SPECT/CT of the thorax, pelvis, and thighs. Seven of the patients also had a whole-body ¹⁸F-FDG PET/CT scan. Forty-nine healthy controls (mean age 59 years) underwent ^99mTc-PYP SPECT/CT and 26 healthy controls (mean age 57 years) had a ¹⁸F-FDG PET/CT scan done. Volumes of interest (VOIs) covering the right biceps, triceps, and quadriceps muscles were drawn manually on each series. Registered ^99mTc-PYP counts, respectively standardized uptake values (SUVs) of ¹⁸F-FDG were obtained from all VOIs. Registered counts were decay- and attenuation-corrected and adjusted for body weight and administered dose of ^99mTc-PYP, yielding a parameter similar to the SUV [g mL^-1].

Results IIM patients had visible tracer uptake in the skeletal muscles of the extremities. The muscular ^99mTc-PYP uptake was significantly higher in upper limbs of patients than the uptake in the same muscle groups in healthy controls (uptake[biceps] 0.46 vs. 0.33 g mL^-1, p=0.01; uptake[triceps] 0.40 vs. 0.27 g mL^-1, p=0.003). The ^99mTc-PYP uptake tended to be higher in the lower limbs of patients than in the lower limbs of controls (uptake[quadriceps] 0.59 vs. 0.48 g mL^-1, p=0.06). The muscular FDG uptake was significantly higher in patients than in controls in both upper limbs (SUV_mean[biceps] 1.35 vs. 0.72 g mL^-1, p=0.006; SUV_mean[triceps] 0.91 vs. 0.44 g mL^-1, p=0.0008) and lower limbs (SUV_mean[quadriceps] 0.84 vs. 0.62 g mL^-1, p=0.0001). The muscular FDG uptake values were consequently higher than the ^99mTc-PYP uptake values, although not by a constant factor.

Conclusions Quantitative ^99mTc-PYP SPECT/CT as well as ¹⁸F-FDG PET/CT imaging revealed muscular inflammation in patients with newly onset, untreated IIM. Patients had higher tracer uptake in skeletal muscles groups than healthy controls. Quantification of muscular tracer uptake with the potential to objectively distinguish physiology from pathophysiology could be a valuable tool in the challenging diagnosis of IIMs.

Disclosure of Interest None declared