Quantitative 177 Lu SPECT (QSPECT) imaging using a commercially available SPECT/CT system

Abstract

Purpose: The combination of single photon emission computed tomography (SPECT) and computer tomography (CT) that incorporates iterative reconstruction algorithms with attenuation and scatter correction should facilitate accurate non-invasive quantitative imaging. Quantitative SPECT (QSPECT) may improve diagnostic ability and could be useful for many applications including dosimetry assessment. Using 177Lu, we developed a QSPECT method using a commercially available SPECT/CT system. Methods: Serial SPECT of 177Lu sources (89–12,400 MBq) were acquired with multiple contiguous energy windows along with a co-registered CT, and were reconstructed using an iterative algorithm with attenuation and scatter correction. Camera sensitivity (based on reconstructed SPECT count rate) and dead-time (based on wide-energy spectrum count rate) were resolved by non-linear curve fit. Utilizing these parameters, a SPECT dataset can be converted to a QSPECT dataset allowing quantitation in Becquerels per cubic centimetre or standardized uptake value (SUV). Validation QSPECT/CT studies were performed on a 177Lu cylindrical phantom (7 studies) and on 5 patients (6 studies) who were administered a therapeutic dose of [177Lu]octreotate. Results: The QSPECT sensitivity was 1.08 × 10−5 ± 0.02 × 10−5 s−1 Bq−1. The paralyzing dead-time constant was 0.78 ± 0.03 µs. The measured total activity with QSPECT deviated from the calibrated activity by 5.6 ± 1.9% and 2.6 ± 1.8%, respectively, in phantom and patients. Dead-time count loss up to 11.7% was observed in patient studies. Conclusion: QSPECT has high accuracy both in our phantom model and in clinical practice following [177Lu]octreotate therapy. This has the potential to yield more accurate dosimetry estimates than planar imaging and facilitate therapeutic response assessment. Validating this method with other radionuclides could open the way for many other research and clinical applications.

Impact of a commercial 3D OSEM reconstruction algorithm on the 177Lu activity quantification of SPECT/CT imaging in a Molecular Radiotherapy trial

“…Several investigations have been reported on the performance of such algorithm on radionuclides frequently used in nuclear medicine, for patient imaging as well as patient specific dosimetry. Some of the more pertinent publications include improvements in correction techniques for quantification in SPECT [4,[17][18][19][20], evaluations of the accuracy of quantitative image reconstruction techniques [21][22][23][24][25] and comparisons of resolution between SPECT/CT systems using machine-specific reconstruction algorithms [26][27][28]. In these studies, the OSEM reconstruction parameters (i.e.…”

MIRD Pamphlet No. 26: Joint EANM/MIRD Guidelines for Quantitative 177Lu SPECT Applied for Dosimetry of Radiopharmaceutical Therapy

“…Better results are obtained when the scatter estimate is incorporated into the projector step in the iterative reconstruction algorithm. Additionally, because a ME collimator the scatter energy window set above the 208-keV photopeak usually contains relatively few counts, this second scatter window can be ignored (45,46).…”

“…These include comparison of different approaches to determine camera normalization factors (54); investigation of dead-time effects and appropriate correction methods (39,46); and comparison of the accuracy of image quantitation, which can be achieved when using 113-or 208-keV photopeaks for the camera equipped with LEHR and ME collimators (30,46).…”

“…Image reconstructions used OSEM algorithms with CDR compensation available on commercial cameras (46) or reconstruction methods developed in-house (30). All reconstructions used CT-based AC, whereas for scatter correction the dual-energy window method (DEW) (30,46), TEW (45,56), ESSE (45), and APDI (30) methods were investigated. Table 4 summarizes the results of these studies.…”

The difference between medicine and magic is that magicians know what they are doing

“…Indeed, it seems inevitable that radiation dose at the cellular level will determine both response in sites of disease and toxicity to normal tissues. While we certainly need better predictive dosimetry to inform safe and effective administered activities of radionuclide therapies, measuring dose delivery following each cycle of PRRT is becoming feasible through quantitative single photon emission computed tomography (SPECT)/CT [32] of cumulative tissue activity of 177 Lu over time or by positron emission tomography (PET)/ CT imaging of pair production associated with 90 Y decay [33].…”