Comparative Performance of NaI/MEGP vs CZT/MEHRS Collimators in SPECT Imaging of Patients Treated with ¹⁷⁷Lu-DOTATATE

Objectives: In order to expand the usability of the CZT camera to image higher energy nuclides such as ¹⁷⁷Lutetium, this study aims to compare the novel CZT/Medium Energy High Resolution and Sensitivity (MEHRS) collimator to the current standard of care NaI/Medium Energy General Purpose (MEGP) collimator for ¹⁷⁷Lu-DOTATATE imaging through quantitative analysis and visual assessment.

Methods: A total of 20 ¹⁷⁷Lu-DOTATATE therapy patients (200mCi therapy dose) are recruited. Patients are scheduled for same-day standard of care SPECT/CT imaging on a GE NM/CT 670 Pro standard NaI gamma camera using a MEGP collimator, and then undergo subsequent SPECT imaging on a GE NM/CT 670 CZT camera using a new MEHRS collimator. Image acquisition parameters were identical on both systems: 2 bed positions, 60 stops/bed position, 15 seconds/stop. The energy window was centered at 208KeV +/- 10%. Images from both the NaI and CZT acquisitions were fused with the CT obtained during the NaI acquisition for attenuation correction and anatomic correlation. Absolute calibration of the scanners was performed by imaging a uniform cylindrical phantom with known ¹⁷⁷Lutetium activity concentration in water. Images were reconstructed using the same parameters for quantitative analysis: 4 iterations, 10 subsets, and no filter, using the ordered -subset estimation -maximization (OSEM algorithm) with resolution recovery, motion, scatter, and attenuation corrections. Images were reconstructed using the following parameters for visual analysis: 6 iterations, 10 subsets, Gauss post-filter 1.0 pixel FWHM, with resolution recovery, motion, scatter, and attenuation corrections. Quantitative analysis consist of comparison of absolute counts from each scanner, and SUV measurements of multiple prominent and representative lesions. Visual analysis consist of two blinded nuclear radiologists reviewing the images and rating each image set on a multi-point rating scale for image quality.

Results: Promising preliminary results from the first patient in our series demonstrated the CZT/MEHRS acquisition absolute counts to be 15% greater. Comparison of the 2 most prominent lesions revealed SUVmax of 66.8 and 51.9 on the CZT/MEHRS acquisition and similar values of 63.0 and 52.9 respectively on the NaI/MEGP acquisition. CZT/MEHRS produces high quality images by visual assessment.

Conclusions: Preliminary results indicate comparable quantitative assessment between new CZT/MEHRS collimator and standard of care NaI/MEGP acquisitions. Higher absolute counts on the CZT/MEHRS acquisition suggest greater absolute system sensitivity. CZT/MEHRS produces high quality images by visual assessment. Further results will be presented including the full subsets of patients and final conclusions. This work suggests that the CZT/MEHRS camera can effectively be used for clinical assessment of ¹⁷⁷Lu-DOTATATE.