Characterisation of a hand-held CZT-based gamma camera for 177Lu imaging

Abstract

Background: Currently, hand-held gamma cameras are being developed for ^99mTc imaging, mainly for sentinel lymph node detection. These cameras offer advantages, such as mobility and ease of access, and may be useful also for other applications such as biokinetic studies in animals or for imaging of small, superficial structures in patients. In this work, the suitability of a CZT-based hand-held camera for ¹⁷⁷Lu imaging is investigated. The energy response of CZT-based detectors combined with the multiple photon emissions of ¹⁷⁷Lu poses new challenges compared to ^99mTc imaging, and a thorough camera characterisation is thus warranted.

Methods: Three collimators (LEHR, LEHS, and MEGP) and three energy windows (55 keV, 113 keV, and 208 keV) are investigated. Characterised camera properties include the system spatial resolution, energy resolution, sensitivity, image uniformity, septal penetration, and temperature dependence. Characterisations are made starting from NEMA guidelines when applicable, with adjustments made when required. The applicability of the camera is demonstrated by imaging of a superficially located tumour in a patient undergoing [¹⁷⁷ Lu]Lu-DOTA-TATE therapy.

Results: Overall, the results are encouraging. Compared to a conventional gamma camera, the hand-held camera generally has a higher sensitivity for a given collimator. For source-collimator distances below 3 cm, the spatial resolution FWHM is within 6 mm for the LEHR and MEGP collimators. Before uniformity correction, the central field-of-view integral uniformity shows best results for the 113-keV window, with values obtained between 11 and 14%. The corresponding values after uniformity correction are within 3%. Effects of septal penetration are observed but are manageable with a proper combination of collimator and energy window setting. Septal penetration and collimator scatter not only affect the 208-keV window but also contribute with counts in lower windows due to energy-tailing effects. The patient study revealed non-uniform uptake patterns in a region that appeared uniform in a conventional gamma camera image.

Conclusions: The results show that the hand-held camera can be used for ¹⁷⁷Lu imaging. A 113-keV energy window combined with LEHR or MEGP collimators provides the best image system characteristics.

Keywords: 177Lu; CZT; Hand-held gamma camera; Molecular imaging.

Fig. 1

Energy resolution as a function of the photon energy for acquisitions made in low-energy mode and high-energy mode. Results determined by fitting of Gaussian functions (a) and by the NEMA interpolation procedure (b) are shown. Markers indicate the measured values, while the lines show the linear regressions (Eq. 6)

Fig. 2

Uniformity-correction matrices for ¹⁷⁷Lu for each energy window and collimator (including open field). Values higher than 1.0 correspond to anode elements whose sensitivity is below the detector average. All uniformity matrices are shown using the same colour scale (lower right corner). Circles and letters A–E indicate positions of the five pixels selected for spectral analysis in Fig. 3

Fig. 3

Spectra for the five selected pixels for the MEGP collimator, where letters correspond to positions indicated in Fig. 2. Energy windows are shown as grey rectangles. A 1-keV-wide mean-value filter has been applied for noise reduction

Fig. 4

System spatial resolution for ¹⁷⁷Lu as a function of the source-collimator distance for each collimator and energy window. The spatial resolution for a conventional gamma camera equipped with an MEGP collimator is also shown, with measured (coloured circles) and simulated (black line) data

Fig. 5

Measured ¹⁷⁷Lu spectra as a function of the detector temperature. The grayscale intensity represents the spectral amplitude; each row in the image represents one acquired spectrum. White lines shows the photopeak centres, and vertical red lines indicate the reference photon energies

Fig. 6

Sensitivities for the three collimators and the three energy windows for ¹⁷⁷Lu, as a function of the ROI radius. The radii corresponding to the source size, and the source size plus 0.5 times the FWHM are shown as vertical dashed lines

Fig. 7

Relative count rate in the FOV as a function of the lateral distance from the detector centre to a ¹⁷⁷Lu source. The FOV border is indicated by the vertical dashed line

Fig. 8

Spectra of ¹⁷⁷Lu acquired using the open field cover and the three collimators. The spectra are normalised to unity at the 113-keV peak. Differences in peak amplitude may be indicative of septal penetration. A spectrum for the Discovery 670 system is shown for comparison

Fig. 9

Patient uptake analysed using the conventional and hand-held gamma cameras. Images acquired using the Tandem Discovery 670 system are shown in a and b. a The fusion of a whole-body and CT scout image. An arrow indicates the uptake selected for imaging. b A magnification of this uptake in the anterior whole-body projection. The enlarged image spans an 4×4 cm² area and has been rotated such that the distal direction is to the right. c A CrystalCam measurement. CrystalCam images acquired with this orientation are shown in d–i, wherein the distal direction is also to the right

Fig. 10

Sensitivities for the three collimators as functions of ROI radius, with TEW correction applied for each energy window. The radii corresponding to the source size, and the source size plus 0.5 times the FWHM are shown as vertical dashed lines