Planar gamma cameras
GP-1 Flood-field uniformity
It is the ability of a gamma camera to respond with a constant surface count density when subjected to a uniformly distributed radiation flux. Uniformity is a key test that reflects the gamma camera’s ability to reproduce the patient’s activity distribution without alterations. It is an indicator that depends on spatial linearity, energy resolution, and the condition of the collimator. Integral non-uniformity (IU) is a global estimator of uniformity change, while differential non-uniformity (DU) is a local estimator of uniformity change.
GP-2 Extrinsic sensitivity
It is the capacity of a gamma camera to convert each radioactive disintegration into an observable event. Sensitivity is expressed as the ratio of the observed count rate with a source of known activity per unit of activity.
GP-3 Extrinsic spatial resolution
Spatial resolution evaluates the imaging system's ability to distinguish two events occurring at a short distance from each other as independent events. Therefore, it indicates the minimum distance at which two sources can be placed to be recorded as two distinct images. It is usually expressed by the full width at half maximum (FWHM) of the count profile in the image of a line. Extrinsic spatial resolution depends on the distance between the source and the collimator, so it must be referenced for a specific distance and always with the same collimator.
GP-4 Spatial linearity
The spatial linearity of a system allows for the evaluation of the distortion or displacement of the measured position of photons relative to the position where they interacted with the detector. It is a measure of the accuracy of the coordinate calculation.
GP-5 Intrinsic energy resolution
Energy resolution defines the ability of a radiation detection system to discriminate photons with similar energies. The gamma camera's ability to distinguish photons of nearby energies is reflected in the width of the photopeak in the energy spectrum: the broader the peak, the poorer the energy resolution. Since a single detector does not perform uniformly across all energy levels, energy resolution must be referenced to a specific energy. Energy resolution is quantified by the full width at half maximum (FWHM) of the photoelectric peak relative to the peak energy.
GP-6 Pixel size
Determining pixel size involves measuring the area corresponding to a single pixel. Each pixel in an image, depending on the matrix used, represents a specific area of the gamma camera's scintillation crystal. This is determined by measuring, in pixels, the distance between the images of two point sources separated by a known distance. The measurement must be performed along both the X and Y axes to verify that the pixel size is consistent in both directions.
GP-7 Multi-window spatial registration
The purpose of this test is to verify that images of the same object acquired with different energy windows overlap. This ensures the gamma camera's ability to assign the same image coordinate to photons of different energies that interact at the same point on the detector surface. The parameter to be determined is the maximum difference in the spatial locations of these images.
GP-8 Temporal resolution
Temporal resolution is the ability of an imaging system to distinguish as separate events those that occur within a very short time interval. The time required by the system to process a scintillation is referred to as dead time, during which the system is unable to accurately detect new scintillations. The parameter used to estimate the temporal resolution of a gamma camera is R-20%, which represents the count rate at which the detected photon rate experiences a 20% loss compared to the incident photon rate.
