As a performance standard, the intrinsic uniformity of the system shall be measured for the CFOV and UFOV.

The intrinsic uniformity is the response of the system without a collimator to a uniform flux of radiation from a point source. Two different uniformity parameters shall be determined: integral uniformity and differential uniformity. Integral uniformity is a measure of the maximum pixel count deviation in the CFOV or UFOV. Differential uniformity is a measure of the maximum deviation over a limited range designed to approximate the size of a photomultiplier tube.

Test Conditions: The radionuclide used to measure intrinsic uniformity is to be Tc-99m. Any other radionuclide used shall be reported separately. The count rate shall not exceed 20,000 counts per second through a symmetric 20% photopeak window. The status of uniformity corrections used shall be stated with the results.

Test Equipment: The test equipment required for this measurement consists of a source holder, a lead mask for the detector, and a computer or multi-channel analyzer. The source holder shall consist of a lead shield to prevent back and side scatter but is open at the front so that it does not restrict the gamma flux from the source to the detector. The lead mask for the detector is a lead aperture of at least the dimensions of UFOV.

Measurement Procedure: The detector shall be masked using a lead mask described above. The source in the source holder shall be placed on the central axis of the detector. The distance from the detector to the source shall be at least five times the largest dimension of the UFOV. The flood field image shall be stored in a matrix size which produces pixel sizes with a linear dimension of 6.4 mm + 30%. The pixels shall be square. A minimum of 10,000 counts shall be collected in the center pixel of the image.

Calculations and Analysis: Prior to performing the uniformity calculations, the pixels for inclusion shall be determined as described below. First, any pixels at the edge of UFOV containing less than 75% of the mean counts per pixel in CFOV shall be set to zero. Second, those pixels that have at least one of their four directly abutted neighbors containing zero counts will be also set to zero. The remaining non-zero pixels are the pixels to be included in the analysis for the UFOV. This step shall be performed only once. Any pixel, which has at least 50% of its area inside the CFOV, shall be included within the CFOV analysis.

DATA PREPARATION: The flood field image, after removing the edge pixels, shall be smoothed once by convolution with a 9-point filter function of the following weightings:

The weighting factor for a pixel outside the analyzed area in the 9-point filter function shall be zero. The smoothed value shall be normalized by dividing by the sum of non-zero weighting factors.

INTEGRAL UNIFORMITY: For pixels within each area (CFOV and UFOV), the maximum and the minimum values are to be found from the smoothed data. The difference between the maximum and the minimum is divided by the sum of these two values and multiplied by 100% Integral Uniformity = + 100% ((Max - Min) / (Max + Min))

DIFFERENTIAL UNIFORMITY: For pixels within each area (CFOV and UFOV) the largest difference between any two pixels within a set of 5 contiguous pixels in a row or column shall be calculated. The calculation shall be done for the X and for the Y directions independently and the maximum change expressed as a percentage using the following: Differential Uniformity = + 100% ((Max - Min) / (Max + Min))

The filtered data are treated as a number of rows (X slices) and columns (Y slices). Each slice is processed by starting at the beginning pixel for the respective field of view. A set of five contiguous pixels is examined to find the maximum and minimum pixels. The differential uniformity is calculated using these values. The next set of five pixels is analyzed by stepping forward one pixel and again determining the percent uniformity. This is repeated until the outermost pixel is reached. The maximum differential uniformity is found in the slice. This process is then repeated for all of the slices.

Reporting: The result is reported as a class standard and expressed as the percentage integral and differential uniformity for both of the CFOV and the UFOV.

MULTIPLE WINDOW SPATIAL REGISTRATION: Multiple window spatial registration shall be measured and reported as a performance standard. Multiple window spatial registration is a measure of the camera’s ability to accurately position photons of different energies when imaged through different photopeak energy windows. Measurements shall be made at nine specified points on the entrance plane of the scintillation camera.

Test Conditions: The radionuclide used to measure multiple window spatial registration shall be Ga-67. The count rate shall not exceed 10,000 counts per second through each symmetric 20% photopeak energy window.

Test Equipment: A lead-lined source holder sha11 co11imate the Ga-67 source through a cylindrical tunnel in the lead. This circular tunnel shall be 5 mm in diameter and 25 mm in length.

Measurement Procedure: Images shall be acquired using the above described collimated Ga-67 source located at nine specific points on the entrance surface of the un-collimated camera. These nine points shall be the central point, four points on the X-axis and four points on the Y-axis. The off central points shall be located 0.4 times and 0.8 times the distance from the central point to the edge of the UFOV of the camera along the respective axes. Separate images of the collimated Ga-67 source shall be acquired through separate energy windows of the Ga-67 photopeaks at each of these images locations. These images shall be acquired with a pixel size of not more than 2.5 mm. For cameras with two energy windows, two images shall be acquired at each point, one using the 93 kev photopeak and the second using the 300 kev photopeak. For cameras with three or more energy windows, the 184 kev photopeak shall also be imaged. At least 1,000 counts shall be acquired in the peak pixel of each photopeak image.

Calculations and Analysis: The displacement of the count centroids from each other in the X and Y directions shall be determined for each measurement point’s photopeak images. A square region of interest (ROI) centered on the maximum count pixel associated with each photopeak image shall be used to analyze the individual photopeak images.

The pixel dimensions of the square ROI shall be approximately four times the FWHM of the image count profile to be analyzed. Each image shall be integrated in the Y direction to determine the X count profile and integrated in the X direction to determine the Y count profile. The centroid of counts in the X and Y directions shall be determined for each image from that direction’s count profile by the method described below. The maximum difference in position of the centroid of counts acquired from each photopeak shall be determined. The largest pixel displacement shall then be converted to millimeters using an accurate millimeter per pixel calibration.

METHOD FOR CENTROID OF COUNTS DETERMINATION: The center of counts in the X and Y directions for each of the photopeak count profiles shall be determined as follows. Find the maximum count pixel in the integrated X or Y profile and calculate the centroid of counts using the following formula: