GS Introduction to Medical Physics IV Laboratory 5 Gamma Camera Characteristics

Transcription

1 GS Introduction to Medical Physics IV Laboratory 5 Gamma Camera Characteristics Purpose: To introduce some of the basic characteristics of a gamma camera. This lab will introduce gamma camera QC and acquiring/evaluating a uniform flood image. This lab will investigate the changes in spatial resolution and sensitivity of the gamma camera as a function of the source to collimator distance and collimation type (LEHR versus MELP). Equipment: Siemens Symbia S gamma cameras: Use four systems with four unique combinations of crystal thickness and collimation 3/8 crystal and LEHR collimator 5/8 crystal and LEHR collimator 3/8 crystal and MELP collimator 5/8 crystal and MELP collimator Materials: Radioactive Sources: - Tc-99m: 2 line sources (0.85 cc) containing ~1 mci 99m Tc each - Tc-99m: 2 flat-bottom sensitivity flasks containing µci 99m Tc (calibrated) Two foam blocks of similar height (~10 cm) for line source Styrofoam sensitivity flask holder I. Extrinsic Spatial Resolution Task: Measure the extrinsic spatial resolution for the gamma camera (in FWHM-mm) with a specific crystal thickness and collimation combination assigned to your group. Spatial resolution will be measured at 4 different source-to-collimator distances. Two separate data acquisitions will be performed as shown in Figure 1. Steps for Data Acquisition: 1. Register New Patient: Patient name: zzpaf_medphys4_lab, extrinsic spatial resolution, Patient ID: today s date, Study: Calibrations, Category/Workflow: Physics/Static Acq for MedPhysIV Lab6 Spatial Resolution. Series name: Static Acq of Tc99m line source <collimator>. Write collimator used in series name. Use preset Tc99m-NMG, matrix size 1024x1024, and zoom Orient the camera heads in the 0/180 degree orientation. Place a chux on top of the bottom collimator/camera face to prevent radioactive contamination of the camera heads. 3. Move the detector heads so that the distance between heads is ~31.5 cm. 4. Setup Figure 1a: Gently place the line source directly on the collimator face and oriented it along the short axis of the camera (i.e. along the bed). 5. Setup Figure 1b: Place the foam blocks on collimator face and the line source on top of the foam block. Orient the line source along the short axis of the camera. Tape the line source to the foam blocks. Measure the horizontal distance from the edge of FOV to the line source to make sure they are parallel. 6. Record the height of the elevated line source from the collimator face and the distance GS , Med Phys IV, Spring 2013 page 1/4

2 between the source and each collimator face. 7. Acquire an image of the phantom using a 10 minute acquisition time. 8. Select acquired series data and launch Processing Activities/Series ROI curve Generic Proc. Use ROI tool to draw lines, create line profile, and compute the FWHM. Analyze one line source at a time. Perform 5 FWHM measurements per line source. Record FWHM data together with crystal thickness and collimation type. Det 1 Det 1 Det 2 Det 2 x (a) (b) Figure 1: Schematic setup for spatial resolution measurements (Section I), x~10.5 cm. Share your measurements with other groups so that each of you has extrinsic resolution values measured at 4 different distances for all four unique combinations of crystal thickness and In your report: 1. Plot the FWHM vs. source to detector distance. 2. Characterize and discuss the change in spatial resolution of the gamma camera at 140 kev (Tc-99m) with crystal thickness and GS , Med Phys IV, Spring 2013 page 2/4

3 II. Sensitivity Task: Measure the sensitivity the gamma camera (in [cps/uci]) with a specific crystal thickness and collimation combination assigned to your group. Sensitivity will be measured at 4 different source-to-collimator distances. Two separate data acquisitions will be performed as shown in Figure 2. Steps for Data Acquisition: 1. Register New Patient: Patient name: zzpaf_medphys4_lab, sensitivity, Patient ID: today s date, Study: Calibrations, Category/Workflow: NM Testing/Tc-99m sensitivity Acq. Series name: Tc99m sensitivity <collimator> distance, Write collimator used in series name. Use preset Tc99m-NMG, matrix size 256x256 and zoom Orient the camera heads in the 0/180 degree orientation. Place a chux on top of the bottom collimator/camera face to prevent radioactive contamination of the camera heads. 3. Setup Figure 2a: Place the flat-bottom flask directly on the chux in the center of the detector. The source to collimator face is ~ 0 cm. Adjust the overhead detector head so that it is at a distance of ~30 cm from the bottom collimator face. 4. Setup Figure 2b: Place the flat-bottom flask into the blue Styrofoam flask holder in the center of the camera. The source to bottom collimator face is ~10 cm and the source to bottom collimator face is ~20 cm. 5. Acquire images of each setup using an acquisition time of one (1) minute. 6. Remove source from scanner room and record 1 background measurement. Place source in location and perform 10 source measurements. Remove source from scanner room and record 1 background measurement. Record the time of each measurement. 7. Double-click the acquired data to view the images. Select statistics option (Σ) in display. Record the total number of counts in each of the 11 views. 8. Background correct the measured counts and compute the mean count rate [cpm]. 9. Decay-correct the assayed activity and determine the activity [in uci] at the time of the 5 th or 6 th (or mean time of) data acquisition. Compute the sensitivity [cpm/uci]. Det 1 Det 1 Det 2 Det 2 x (a) (b) Figure 2: Schematic setups for sensitivity measurements (Section II), x~10cm. GS , Med Phys IV, Spring 2013 page 3/4

4 Share your measurements with other groups so that each of you has sensitivity values measured at 4 different distances for all four unique combinations of crystal thickness and In your report: 1. Plot the sensitivity vs. source to detector distance. 2. Characterize and discuss the change in sensitivity with crystal thickness and GS , Med Phys IV, Spring 2013 page 4/4

5 Date: Group Members: Camera Name: Crystal Thickness: Collimation: Extrinsic Spatial Resolution Data: Setup A Setup A Setup B Setup B Distance [cm] (~0.5) (~31.5) (~10.5) (~21) FWHM-mm #1 FWHM-mm #2 FWHM-mm #3 FWHM-mm #4 FWHM-mm #5 Sensitivity Data: Source Activity: Calibration Time: Time Setup A Setup A Time Setup B Setup B Distance [cm] (~0) (~30) (~10) (~20) Background 1 Measurement 1 Measurement 2 Measurement 3 Measurement 4 Measurement 5 Measurement 6 Measurement 7 Measurement 8 Measurement 9 Measurement 10 Background 2 GS , Med Phys IV, Spring 2013 page 5/4