Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
University of Washington School of...
Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
X-ray Output
II. Radiation Output ...
Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
Set Up:
1) Set to TT exposure on t...
Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
1) Set exposure to Table Bucky on ...
Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
V. Relation between kVp, mAs, pati...
Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
4) DR Table detector with scatter
...
Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability
4) DR Table detector with scatter
...
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Introduction to Radiology

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Transcript of "Introduction to Radiology"

  1. 1. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability University of Washington School of Medicine, Department of Radiology Diagnostic Radiology Imaging Physics Course 2007-2008 Name: ______________________________________________________________________ Today’s Date: ________________________________________________________________ Goals: To determine how changes in kVp and mAs affect x-ray output. How does patient thickness affect x-ray technique and patient dose? The participant will be: 1. Familiar with the controls of an x-ray unit and how to obtain a CR and DR image. 2. Able to identify some factors influencing contrast resolution 3. Able to identify the effects window and level changes on the displayed image Items Needed: • Unfors XI radiation output meter • Measuring tape • Lucite plastic slabs • Contrast Detail phantom I. Describe equipment 1. X-ray tube 2. Collimator/light field 3. Bucky tray and autocollimator 4. Phototimer 5. Wall stand 6. Console- knobs for kVp, mA, mAs 7. Computed radiography reader 8. Computed radiography QA workstation 9. Digital radiography unit 10. Contrast-detail phantom introduction-to-radiology61.doc Page 1 of 6
  2. 2. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability X-ray Output II. Radiation Output vs. mAs Set Up: 1) Set to Table Top (TT) exposure on the control panel—no Automatic Exposure Control (AEC) in order to manually control the mAs. 2) Set up the Unfors in the center of the table. 3) Orient the long axis of the Unfors perpendicular to the tube axis to minimize the heel effect. 4) Set the tube 40 inches above the Unfors using either the collimator tape or tape measure 5) Center light field to Unfors, collimate to 7” x 7” (check collimation after each exposure). 6) Set the technique to 80 kVp at 5, 50, and 100 mAs on the control panel. Measure and record the exposure output (mR) as a function of mAs. Table 1 Output vs. mAs kVp mAs Output (mR) mR/mAs 80 10 80 20 80 40 Graph 1 Output vs mAs 0 100 200 300 400 500 0 10 20 30 40 50 mAs mR How does the output change with changes in mAs? ____________________________ III. Radiation Output vs. kVp Setting introduction-to-radiology61.doc Page 2 of 6
  3. 3. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability Set Up: 1) Set to TT exposure on the control panel—no AEC. 2) Set up the Unfors in the center of the table. 3) Orient the long axis of the Unfors perpendicular to the tube axis. 4) Set the tube 40 inches above the Unfors using either the collimator tape or tape measure. 5) Center to Unfors, collimate to 7” x 7” (check collimation after each exposure). 6) Set the technique to 50, 80, and 110 kVp at 20 mAs on the control panel. Measure and record the exposure output (mR) as a function of kVp. Calculate mR/mAs. Table 2 Output vs. kVp kVp mAs Output (mR) mR/mAs 50 20 80 20 110 20 Graph 2 Output vs kVp 0 100 200 300 400 500 40 60 80 100 120 kVp mR How does the radiation output change with changes in kVp? _____________________ IV. Relation between entrance and exit dose to patient. Set Up: introduction-to-radiology61.doc Page 3 of 6
  4. 4. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability 1) Set exposure to Table Bucky on the control panel. 2) Turn on all three AECs at the control panel. 3) Set the tube to the center of the table. 4) Place a 14” x 17” dummy cassette (without Image Receptor) in the table Bucky. 5) Align the Bucky to the tube and close the tray. 6) Place the 6 inch plastic phantom on the floats in the center of the light field on the TT. a) If the tube is correctly set and centered to the Bucky, the light will automatically show and collimate to the cassette size. 7) Set up the Source to Surface Distance (SSD) to 29.5 inches from the x-ray source. a) SSD is to the top of the phantom. 8) For entrance dose, place the Unfors on top of the phantom perpendicular to the tube axis. 9) For exit dose, place the Unfors under the phantom perpendicular to the tube axis. 10) Set technique to 70, 80, and 110 kVp on the control panel. 11) Reset the Bucky tray between exposures (leave the dummy cassette in the tray). Measure and record the entrance and exit doses and the mAs. Calculate mR/mAs. Calculate the ratio of entrance to exit dose for the three kVp values. Table 3 Location kVp mAs mR Ent/Exit Ratio mR/mAs Entrance 70 Exit 70 Entrance 80 Exit 80 Entrance 100 Exit 100 Did the AEC unit maintain approximately the same radiation intensity on the detector regardless of kVp? __________________________________ introduction-to-radiology61.doc Page 4 of 6
  5. 5. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability V. Relation between kVp, mAs, patient dose A unit equipped with an AEC circuit automatically sets the correct mAs. Portable units do not have AEC so adjustments in the kVp and mAs have to be made manually. If the contrast is not correct the kVp must be adjusted. There is a rule of thumb that a 15% change in kVp is equivalent to a factor of two change in mAs. An increase in kVp from 70 kVp to 80 kVp is about a 15% increase. Comparing the data from Table 3, what can you conclude about the 15% rule? ____________________________________ __________________________________________________________________________________ The AEC unit adjusts the mAs to maintain a constant intensity on the detector. How do the mAs track with the 15% rule? ___________________________________________________________________ __________________________________________________________________________________ Factors Affecting Low-contrast Detectability Using the Fuji CR reader: process all cassettes in the Chest PA reading mode. Send all the images to PACS with the Patient Name = QAPHYSICS, CR and MRN = UQA001CR. Using the DR system, send all images to PACS with Patient Name = QAPHYSICS, DR and MRN = UQA001DR. It will be necessary to evaluate the images on the PACS (look for images with today’s date). VI. Effect of Scatter and Imaging Receptor on Contrast Visibility 1) CR Table Top (TT) with scatter a) Set to Table Top (Screen) on the control panel—no AEC. b) Set cassette, contrast detail phantom, tube, SID, and collimation as for TT without scatter. c) Place 3 inches of tissue phantom on top of the contrast detail phantom. d) Set the technique to 110 kVp at 0.5 mAs on the control panel. i) After each exposure process the cassette in the Chest PA reading mode. ii) Annotate each image with the technique used and “CR TT with Scatter – Table 4.” 2) CR Table Bucky with scatter a) Set exposure to Table Bucky on control panel. b) Turn on all AECs at the control panel. c) Lock the tube to the center of the table. d) Set tube to 40 inches SID. e) Place a 14” x 17” CR cassette in the Bucky. f) Align the Bucky to the tube and close the tray. g) Set the contrast detail phantom on the table in the center of the light field. h) Place 3 inches of tissue phantom on top of the contrast detail phantom. i) Collimate to the phantom size after each exposure. j) Set technique to 110 kVp on the control panel. i) After each exposure process the cassette as Test/Control algorithm. ii) Annotate each image with the technique used and “CR Bucky with Scatter – Table 4.” 3) Send the images to PACS. introduction-to-radiology61.doc Page 5 of 6
  6. 6. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability 4) DR Table detector with scatter a) Set exposure to Table on control panel – tube head will automatically align and set to 100 cm. b) Turn on all AECs at the control panel. c) Set the contrast detail phantom on the table in the center of the light field. d) Place 3 inches of tissue phantom on top of the contrast detail phantom. e) Collimate to the phantom size after each exposure. f) Set technique to 110 kVp on the control panel. i) Notice that the DR unit automatically processes the images with no transfer of the receptor. ii) Annotate each image with the technique used and “DR with Scatter – Table 4.” 5) Send the images to PACS. Record the smallest hole size visible at the indicated hole depth in Table 4. There are two holes in each square at 0.4, 0.8, 1.6, 2.5; single holes at 6.3, 8.0. Table 4 Object visibility as a function of hole size with and without scatter Hole depth: 0.4 0.8 1.6 2.5 6.3 8.0 110 kVp @ 0.5 mAs CR TT with scatter 110 kVp w/ Photo- timers CR Bucky with Scatter 110 kVp w/ Photo- timers DR with Scatter How much does a grid improve the contrast visibility? ____________________________________ _______________________________________________________________________________ Is there a marked improvement with DR over CR? ____________________________________ _______________________________________________________________________________ VII. Effect of window and level controls on object visibility For the images created above, change the window and level controls and observe how objects can be made to appear or disappear depending on the window and level settings. How does window and level affect visibility of objects on screen? ___________________________ ________________________________________________________________________________ introduction-to-radiology61.doc Page 6 of 6
  7. 7. Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability 4) DR Table detector with scatter a) Set exposure to Table on control panel – tube head will automatically align and set to 100 cm. b) Turn on all AECs at the control panel. c) Set the contrast detail phantom on the table in the center of the light field. d) Place 3 inches of tissue phantom on top of the contrast detail phantom. e) Collimate to the phantom size after each exposure. f) Set technique to 110 kVp on the control panel. i) Notice that the DR unit automatically processes the images with no transfer of the receptor. ii) Annotate each image with the technique used and “DR with Scatter – Table 4.” 5) Send the images to PACS. Record the smallest hole size visible at the indicated hole depth in Table 4. There are two holes in each square at 0.4, 0.8, 1.6, 2.5; single holes at 6.3, 8.0. Table 4 Object visibility as a function of hole size with and without scatter Hole depth: 0.4 0.8 1.6 2.5 6.3 8.0 110 kVp @ 0.5 mAs CR TT with scatter 110 kVp w/ Photo- timers CR Bucky with Scatter 110 kVp w/ Photo- timers DR with Scatter How much does a grid improve the contrast visibility? ____________________________________ _______________________________________________________________________________ Is there a marked improvement with DR over CR? ____________________________________ _______________________________________________________________________________ VII. Effect of window and level controls on object visibility For the images created above, change the window and level controls and observe how objects can be made to appear or disappear depending on the window and level settings. How does window and level affect visibility of objects on screen? ___________________________ ________________________________________________________________________________ introduction-to-radiology61.doc Page 6 of 6

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