Loading…

Flash Player 9 (or above) is needed to view presentations.
We have detected that you do not have it on your computer. To install it, go here.

Like this document? Why not share!

Introduction to Radiology

on

  • 738 views

 

Statistics

Views

Total Views
738
Views on SlideShare
738
Embed Views
0

Actions

Likes
0
Downloads
6
Comments
0

0 Embeds 0

No embeds

Accessibility

Upload Details

Uploaded via as Microsoft Word

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

Introduction to Radiology Introduction to Radiology Document Transcript

  • 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 2007-2008Name: ______________________________________________________________________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 imageItems Needed: • Unfors XI radiation output meter • Measuring tape • Lucite plastic slabs • Contrast Detail phantomI. Describe equipment 1. X-ray tube 7. Computed radiography reader 2. Collimator/light field 8. Computed radiography QA 3. Bucky tray and autocollimator workstation 4. Phototimer 9. Digital radiography unit 5. Wall stand 10. Contrast-detail phantom 6. Console- knobs for kVp, mA, mAsintroduction-to-radiology61.doc Page 1 of 6
  • Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability X-ray OutputII. Radiation Output vs. mAsSet 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 measure5) 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/mAs (mR) 80 10 80 20 80 40Graph 1 Output vs mAs 500 400 300 mR 200 100 0 0 10 20 30 40 50 mAsHow does the output change with changes in mAs? ____________________________III. Radiation Output vs. kVp Setting introduction-to-radiology61.doc Page 2 of 6
  • Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast DetectabilitySet 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 20Graph 2 Output vs kVp 500 400 300 mR 200 100 0 40 60 80 100 120 kVpHow 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
  • Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability1) 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 100Did the AEC unit maintain approximately the same radiation intensity on the detector regardless ofkVp? __________________________________ introduction-to-radiology61.doc Page 4 of 6
  • Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast DetectabilityV. Relation between kVp, mAs, patient doseA unit equipped with an AEC circuit automatically sets the correct mAs. Portable units do not haveAEC so adjustments in the kVp and mAs have to be made manually. If the contrast is not correct thekVp must be adjusted. There is a rule of thumb that a 15% change in kVp is equivalent to a factor oftwo change in mAs.An increase in kVp from 70 kVp to 80 kVp is about a 15% increase. Comparing the data from Table3, 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 trackwith the 15% rule? _____________________________________________________________________________________________________________________________________________________ Factors Affecting Low-contrast DetectabilityUsing 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’sdate).VI. Effect of Scatter and Imaging Receptor on Contrast Visibility1) 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
  • Laboratory Exercise No 1: X-ray Output and Factors Affecting Low-contrast Detectability4) 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 8.0110 kVp CR TT with @ 0.5 scatter mAs110 kVp CR Bucky w/ with Scatter Photo- timers110 kVp DR with w/ Scatter Photo- timersHow 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 visibilityFor the images created above, change the window and level controls and observe how objects can bemade 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