NM Module Section 2 6 th Edition Christian, Ch. 3

Transcription

1 NM 4303 Module Section 2 6 th Edition Christian, Ch. 3

2 Gas Filled Chamber Voltage

3 Gas filled chamber uses Hand held detectors cutie pie Geiger counter Dose calibrators

4 Cutie pie Chamber voltage in Ionization range which provided counting proportionality

5 Geiger Counter Voltage in GM range which causes an avalanche of electrons in chamber characteristic x-rays Very sensitive, in low level activity, but no proportionality Makes characteristic clicking sound

6 Dose calibrator Why do you change dose names when detecting different radionuclide?

7 Dose calitrator

8 Cutie Pie

9 Scintillation Detector

10 Detector Crystal NaI- sodium Iodide, thallium activtated ¼ to ½ in. thick Light conversion process- Comptons ending in a photoelectic interaction to convert to visible light Hygorscopic Heat/cold sensitive Light pipe

11 Photomultiplier 35 to 100+ pm tubes in hexagon, square, round Photocathode in intimate contact with crystal 10 dynodes amplify current 50x ea. to create a weak current and maintain proportionality to input Discuss fluctuation, noise, drift, power supply stability, gain.

12 Pre-amplifier Amplifies signal 4-5 times Must have linear amplification of input to output Connected closely to PM tube to prevent signal loss

13 Amplifier Provides linear amplification of current by appx times proportionate to the energy input. Maintains X & Y positioning with Z signal intensity

14 Positioning summing matirx (Signal from gamma event) Gamma event spreads 4 pie. Closest PM tube collects greatest amt. of signal. 4 signals are created -x,+x,-y,+y, they are summed to give Z intensity. Correction Circuitry used.

15 Pulse Height Analyzer LL---UL Single or dual channel analyzer Anticoincidence circuit

16 Pulse Height

17 FWHM energy resolution See p. 64 in Christian FWHM =.5 max. voltage lower voltage peak X % good for Tc99m 7-9% food for Cs137

18 FWHM of Cesium-137

19 Why isn t the photopeak a straight line? Pt size, movement, distance, time, etc Where gamma event is absorbed in crystal Attenuators Energy of photons # on pm tubes Count density FWHM of equipment collmimation

20 Spatial Resolution FWHM (Christian, p71) See Mettler 5 th Ed. P21-2. FWHM spatial resolution is a measure of the spread of a gamma event. 3-5 mm is good FWHM spatial resolution

21 Resolution Intrinsic- W/0 collimator Extrinsic- With collimator on. (Remember--- Sensitivity and Resolution are inversely proportional.)

22 Windowing 10% window means 5% on each side of the 140 Kvp photo peak. Synetric window Asymetric window

23 Well Counter

24 Thyroid Probe

25 Not for imaging. Why?

26

27 Anger Camera

28

29 Collimators

30 Increased Distance from Collimator face Collimator Field of view Image Size resolution Detector Sensitivity Flat field NA NA (THYROID) Parellel SAME SAME APPX SAME Focus or converging Diverging more than parallel Pinhole (aperature opening)

31 Collimator resolution vs sensitivity Sensitivity may be increased by: 1. Increasing bore, (hole) size 2. Decreasing bore length Resolution may be increased by: 1. Smaller holes 2. Longer holes

32 Cont. Distance affects resolution. (short dis. better) Septal thickness: Increased thickness decreases sensitivity. What is cross talk, or septal penetration?

33 Converging Collimator Provides magnification, spatial resolution, and sensitivity. See p. 72 Christian, column 2 top of page.

34 Analog & Digital Cameras P. 74 Christian. Analog- 4 pulses summed to give Z pulse based on total energy deposited Digital- Use an A to D converter, digitizes signal. Newer cameras allow small cluster processing.

35 Nonuniformity 1. Comes primarily form mis-positioning of gamma events. 2. May also occur due to spreading of the pulse in the window (p.75)

36 Uniformity Correction Reference images collected to create correction maps or algarithms.

37 Linearity Correction

38 100 M flood uniformity correction

39 Image formation 2 Methods: 1. Photographic image formation- old system. Dots recorded by use of CRT with X & Y positioning and Z intensity.

40 Cont. 2. Digital image formation A. Frame mode acquisition- Each image pixel records a signal by use of X & Y positioning. (An image is created in the computer.) B. List mode acquisition- List of digitized addresses are recorded. May also record time markers.

41 Cont. Frame mode may also gate an image. P Christian

42 Solid State & Pixellated Cameras-- Using Zinc telluride crystals & collection of electron hole pairs

43 Emission Computed Tomography Uses tomography to determine the distribution of radioactivity in the patient. (rubix cube)

44 SPECT Uses imaging angles from 180 to 360 degrees to create images for reconstruction. (What is filtered back projection?)

45 Filtering Process of image manipulation to increase image resolution while reducing noise.

46 Ramp filter-- preserves spatial resolution (views the edges of the organ) Butterworth filter low pass, smooths the image & reduces noise

47 PET positron travels mm prior to a annihilation 511 KEV photons travel in straight lines 180 degrees apart C-11, N-13, O-15, F-18 are in organic molecules

48 Resolution problems in ECT 1. Attenuation in tissue 2. Transaxial spatial resolution loss 3. Scatter radiation 4. Accidental counting of unpaired photons 5. Noise from ramdomness of radioactive decay.

49 Hand held detector Quality Control Accuracy annual calibration required. Must be accurate within +/- 5% of Nat. Institute standards sources Constancy (day by day) test with long lived radionuclides and must be within 10%

50 Dose Calibrator Accuracy- annually as per NRC. must be accurate within +/- 10%. Energy linearity accuracy. Cs 137 (662KV, t1/2 30yr.) & Co57 (122KV, T1/2 270Days) Constancy- Daily (same as above) Linearity- quarterly Lowest to highest dose given to pt. must be accurate within +/- 10%. Geometry- Must be performed 2 installation and if equipment is modified

51 QC for Probes Peaking and counting background.

52 Scintillation Camera QC Flood field uniformity- Daily PHA (Peaking)- Daily Spatial Resolution & linearity Weekly (Your license may require additional tests and different time intervals.) Intrinsic or Extrinsic tests? Sheet source, flood source, or point source (See p. 92 for steps in QC)

53 Phantoms See p. 91 for (2) phantom requirements. A. PLES B. Four Quadrant C. Orthogonal hole

54 SPECT QC Uniformity- must be no greater than +/- 1% Extrinsic uniformity flood 5 million counts for 64x64 matrix done daily 60 million weekly (Different collimators require separate floods for separate uniformity correction & saved on computer.)

55 Cont. SPECT Center of Rotation done Weekly

56 End Chapter 3