No Slide TitleRadiation Protection and Safe Use of Radiation Sources
Part II Quantities and Measurements
Module 3 Principles of Radiation Detection and Measurement
Session 10 Imaging Detectors
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Introduction
Radiation imaging detectors and their principles of operation will be discussed
Students will learn about film radiographic imaging, rectilinear scanners, total performance phantoms, gamma cameras, cardiothoracic imaging, bone scans, computerized tomography, SPECT nuclear imaging, ECG-gated tomography, QA/QC issues, and positron emission tomography (PET)
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Principles of nuclear imaging will be described
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Film Radiographic Imaging
X-ray imaging is a method of illuminating the body with a penetrating high energy ionizing radiation. The differential absorption of this radiation by the various tissues of the body creates on film an inverse shadow of the body.
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Nuclear Medicine
Imaging Detectors
Nuclear Medicine is a method of obtaining diagnostic images by giving the patient a small dose of radioactive isotope
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Nuclear Medicine
Imaging Detectors
The radioisotope dose is given either by an IV injection in the arm, breathing an aerosol, or swallowing a capsule
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Nuclear Medicine
Imaging Detectors
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A camera will be used to take pictures
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Nuclear Medicine
Imaging Detectors
Some exams require a delay after the dose is given and before the pictures are started. This delay is required to give the dose time to collect in the organ being studied
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Nuclear Medicine
Imaging Detectors
The patient may be asked to lie down or sit in front of the camera. The technologist will position the camera close to the area of the body that is to be imaged
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Nuclear Medicine
Imaging Detectors
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distribution of a radiopharmaceutical
Collimator
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Collimator
Energy resolution P P
Sensitivity P T P
Counting precision P P
Test of integral background P T P
Test of preset analyzer facilities P P
System linearity P T P
Background subtraction P P
Contrast enhancement P P
Scanner drive P P
P = physicist T = technician
distribution of a radiopharmaceutical
Siemens
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Gamma Camera
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Nuclear medicine images arise from injected radioactive tracers which subsequently emit radiation from within body organs. The radioactive compounds tend to be designed to accumulate selectively in specific tissues.
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Dynamic Acquisition
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Coronary angiography requires multiple separate views to completely examine coronary anatomy and resolve potential vessel overlap. Several separate sequential injections of left (LCA) and right coronary arteries (RCA) are shown.
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Bone Scans
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Tomographic Imaging
Tomography is a "slicing" of the body into various sections and in various view planes. The tomographic sections when viewed in sequence or integrated by a computer allow the display and understanding of 3-dimensional anatomy.
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Computed Tomography
Computed tomography is a digitally based x-ray technique. Like x-ray, the resulting images arise from differential x-ray absorption of tissue, a feature that rests primarily on atomic weight (and thus the electron density) of the various tissues.
The technique uses a narrowly collimated x-ray beam to irradiate a slice of the body. The amount of radiation transmitted along each projection line is collected by photo-multiplier tubes and counted digitally.
By rapidly acquiring views from numerous different projections, achieved by quickly rotating the tube and detectors around the body, the transmissivity of the body from different angles can be established externally.
Once these transmission values are collected, they can be digitally filtered and back-projected mathematically (by a technique known as Fourier transformation) onto a matrix which represents fine differentiation of tissue densities. Mapping this density onto what is called the Hounsfield scale where bone is +1000 and air is -1000, resolves as subtle as 1/2 a percent electron density within half millimeter volumes of tissue.
This distinction is enough to discriminate most of the soft tissue organs of the brain and abdomen, not to mention the lungs and mediastinum. Since the image is digital and represents a slice, multiple slices can be obtained and a volume estimated and displayed as a three-dimensional structure on a video display tube or film.
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CT Scanner
The picture on top on this slide shows the Phillips 6000 Computerized Tomography (CT) imaging camera.
Conventional CT can be used to scan a complete anatomical volume by making successive, stepwise scans of one slice after another. However, scanning a complete volume with this technique requires a delay between each scan, so that the table can be moved into position for the following scan.
New technology avoids these delays by using Volumetric CT which combines continuous rotation and continuous radiation data collection with continuous patient table transportation.
The data obtained can be submitted directly to the image reconstruction process resulting in 3D images of the anatomical site. Individual anatomical scans can be used to prescribe the radiation dose to the tumor site with minimal exposure to critical surrounding normal tissues.
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SPECT Nuclear Imaging
The Single Photon Emission Computed Tomography (SPECT) camera is a large scintillation crystal connected to multiple photo-multiplier tubes which detect radiation emanating from the body. The technology of single photon emission tomography arises from positioning the camera head at multiple angles around the body accumulating as many as 180° of views at specific angular intervals.
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This slide shows an example of SPECT perfusion imaging.
Nuclear myocardial perfusion tomograms using the radioactive compound technetium-99m sestamibi are shown compared to illustrations of the heart from similar views. Note that most of the myocardial wall activity arises from the left ventricular myocardium since it is considerably thicker (11 mm) than the right ventricular free wall (3 mm).
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Tomographic Reconstruction
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Tomographic Planes
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Myocardial Scintigrafi
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ECG Gated Tomography
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Image n
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ECG-Gated Bloodpool Scintigraphy
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Center of rotation
Radiopharmaceutical
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Intrinsic resolution depends on the positioning of the scintillation events (detector thickness, number of PM-tubes, photon energy)
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(Contamination of Collimator)
The image to the right is aquired after cleaning of the collimator
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Difference
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Defect Collimator
The images in the lower row are acquired using a collimator with 50% lower sensitivity in an 1 cm3 area in the center of the field of view. The images in the upper row are from the same patient acquired with a good collimator. It is important to point out the risk of false positive results if the camera is not working perfectly.
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at Different Energies
Intrinsic spatial resolution with Ga-67 point source (count rate < 20k cps); quadrant bar pattern; 3M counts; preset energy window
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at Different Energies
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photon
electron
Scattered
photon
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Window setting
Patient Size
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Pulse Height Distribution
The width of the full energy peak (FWHM) is determined by the energy resolution of the gamma camera. There will be an overlap between the
scattered photon distribution and the full energy peak, meaning that some scattered photons will be registered
Energy
Counts
0
20
40
60
80
100
120
140
20
60
100
120
140
160
Tc-99m
20%
10%
40%
Increased window width will result in an increased number of registered scattered photons and hence a decrease in contrast
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Scatter Correction


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I = I0 e(-µx)
This figure explains the origin of the detected photons. It is purely theoretical. Assume an object 20 x 20 x 20 cm3 filled with Tc-99m. The diagram shows that 12% of the registered photons come from the first cm of the object. Only 4% of the registered photons comes from 10 cm and 1% from 20 cm.
Diagr1
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Blad4
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1.2295585885
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Blad1
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23% 7% 2%
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Uniformity, tomography P P
Energy resolution P P
Sensitivity P T P
Center of rotation P T P
Linearity P P
Resolution P P
P = physicist T = technician
Bar phantom or orthogonal-hole phantom
Subjective evaluation of the image
Calculate absolute (AL) and differential (DL) linearity
AL: Maximum displacement from ideal grid (mm)
DL: Standard deviation of displacements (mm)
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Acquire an image of 10,000,000 counts
With collimator:
Acquire an image of 10,000,000 counts
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Calculate
Integral uniformity (IU)
Differential uniformity (DU)
IU = x 100 where Max is the maximum and Min is the minimum counts in a pixel
DU = x 100 where Hi is the highest and Low is the lowest pixel value in a row of 5 pixels moving over the field of view. Matrix size 64x64 or 128x128
Max-Min
Tomographic Uniformity
Tomographic uniformity is the uniformity of the reconstruction of a slice through a uniform distribution of activity.
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Incorrect Measurement
Two images of a flood source filled with a solution of Tc-99m, which had not been mixed properly
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Bar phantom
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Intrinsic: Collimated line source on the detector
System: Line source at a certain distance
Calculate FWHM of the line spread function
FWHM: 7.9 mm
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in air
SPECT phantom
Expressed as counts/min/MBq and should be measured for each collimator
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Multiple Window
Spatial Registration
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Multiple Window
Spatial Registration
Collimated Ga-67 sources are used at central point, four points on the X-axis and four points on the Y axis
Perform acquisitions for the 93, 184 and 300 keV energy windows
Displacement of count centroids from each peak is computed and maximum is retained as MWSR in mm
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Count Rate Performance
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Count Rate Performance
Use of decaying source or calibrated copper sheets to compute the observed count rate for a 20% count loss and the maximum count rate without scatter
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Pixel Size
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Make a tomographic acquisition
In y-direction a straight line.
Calculate the offset from a fitted cosine and linear function at each angle.
Cosine
function
Center Of Rotation
These are transversal slices of a myocardial tomography. They show the possible effects of an offset in center of rotation. The matrix size is 64 x 64 and the pixel size is 5 mm. Equivalent to an offset in center of rotation is patient movement during acquisition.
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Compare result with reference image.
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base & fog, sensitivity, contrast
increase the sensitivity and
specificity of an examination.
clinically tested methods
well documented algorithms
Positron Emission Tomography
What is a PET scan? A Positron Emission Tomography (PET) scan is a procedure used to observe the brain, the heart, and tumors/cancers. For much of the past 20 years, PET scans have been a superb research tool for probing brain function and cardiac metabolism/blood flow.
However, in the past five years, the clinical use of PET imaging has become apparent, especially in cancer imaging. The major reason PET scanning is assuming increased importance is because it can trace metabolic changes present in cancer cells which are different from normal tissues.
In brief, cancers often have increased rates of blood flow, amino acid transport, protein synthesis, DNA synthesis, and glucose transport and use as compared with normal tissues. These qualitative metabolic changes can be detected by PET scanning with considerable efficiency. Therefore, the PET scan imaging method is assuming a growing role in the clinical practice of cancer imaging.
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PET
When is a PET scan used? Presently, doctors have been ordering PET scans in a growing number of clinical cases in oncology, cardiology, and neurology. A cancer doctor (oncologist) may order a PET scan to find: 1) abnormal tissue or tumors, 2) to determine appropriate treatment for a tumor, 3) to monitor response of cancers to therapy – is the cancer still alive or dead, and 4) to detect if a cancer has returned.
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Radionuclide
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yes!
Coincidence?
PET Detectors
A large number of scintillation crystals are coupled to a smaller number of PM-tubes. In the block detector, a matrix of cuts are made to define the detector elements. The light produced in each crystal will produce a unique combination of signals, which will allow the detector to be identified.
Flood response for a block detector
M Dahlbom, UCLA
II.3.10 – slide * of 126
Types of Coincidence Events
“True” events result from coincidence between 2 photons from the same annihilation. Such events provide valid data.
“Random” and “Scatter” events represent invalid data. These events are recorded by the system as misplaced “trues”, resulting in background noise
that reduces image contrast and resolution.
Siemens
Factors Affecting
Image Formation
Detector efficiency
(the probability that the detector registers an event when a gamma ray path intersects the detector. Depends on detector size and material)
System sensitivity
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Count-rate capability
(the ability of the scanner to record events at high count rates. Depends on detector material and the properties of the electronic components)
Spatial resolution
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Scanner cross calibration
Removable septa positioning
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Superior Image Quality is the result of superior Count Quality
Siemens
trues
randoms
trues
randoms
counts but a positive change in the
ratio of trues to randoms & scatter
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(e.g. brain, heart, bladder)
Specially designed lead strips
optimized for NaI coincidence
lesion detectability
Randoms
(rejected)
Trues
(counted)
Scatter
(rejected)
Siemens
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The probability of photon interaction increases with the crystal thickness
The spatial resolution decreases with the thickness of the crystal
Can this be optimized?
Radiation imaging detectors and their principles of operation were discussed
Students learned about film radiographic imaging, rectilinear scanners, total performance phantoms, gamma cameras, cardiothoracic imaging, bone scans, computerized tomography, SPECT nuclear imaging, ECG-gated tomography, QA/QC issues, and positron emission tomography (PET)
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II.3.10 – slide * of 126
Knoll, G.T., Radiation Detection and Measurement, 3rd Edition, Wiley, New York (2000)
Attix, F.H., Introduction to Radiological Physics and Radiation Dosimetry, Wiley, New York (1986)
International Atomic Energy Agency, Determination of Absorbed Dose in Photon and Electron Beams, 2nd Edition, Technical Reports Series No. 277, IAEA, Vienna (1997)
Where to Get More Information
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International Commission on Radiation Units and Measurements, Fundamental Quantities and Units for Ionizing Radiation, Report No. 60, ICRU, Bethesda (1998)
Where to Get More Information
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II.3.10 – slide * of 126
Hine, G. J. and Brownell, G. L., (Ed. ), Radiation Dosimetry, Academic Press (New York, 1956)
Bevelacqua, Joseph J., Contemporary Health Physics, John Wiley & Sons, Inc. (New York, 1995)
International Commission on Radiological Protection, Data for Protection Against Ionizing Radiation from External Sources: Supplement to ICRP Publication 15. A Report of ICRP Committee 3, ICRP Publication 21, Pergamon Press (Oxford, 1973)
Where to Get More Information
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Where to Get More Information
Cember, H., Introduction to Health Physics, 3rd Edition, McGraw-Hill, New York (2000)
Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8th Edition, 1999 update), Wiley, New York (1999)
International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)
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