Lecture 12Lecture 12

Last Week Summary Last Week Summary Sources of Image DegradationSources of Image Degradation

Quality ControlQuality ControlThe SinogramThe Sinogram

Introduction to Image ProcessingIntroduction to Image Processing

PET TomographPET Tomograph

DetectorDetector

Photomultiplier tubesPhotomultiplier tubes

• Photomultiplier tubes (or PMTs) are Photomultiplier tubes (or PMTs) are photodetectors which provide extremely high photodetectors which provide extremely high sensitivity and ultra-fast response. A typical sensitivity and ultra-fast response. A typical PMT consists of a photoemissive cathode PMT consists of a photoemissive cathode (photocathode) followed by focusing (photocathode) followed by focusing electrodes, an electron multiplier and an electrodes, an electron multiplier and an electron collector (anode) in a vacuum tube.electron collector (anode) in a vacuum tube.

PMT DiagramPMT Diagram

How the PMT worksHow the PMT works

• When light enters the tube and strikes the photocathode, the When light enters the tube and strikes the photocathode, the photocathode emits photoelectrons into the vacuum. These photocathode emits photoelectrons into the vacuum. These photoelectrons are then directed by the focusing electrode photoelectrons are then directed by the focusing electrode voltages towards the electron multiplier where electrons are voltages towards the electron multiplier where electrons are multiplied by the process of secondary emission. The multiplied multiplied by the process of secondary emission. The multiplied electrons are collected by the anode as an output signal. Because electrons are collected by the anode as an output signal. Because of secondary-emission multiplication, photomultiplier tubes of secondary-emission multiplication, photomultiplier tubes provide extremely high sensitivity and exceptionally low noise provide extremely high sensitivity and exceptionally low noise among the photosensitive devices currently used to detect among the photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared radiant energy in the ultraviolet, visible, and near infrared regions. A photon striking the photocathode would usually yield regions. A photon striking the photocathode would usually yield the emission of a single electron but the multiplier can create a the emission of a single electron but the multiplier can create a final output of one million electrons for each electron emitted. final output of one million electrons for each electron emitted. This is the gain of the PMT and it is enormous!This is the gain of the PMT and it is enormous!

PMT ContinuedPMT Continued

• The electron multiplier section consists of nine (or The electron multiplier section consists of nine (or more, some PMTs use up to 19) electrodes called more, some PMTs use up to 19) electrodes called dynodes. Each dynode is charged with about 100 volts dynodes. Each dynode is charged with about 100 volts more positive charge than the previous dynode in the more positive charge than the previous dynode in the chain. As electrons are emitted from a previous dynode chain. As electrons are emitted from a previous dynode they are focused to the next dynode by means of this they are focused to the next dynode by means of this increasing positive voltage. The electrons strike that increasing positive voltage. The electrons strike that dynode and are multiplied and the cascade of emitted dynode and are multiplied and the cascade of emitted electrons continues to grow at each dynode. Finally the electrons continues to grow at each dynode. Finally the stream of electrons, which began perhaps as a single stream of electrons, which began perhaps as a single electron, is collected by an anode where it appears as electron, is collected by an anode where it appears as an electrical current. an electrical current.

PMT ContinuedPMT Continued

• The entire PMT is powered by a source of about 1000 The entire PMT is powered by a source of about 1000 Volts. The photocathode being the most negative Volts. The photocathode being the most negative electrode, each dynode is succession is more positive electrode, each dynode is succession is more positive than the last. The potential difference required is than the last. The potential difference required is easily derived using a resistive voltage divider easily derived using a resistive voltage divider consisting of a chain of one Megaohm resistors in consisting of a chain of one Megaohm resistors in series. The final dynode has a potential of 1000 volts series. The final dynode has a potential of 1000 volts positive relative to the photocathode. Finally the positive relative to the photocathode. Finally the electron stream is collected by the anode.electron stream is collected by the anode.

  

Detector MaterialsDetector Materials

Ring and Block DiagramRing and Block Diagram

Detector BlocksDetector Blocks

Range in mm before annihilationRange in mm before annihilation• What is the mean distance traveled before annihilation of the following radionuclidesWhat is the mean distance traveled before annihilation of the following radionuclides•   •   • Rb-82Rb-82 _______________2.6mm________________________________2.6mm_________________•   •   • N-13 N-13 _______________1.4 mm_______________________________1.4 mm________________•   •   • O-15O-15 _______________1.5mm________________________________1.5mm_________________•   •   • C-11C-11 _______________0.3mm________________________________0.3mm_________________•   •   • F-18F-18 _______________0.2mm_________________ _______________0.2mm_________________ •   •   •   

Image formation In PETImage formation In PET

PET Data AcquisitionPET Data Acquisition

• If 2 photons are simultaneously detected by 2 small If 2 photons are simultaneously detected by 2 small detectors, we can infer that the annihilation must have detectors, we can infer that the annihilation must have occurred along the line connecting the 2 detectors. occurred along the line connecting the 2 detectors. This line is referred to as the "line of response," or This line is referred to as the "line of response," or "LOR." To increase the sensitivity of the scanner, the "LOR." To increase the sensitivity of the scanner, the object is surrounded by a "ring" of small detectors object is surrounded by a "ring" of small detectors rather than only 2. Such a ring is shown in rather than only 2. Such a ring is shown in Figure 1AFigure 1A. . To image multiple planes simultaneously, several To image multiple planes simultaneously, several such rings are placed back-to-back.such rings are placed back-to-back.

PET Data AcquisitionPET Data Acquisition

• It is therefore convenient to organize the It is therefore convenient to organize the detectors into 2D arrays, called detector detectors into 2D arrays, called detector "blocks," where the detectors along the "blocks," where the detectors along the xx-axis -axis go around the ring and those in the go around the ring and those in the yy-direction -direction go axially into the ring. An array of go axially into the ring. An array of photomultiplier tubes are placed behind the photomultiplier tubes are placed behind the detector block to collect the scintillation light detector block to collect the scintillation light and determine within which detector the event and determine within which detector the event occurred occurred

Annihalation EventAnnihalation Event

Lines of ResponseLines of Response

Parallel LORsParallel LORs

Quality ControlQuality Control(Performance Measurements)(Performance Measurements)

We need to define some basic principles:We need to define some basic principles:• Spatial resolution : The ability to differentiate two points in space.Spatial resolution : The ability to differentiate two points in space.• Temporal resolution : The ability to differentiate information in time.Temporal resolution : The ability to differentiate information in time.• Sensitivity ;Sensitivity ; Sensitivity is defined as the ability of a detection system to Sensitivity is defined as the ability of a detection system to

register counts per unit time per dose administered : cts/min/mCi. It can also register counts per unit time per dose administered : cts/min/mCi. It can also be measured in counts per second/ unit dose.be measured in counts per second/ unit dose.

• Coincidence window : The time during which one coincidence event can be Coincidence window : The time during which one coincidence event can be counted.counted.

• Scatter : Interaction of radiation with matter, which results in a change of Scatter : Interaction of radiation with matter, which results in a change of direction of the photon.direction of the photon.

• Randoms : Two unrelated coincidence events that fall within the coincidence Randoms : Two unrelated coincidence events that fall within the coincidence windowwindow

Image Degredation In PETImage Degredation In PETSingles EventsSingles Events

Image Degradation In PET RandomsImage Degradation In PET Randoms

Statistical AspectsStatistical Aspects

• Not only does the sampling pose a problem, Not only does the sampling pose a problem, but the statistical density, or lack thereof but the statistical density, or lack thereof (especially in nuclear medicine) poses a (especially in nuclear medicine) poses a “noise problem”.“noise problem”.

• Noise can be thought of as the small, Noise can be thought of as the small, random fluctuations that appear across the random fluctuations that appear across the image. image.