Module DModule D

Computed Tomography Computed Tomography Physics, Instrumentation, Physics, Instrumentation,

and Imagingand Imaging

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CT Imaging SystemsCT Imaging Systems

High Voltage GeneratorHigh Voltage Generator

3 Phase system - more efficient production 3 Phase system - more efficient production of x-rays are possibleof x-rays are possible

The high voltage cables were eliminated The high voltage cables were eliminated with helical technology.with helical technology.

CT Imaging SystemsCT Imaging Systems

Generators today are high-frequency and tri-Generators today are high-frequency and tri-phasicphasic

In contrast to earlier generators today's’ are:In contrast to earlier generators today's’ are: Small and compactSmall and compact More efficientMore efficient Take low voltage 5 – 50 HzTake low voltage 5 – 50 Hz Flow frequency current is converted into high Flow frequency current is converted into high

voltage, high frequency of between 500 – voltage, high frequency of between 500 – 25,000 Hz.25,000 Hz.

Power ratings for CT generators range Power ratings for CT generators range between 30-60 kilowatts (KW) between 30-60 kilowatts (KW)

Power ratings for CT generators range between Power ratings for CT generators range between 30-60 kilowatts (KW) allowing for a wide range 30-60 kilowatts (KW) allowing for a wide range

of exposure techniquesof exposure techniques

kVp selectionskVp selections8080

100100

120120

130130

140140

Milliamp selectionsMilliamp selections3030

5050

6565

100100

125125

150150

175175

200200

CT Imaging SystemsCT Imaging Systems

Generators are mountedGenerators are mounted On the orbital scan frame with the tubeOn the orbital scan frame with the tube In the corner of the gantry (stationary)In the corner of the gantry (stationary)

Slip ring technology make this possible.Slip ring technology make this possible.

Slip Ring TechnologySlip Ring Technology

Electromechanical devicesElectromechanical devices Transmit energy across a rotating surfaceTransmit energy across a rotating surface Rings and brushesRings and brushes Made spiral or volume scanning possibleMade spiral or volume scanning possible Two slip ring designsTwo slip ring designs

DisksDisks CylindersCylinders PAGES 81 and 82 in SeeramPAGES 81 and 82 in Seeram

CT Imaging SystemsCT Imaging Systems

Wire brushes are used to transmit Wire brushes are used to transmit power to the CT componentspower to the CT components– Brushes glide in contact grooves along the Brushes glide in contact grooves along the

slip ringsslip rings– Two types of brushesTwo types of brushes

– CompositeComposite– WireWire

Low voltage slip ring scannersLow voltage slip ring scanners

480 AC current480 AC current Slip-ring provides power to high-Slip-ring provides power to high-

voltage transformer then to voltage transformer then to radiographic tuberadiographic tube

X-ray generator and tube are X-ray generator and tube are positioned on the orbital scan framepositioned on the orbital scan frame

High-Voltage Slip-Ring ScannersHigh-Voltage Slip-Ring Scanners

AC delivers power to the high-voltage AC delivers power to the high-voltage generatorgenerator

High-voltage generator then supplies high-High-voltage generator then supplies high-voltage to the slip-ringsvoltage to the slip-rings

High-voltage from the slip ring is transferred High-voltage from the slip ring is transferred to the x-ray tubeto the x-ray tube

High-voltage generator does not rotate with High-voltage generator does not rotate with the x-ray tubethe x-ray tube

CT Imaging SystemsCT Imaging Systems

Main difference is the way in which the images are Main difference is the way in which the images are gathered and how the reconstruction algorithms gathered and how the reconstruction algorithms are used.are used.

Conventional CTConventional CT– One row of detectorsOne row of detectors– SDCTSDCT

Spiral CTSpiral CT– 164 rows of detectors164 rows of detectors– MDCTMDCT

Slice by Slice ScanningSlice by Slice Scanning

Also called “step and shoot”Also called “step and shoot” Tube rotates around the gantryTube rotates around the gantry Attenuated radiation from the patient is Attenuated radiation from the patient is

captured by detectorscaptured by detectors Tube stops and the couch movesTube stops and the couch moves Process repeated until the entire exam is Process repeated until the entire exam is

completedcompleted

Z-AxisZ-Axis

CT is often referred to as Axial CTCT is often referred to as Axial CT This is incorrectThis is incorrect

CT is an axial scanning TECHNIQUE onlyCT is an axial scanning TECHNIQUE only CT images are acquired along the Z-axis of CT images are acquired along the Z-axis of

the patient’s bodythe patient’s body The Z-axis is along the transverse or axial The Z-axis is along the transverse or axial

planeplane

Volume CTVolume CT

Tube rotates continuously while making an Tube rotates continuously while making an exposure as the patient moves through the exposure as the patient moves through the gantrygantry

The continuous rotation of the x-ray tube The continuous rotation of the x-ray tube and the couch top movement equal the and the couch top movement equal the spiral geometry acquisitionspiral geometry acquisition

THUS- the name - THUS- the name - Spiral CTSpiral CT

Original spiral CT scannersOriginal spiral CT scanners

Had only 1 row of detectors Had only 1 row of detectors

(SDCT) single detector computer (SDCT) single detector computer

tomographytomography

Used different algorithms than what are Used different algorithms than what are used today with 164 rows of detectorsused today with 164 rows of detectors

CT computer systemsCT computer systems

Processes information for the DASProcesses information for the DAS windowingwindowing Image enhancementImage enhancement Image magnificationImage magnification ROIROI Quantitative measurementsQuantitative measurements MPR or multiplanar reconstructionMPR or multiplanar reconstruction 3-D imaging3-D imaging MIP maximum intensity projections (vascular imaging)MIP maximum intensity projections (vascular imaging) Volume renderingVolume rendering Surface renderingSurface rendering Tasks surrounding image manipulationTasks surrounding image manipulation

CT Computer SystemCT Computer System

MinicomputerMinicomputer

Capable of performing complex computations Capable of performing complex computations while receiving high-level input and out putwhile receiving high-level input and out put

Used in MRI alsoUsed in MRI also Computer processing architecture Computer processing architecture

PipelinePipeline ParallelParallel distributeddistributed

Array ProcessorArray Processor

?most important part??most important part? Dedicated electronic circuitry Dedicated electronic circuitry Rapid calculationsRapid calculations Receives information from the detectorsReceives information from the detectors Measurements from hundreds of projections Measurements from hundreds of projections

are used to piece information back togetherare used to piece information back together

Array ProcessorArray Processor

Microprocessors assistMicroprocessors assist Speeds of 1 nanosecondSpeeds of 1 nanosecond Number of array processors determines the Number of array processors determines the

speed of reconstructionspeed of reconstruction RetrospectiveRetrospective Post-processingPost-processing

Scanning ProcessScanning Process

Voltage causes electrons to “boil off”Voltage causes electrons to “boil off” Electrons are accelerated striking the cathodeElectrons are accelerated striking the cathode X-ray productionX-ray production Beam travels through tube and the “bow tie” filterBeam travels through tube and the “bow tie” filter

Filter shapes and Filter shapes and Defines the beamDefines the beam

After filtration comes collimationAfter filtration comes collimation (Filtration occurs twice)- prior to entering the (Filtration occurs twice)- prior to entering the

patient and again after it is attenuated by the patient and again after it is attenuated by the patient.patient.

AttenuationAttenuation

Depends on:Depends on:1. electrons per gram1. electrons per gram2. affective atomic density of the absorber2. affective atomic density of the absorber3. atomic number of the absorber3. atomic number of the absorber4. energy of the transmitted photons4. energy of the transmitted photons

What does the “Z” number of the absorber What does the “Z” number of the absorber mean?mean?

Lambert-Beer LawLambert-Beer Law

-is “an exponential relationship that explains -is “an exponential relationship that explains what happens to x-ray photons as they what happens to x-ray photons as they travel through body tissue”.travel through body tissue”.

Lambert-Beer Law incorporates the Lambert-Beer Law incorporates the principles of:principles of:– Photoelectric effectPhotoelectric effect– Compton ScatteringCompton Scattering

Lambert-Beer LawLambert-Beer Law

IIinin = = IIoutout ee -μx-μx

I out = the transmitted intensity I out = the transmitted intensity I in = the original intensity I in = the original intensity e = Euler’s Constant (2.718) (Base of the Natural Logarithm) e = Euler’s Constant (2.718) (Base of the Natural Logarithm) Φ = Linear Attenuation Coefficient Φ = Linear Attenuation Coefficient x = thickness of the objectx = thickness of the object

Continue…..Continue….. Lambert-Beer Law equation Lambert-Beer Law equation

Values are known for the transmitted and Values are known for the transmitted and original beam intensities and for the original beam intensities and for the thickness of the object. thickness of the object.

Because these values are known, the linear Because these values are known, the linear attenuation can be derived.attenuation can be derived.

Linear attenuation coefficientLinear attenuation coefficient

- Linear attenuation coefficient represents the - Linear attenuation coefficient represents the

raterate that x-rays are attenuated that x-rays are attenuated (diminished) as they travel through the body.(diminished) as they travel through the body.

Image DisplayImage Display

CRT (Cathode Ray Tube)CRT (Cathode Ray Tube) Display matrixDisplay matrix Pixel size determinantPixel size determinant Bit depthBit depth CT number value scaleCT number value scale Window width and level Window width and level Primarily a factor related to the DFOV (display field of view) Primarily a factor related to the DFOV (display field of view)

(DFOV has the GREATEST IMPACT on image (DFOV has the GREATEST IMPACT on image resolution and image noise)resolution and image noise)

ProtocolProtocol Technologist controls or Selectable scan factorsTechnologist controls or Selectable scan factors

Monitor MatrixMonitor Matrix

Resolution is based on the size of the matrix Resolution is based on the size of the matrix – 512 x 512512 x 512– 1024 x 10241024 x 1024– 2048 x 20482048 x 2048

Newer scanners allow for the selection of these Newer scanners allow for the selection of these larger maxtrices during scanning.larger maxtrices during scanning.