Module CModule C

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

and Imagingand Imaging

DisclaimerDisclaimer

This workforce solution was funded by a grant awarded This workforce solution was funded by a grant awarded under the President’s Community-Based Job Training Grants under the President’s Community-Based Job Training Grants as implemented by the U.S. Department of Labor’s as implemented by the U.S. Department of Labor’s Employment and Training Administration.  The solution was Employment and Training Administration.  The solution was created by the grantee and does not necessarily reflect the created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor.  The official position of the U.S. Department of Labor.  The Department of Labor makes no guarantees, warranties, or Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to assurances of any kind, express or implied, with respect to such information, including any information on linked sites such information, including any information on linked sites and including, but not limited to, accuracy of the and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership.  This adequacy, continued availability, or ownership.  This solution is copyrighted by the institution that created it.  solution is copyrighted by the institution that created it.  Internal use by an organization and/or personal use by an Internal use by an organization and/or personal use by an individual for non-commercial purposes is permissible.  All individual for non-commercial purposes is permissible.  All other uses require the prior authorization of the copyright other uses require the prior authorization of the copyright owner.owner.

Data AcquisitionData Acquisition

Three step process:Three step process:

1. Data Acquisition1. Data Acquisition

2. Image Reconstruction2. Image Reconstruction

3. Image Display3. Image Display

CT componentsCT components

GantryGantry

Gantry Apertures range from Gantry Apertures range from 50 – 85 cm50 – 85 cm

Current table limit Current table limit 450 lbs.450 lbs.

Diagnostic Ct tables are curvedDiagnostic Ct tables are curved Radiation Therapy tables are flatRadiation Therapy tables are flat

CT componentsCT components

Tilt 30 degrees cephalad or caudalTilt 30 degrees cephalad or caudal

Tilt capabilities absent using Positron Tilt capabilities absent using Positron Emission Tomography (PET)Emission Tomography (PET)

Table and scanning controls Table and scanning controls GantryGantry tabletable

CT X-Ray TubeCT X-Ray Tube

More than one filamentMore than one filament tungstentungsten

Focusing cupFocusing cup

AnodeAnode Stationary Stationary rotatingrotating

AnodeAnode

Rotating most common todayRotating most common today Shorter exposure timesShorter exposure times Small focal spots for greater detailSmall focal spots for greater detail High heat load capacityHigh heat load capacity Produces heterogeneous beam from Produces heterogeneous beam from

large rotating anodelarge rotating anode Large anode disc and small focal spot Large anode disc and small focal spot

facilitate spatial resolution facilitate spatial resolution requirementsrequirements

AnodeAnode

Made of rheniumMade of rhenium TungstenTungsten Molybdenum alloy (RTM)Molybdenum alloy (RTM)

Additional types of discs-Additional types of discs- All metal diskAll metal disk Brazed graphite diskBrazed graphite disk Chemical vapor diskChemical vapor disk

AnodeAnode

Target angle is generally 12 degreesTarget angle is generally 12 degrees

Rotational speed is eitherRotational speed is either 3,600 rpm3,600 rpm 10,000 rpm10,000 rpm

Heat storage capacityHeat storage capacity 2-5 million heat units (HU)2-5 million heat units (HU)

AnodeAnode

Heat dissipation rates are in the area Heat dissipation rates are in the area of 400,000 HU/m.of 400,000 HU/m. Achieved usingAchieved using

Oil cooledOil cooled Liquid cooledLiquid cooled Fan cooledFan cooled

X-ray BeamX-ray Beam

PolychromaticPolychromatic

CT has to have MonochromaticCT has to have Monochromatic

Bowtie Filter Bowtie Filter Remove the long wavelength x-rays from Remove the long wavelength x-rays from

the beamthe beam Shape the beamShape the beam

CollimationCollimation

Two sets of collimatorsTwo sets of collimators

Patient radiation dosePatient radiation dose

Quality of the CT imagesQuality of the CT images

Together called PRE-detectorTogether called PRE-detector Separately called PRE and POST patientSeparately called PRE and POST patient

CT DetectorsCT Detectors

Positioned within the gantryPositioned within the gantry

Generation determines the type Generation determines the type RotatingRotating

oror Stationary Stationary

Detector CharacteristicsDetector Characteristics

1.1. EfficiencyEfficiency

2.2. Response timeResponse time

3.3. Dynamic RangeDynamic Range

4.4. High ReproducibilityHigh Reproducibility

5.5. StabilityStability

Types of DetectorsTypes of Detectors

Gas IonizationGas Ionization

Solid-StateSolid-State

Xenon DetectorsXenon Detectors

Inert gasInert gas Fast rate of decayFast rate of decay Used in third generation scannersUsed in third generation scanners

Fourth generation scanners have non-Fourth generation scanners have non-rotating detectors. Fourth generation rotating detectors. Fourth generation detectors rock back and forth.detectors rock back and forth.

Continue…….Continue……. Detectors Detectors

Solid State DetectorsSolid State Detectors Also called Scintillation detectorsAlso called Scintillation detectors

Made of:Made of: Bismuth germinateBismuth germinate Cesium iodideCesium iodide Cadmium tungstateCadmium tungstate

SamplingSampling

Governed by Nyquist TheoremGoverned by Nyquist Theorem

Increasing sampling methodsIncreasing sampling methods Adjustments in slice thicknessAdjustments in slice thickness Use of closely packed detectorsUse of closely packed detectors Quarter-shifted detectorsQuarter-shifted detectors

Array ProcessorArray Processor

Primary function – reconstruct the projected Primary function – reconstruct the projected raw data raw data

Solves all mathematically complex problemsSolves all mathematically complex problems

Used in retrospective reconstruction and post Used in retrospective reconstruction and post processingprocessing

Number and type used affects the Number and type used affects the reconstruction time of the scannerreconstruction time of the scanner

DASDAS

Data Acquisition systemData Acquisition system A set of electronics that are located between A set of electronics that are located between

the detector array and the CT host computerthe detector array and the CT host computer DAS functions:DAS functions:

Measures the transmitted beam Measures the transmitted beam Logarithmic conversionsLogarithmic conversions Changes the measurements into digital formChanges the measurements into digital form Sends the digital signals to the array processor for Sends the digital signals to the array processor for

processingprocessing