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184 Cards in this Set

  • Front
  • Back

Data acquisition refers to the method by which the

patient is scanned to obtain enough datafor image reconstruction

Scanning is defined by the

beam geometry - which characterizes the particular CT system and also plays a central role in spatial resolution and artifact production

Basic scheme - two elements in a basic scheme for data acquisition

1. beam geometry

2. components - comprising the scheme

Beam geometry

Refers to the size, shape, and motion ofthe beam and its path


Refers to those physical devices thatshape and define the beam, measure its transmission through the patient, andconvert the information into digital data for input into the computer


The part of the beam that falls on onedetector


A collection of rays from one translationacross the object


An electrical signal that represents asignature of the attenuation generated by the detector

Data sample

A collection of views transmitted formeasurement

Scanning also includes the movement of the patient

through the gantry to scan the next slice

The x-ray beam that emanates from the tube consists of

several rays

Projection data are collected by the

detector because each ray is attenuated by the patient and subsequently transmitted and projected on the detector

Three primary types of acquisition geometries

1. parallel beam geometry

2. fan beam geometry

3. spiral/helical geometry - most recently developed

A simple categorization of CT equipment evolved based on

1. scanning geometry

2. scanning motion

3. # of detectors

^ this sort of categorization is known as generations

Scanner generations


First generation scanners

1. Used parallel beam geometry

2. Defined by a set of parallel rays thatgenerates a projection profile

3. Translate-rotate principle

4. Rectilinearpencil beam scanning

5. Tube and detector rotate 1 degree betweentranslations

6. 180-degree rotation around the patient

7. 4.5 to 5.5 minutes per scan

Second generation scanners

1. Fan beam geometry

2. Differed from first-generation scannersby the addition of the linear detector array and multiple pencil beams

3. Resulted in fan beam effect

4. Rectilinearmultiple pencil beam scanning

5. Larger increments between translationwhen compared to first generation

6. 180-degree rotation

7. 20 seconds to 3.5 minutes per scan

Second generation scanners- xray tube traces a

semicircular path during scanning

Second generation scanners - the time decrease is

inversely proportional to the # of detectors

Second generation scanners - the more detectors, the

shorter is the total scan time

Third generation scanners

1.Fan beam geometry

2. 360-degree rotation

3. Curved detector array

4. Continuouslyrotating fan beam scanning

5.Scan time = seconds

Curved detector array

xray tube coupled to a curved detector array that subtends an arc of 30-40 degree or greater from the apex of the fan

Continuously rotating fan beam scanning

xray tube and detectors rotate, projecting profiles are collected and a view is obtained for every fixed point of the tube and detector

Third generation- path traced by tube

circle rather than semicirccle

Third generation scan time

increases patient throughput and limits the production of artifacts caused by respiratory motion

Fourth generation scanners

Two types of beam geometries

1. Rotating fan beam within a circulardetector array

2.Rotating fan beam outside a nutatingdetector ring

Rotating fan beam within a circular detector array

stationary ring of detectors

Rotating fan beam outside a nutating detector ring

in which the apex of the fan (xray tube) is located outside a nutating ring of detectors

RotatingFan Beam Within a Circular Detector Array

1.Wide fan beam geometry

2. X-ray tube rotating in a circular pathwithin a stationary, circular detector array

3. Very short scan times

4. Image reconstruction algorithm is at thedetector opposed to the x-ray tube

RotatingFan Beam Within a Circular Detector Array- xray tube traces

circular path

RotatingFan Beam Within a Circular Detector Array- as the tube moves from the point to the point within the circle, single

rays strike a detector

RotatingFan Beam Outside a NutatingDetector Ring

1. Fan beam geometry

2. X-ray tube rotates outside the detectorring

3. Detector ring tilts (nutates)during data collection so that detectors closest to the x-ray tube move out ofthe x-ray beam path

4. Currently not manufactured

Rotating Fan Beam Outside a Nutating Detector Ring - the term nutating describes the

tilting action of the detector ring during data collection

Rotating Fan Beam Outside a Nutating Detector Ring - scanners with this type of scanning motion

eliminate the poor geometry of other schemes in which the tube rotates inside its detector ring near the object


1. Spiral-helical geometry

2. Continuous rotation (volume) scanners

3. Data acquisition collected in volumesrather than individual slices

4. Based on slip-ring technology

5. X-ray tube (fan beam) described as aspiral or helix

6. Patient is transported through the gantryaperture for a single breath hold

Spiral/Helical geometry scanners

1. Cone-beamgeometry

2. Produces multiple slices per revolutionof the x-ray tube and detectors

3. Requires cone-bean algorithms toreconstruct images

4. Utilized in scanners capable of imaging16 or greater slices per 360-degree rotation

5. covers entire heart in a single rotation

6. detectors are 2D

Fifth generation scanners - Classified as high-speed CT scanners

Scan time is in the milliseconds

Fifth generation scanners

Two types of scanners in this generation:

1. Electron beam CT scanner (EBCT)

2. Dynamic spatial reconstructor(DSR)Scanner is now obsolete

EBCT scanner - developed for

high-speed CT scanning of heart andcirculation

EBCT scanner- overall goal is to

produce high-resolution images of movingorgans that are free of artifacts caused by motion

EBCT distributed by

Siemens Medical Systems under the nameEvolution

EBCT scanners - its design enables it to

acquire CT data 10x faster than conventional CT scanners

EBCTversus Conventional Scanner -Design configuration differences

1. EBCT is based on electron beam technologyand no x-ray tube is used

2. EBCT has no mechanical motion of thecomponents

3. Acquisition geometry of the EBCT scanneris fundamentally different compared with conventional systems

EBCT acquires CT data

10 times faster than conventional CT scanners

Silicon photodiodes

convert light into current

The output from detectors is sent to the

data acquisition system

data acquisition system

consists of ADCs or digitizers that sample and digitize the output signals from the detectors

Sixth generation scanners

1. Dual source CT scanner (DSCT)

2. Two x-ray tubes and two sets of detectorsoffset by 90 degrees

3. Created to deal with artifacts producedin CT angiography studies

4. Designed for cardiac CT imaging due toimproved temporal resolution

Seventh generation scanners

1. Flat-panel digital detectors

2. Consists of a cesium iodide (CsI)scintillator coupled to an amorphous silicon thin-film transistor (TFT) array

3. Excellent spatial resolution butlow-contrast resolution

4. May be used for angiography or breastimaging, where image sharpness is of primary importance

5. Still in prototype development and not currentlyavailable for clinical imaging use

7th generation scanners -also used in

angiography to image blood vessels

Slip ring technology allows

for continuous gantry rotation throughthe elimination of the long high-tension cables

Slip ring technology is an

“Electromechanical devices consisting of circularelectrical conductive rings and brushes that transmit electrical energy acrossa rotating interface”

Spiral/helical CT made possible through

use of slip ring technology

Slip ring design

1. disk

2. cylinder

3. contactless slip ring


Conductive rings form concentric circlesin the plane of rotation


Conductive rings positioned along theaxis of rotation to form a cylinder

Contactless slip ring

Electrical energy transferred acrossrotating interface without use of electrical contact

Slip ring brush design

1. wire

2. composite


Uses conductive wire as a sliding contact


Uses a block of some conductive materialas a sliding contact

Power supply

1. low voltage slip ring

2. high voltage slip ring

Low-voltage slip ring

1. Alternating current (AC)power -> slip ring -> high-voltage generator -> x-ray tube

2.The x-ray generator, x-ray tube, andother controls are positioned on the orbital scan frame

High-voltage slip ring

AC power -> high-voltage generator -> slip ring -> x-ray tube

High-voltage generator does not rotatewith the x-ray tube

Slip ring advantages

1. Continuous rotation of the x-ray tube

2. Faster scan times and minimal interscandelays

3. Capacity for continuous acquisitionprotocols

4. Elimination of the start-stop processfound in conventional scanners

5. Removal of cable wraparound process

In initial experiments, Hounsfield used

low energy monochromatic gamma ray radiation

Xray system - components include

1. x-ray generator

2. xray tube

3. xray beam filter

4. collimators

X-ray generator - Ct scanners use

high frequency generators

X-ray generator located

within the gantry

X-ray generator has

High-frequencyinverter circuit

X-ray generator

Low-voltage, low-frequency AC current ischanged to high-voltage, high-frequency direct current (DC) current for use bythe x-ray tube

X-ray generator- power ratings range from

20 to 120 kW

X-ray tubes

1. Fixed-anode, oil-cooled x-ray tubes

2. Used in first- and second-generationscanners

3. Rotating anode x-ray tubes

Rotating anode x-ray tubes - produced

heterogeneous beam of radiation from alarger-diameter anode disk with focal spot sizes to facilitate the spatialresolution requirements of the scanner

Rotating anode x-ray tubes - made of

rhenium,tungsten, and molybdenum (RTM) alloy

Rotating anode x-ray tubes - tubes for spiral imaging may include

graphite base body for high thermalcapacity

Rotating anode x-ray tubes - angle?

Small target angle (12 degrees)

Rotating anode x-ray tubes - rotation speed? working life?

1. Rotation speed of 3,600 to 10,000revolutions per minute

2. Working life of 10,000 to 40,000 hours

X-RayTubes in Spiral/Helical CT Scanners:

Helical/spiral scanners have additionaltube requirements and concerns

1. Higher power levels

2. Additional heat generation

3. Heat storage

4. Heat dissipation

CTX-Ray Tube Glass Envelope - ensures


CTX-Ray Tube Glass Envelope - provides structural

support of anode and cathode structures

CTX-Ray Tube Glass Envelope - provides high

voltage insulation between the anode andcathode

CTX-Ray Tube Glass Envelope - consists of

tubes with metal envelopes which

1. Prevents arcing

2. Allows higher tube currents

3. Increases storage capacity

4. Heat dissipation rate improvement

Anode assembly consists of


2. Rotor stud and hub

3. Bearing assembly

Anode assembly - three basic designs

1.All-metal disk

2. Brazed graphite disk

3. Chemical vapor deposition (CVD) graphitedisk

ConventionalAll-Metal Disk - consists of a

base body made of titanium, zirconium,and molybdenum with a focal track layer of 10% rhenium and 90% tungsten

Conventional All-Metal Disk - transfers

heat from the focal track very quickly

Conventional All-Metal Disk - unable to meet the needs of

spiral/helical CT imaging due to weight

BrazedGraphite Disk - consists of a

tungsten-rhenium focal track brazed to agraphite base body

BrazedGraphite Disk - provides

increased heat storage capacity due tohigh thermal capacity

BrazedGraphite Disk - tubes for

spiral/helicalCT scanning are based mostly on this type of design

CVDGraphite Disk - consists of a

graphite base body with atungsten-rhenium layer deposited on the focal track by a chemical vapor process

CVDGraphite Disk - accommodates

large, lightweight disks with large heatstorage capacity and fast cooling rates

CVDGraphite Disk - intended for use in

spiral/helical CT x-ray tubes

Anode assembly - Bearing assembly is to

provide and ensuresmooth rotation of the anode disk

Anode assembly - rotation speeds of

10,000 rpm arepossible

Anode assembly- liquid bearing method

1. Provides lubrication

2. Improves anode disk rotation

3. Improves tube cooling by conducting heataway from x-ray tube more efficiently

4. Free of vibrations and noise


A copper cylinder brazed to an inner steelcylinder with a ceramic coating around the outside

Rotor- the hub and stud

prevent the transmission of heat from thedisk to the bearings

Straton X-ray tube - useful for

multislice CT (MSCT) scanners

Straton X-ray tube - anode is

immersed in oil which:

1. Provides higher cooling rates

2. Tolerates higher mA

3. Allows long exposure times for increasinganatomical coverage

Straton X-ray tube - smaller size

ensures faster gantry rotation

Straton X-ray tube - cathode consists of an

electron beam that is deflected to strikethe anode at two focal spots

AlternativeX-Ray Tube Designs for MSCT - Two directlycooled x-ray tube designs

1. iMRC 800 x-ray tube

2. Vectron x-ray tube

iMRC 800 x-ray tube - facilitates very high

heat loading capacity and rapid heatdissipation

iMRC 800 x-ray tube - features a dynamic

focal spot (DFS) that increases the datasampling and generates artifact-free ultra-high spatial resolution

Vectron x-ray tube - tube based on the

Stratonx-ray tube direct anode cooling technology

Vectron x-ray tube - operates with tube

voltages ranging from 70 to 150 kV in increments of 10 kV

Vectron x-ray tube - electron beam that can

create two focal spots, thus increasingin-plane resolution without increasing patient dose

Filtration -removes

long-wavelength x-rays that do not play arole in CT image formation

Filtration produces a

“harder” beam

Filtration reduces

patient dose

Filtration shapes the

energy distribution across the radiationbeam to produce uniform beam hardening

Bowtie filters - applies to a

class of filter shapes featuringbilateral symmetry with a thickness that increases with the distance from thecenter

Bowtie filters - allows a more

uniform beam to be delivered to thedetector

Collimation - purpose is to

protect the patient by restricting thebeam to the anatomy of interest only

In CT, collimation is

equally important because it affectspatient dose and image quality

Basic CT collimation scheme includes

1. Prepatient collimators

2. Postpatient collimators

3. Predetector collimators

AdaptiveSection Collimation - MSCT collimation scheme issues that result inan

increase of patient dose


Exposure of the patient outside theimaged range, which occurs for spiral CT with multirow detectors at the startand the end of the scan


Relates to x-ray beams being slightlywider than the detector, which means that patients are exposed over a smallarea without the signal being detected

Adaptive section collimation

correctsthese problems by blocking the x-ray beam from exposing tissue outside theimaged volume in the z-direction by dynamically adjusted collimators at thebeginning and at the end of the CT scan

CTDetector Technology - CT detector functions


Capture the

radiation beam from the patient

Convert radiation

beam into an electrical signal

Convert electrical signal

into binary coded information

Detector characteristics - detectors exhibit several characteristics essential for CT

image production affecting good image quality

Capture efficiency

the effectiveness with which detectors can obtain photons transmitted from the patient

Absorption efficiency

the # of photons absorbed by the detector, dependent on the atomic #, physical density, size, and thickness of the detector face


the steadiness of the detector response

Response times

the speed with which the detector can detect an x-ray event and recover to detect another event

Dynamic range

the ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated by a CT detector


the persistence of the image even after the radiation has been turned off

Types of detectors - The conversion of x-rays to electricalenergy in a detector is based on two fundamental principles

1. Scintillationdetectors

2. Gas ionization detectors

ScintillationDetectors - convert x-ray energy into

light,after which the light is converted into electrical energy by a photodetector

ScintillationDetectors - solid-state detectors that

consistof a scintillation crystal coupled to a photodiode tube

ScintillationDetectors - Excellent conversion

efficiency and photon capture efficiency

ScintillationDetectors - exhibit


Gas Ionization Detectors - consists of a series of

individual gas chambers, usuallyseparated by tungsten plates carefully positioned to act as electron collectionplates

Gas Ionization Detectors - x-ray falls on

individual chamber, and ionization of thegas results in a positive and negative ion

Gas Ionization Detectors - promote excellent

stability, fast response times, andexhibit no afterglow problems

Gas Ionization Detectors - poor efficiency

and unable to be used in MSCT

As CT strives in its efforts to improveclinical image quality and reduce patient dose, manufacturers have introduced

detectors that are more efficient in these respects

Recent detector improvementclassifications

1. Conventional energy integration (EI) detector

2. Dual-layer detector

3. Direct conversion detector

The Stellar Detector - falls in the

category of the firstthird-generation CT detectors

The Stellar Detector - its innovation is that

the electronics are totally integrated inthe photodiode

The Stellar Detector - can measure

small signals over a wide dynamic range,which reduces and enhances CT image quality

The NanoPanel Prism Detector

A dual-layerdetector

The NanoPanel Prism Detector - used in

spectralCT imaging

The NanoPanel Prism Detector - the configuration allows for

acquisition of both low and high energiesfor every exposure used in CT imaging

The NanoPanel Prism Detector - ensures optimum

performancein all characteristics that influence CT image quality

Detector-BasedSpectral CT - Involves exploiting the transmitted x-rayphotons

through the patient

Two approaches

1. Energyweighting

2. Materialdecomposition

Two dual-energymethods areused to extract spectral information from the

x-rays transmitted through the patient during imaging

1. Dual-sourceCT

2. Fast-kVswitching CT


Twin beam technology results in thesimultaneous scan of two contiguous slices

Dual-Row/Dual-SliceDetectors - results in faster

volume coverage compared with single-rowCT systems

Dual-Row/Dual-SliceDetectors - doubling the

sampling density and total number ofmeasurements results in excellent resolution

Multirow/MultisliceDetectors - introduced to

increase the volume coverage speed andthus decrease the time for data collection

Multirow/MultisliceDetectors - array consists of

multiple separate detector rows

Multirow/MultisliceDetectors - can acquire

4 to 320 slices per 360-degree rotation

Multirow/MultisliceDetectors - fall into two categories

1. Matrixarray detectors

2. Adaptivearray detectors

MatrixArray Detector - often referred to as a

fixed array detector

MatrixArray Detector - design?


MatrixArray Detector - Isotropic design

1. All cells equal in all dimensions

2. Perfect cubes

3. Produces improved spatial resolution inboth longitudinal and transverse planes

AdaptiveArray Detectors - design?

Anisotropic in design

- cells are not equal but rather they have different sizes

DetectorArray Configuration - regardless of category, during scanning, the detector configuration is used to

determine two things

1. Number of slices

2. Thickness of each slice

AreaDetectors - Two CT detector prototypes currentlyundergoing clinical testing

1. 256-slice CT scanner prototype

2. Flat-panel CT scanner prototype

256-SliceCT Prototype Detector - promotes a

wide area multirow array that allows for a 128-mm beamwidth

256-SliceCT Prototype Detector - large beam width makes it possible to

scan larger volumes such as the entireheart in a single rotation

Flat-PanelDetectors - similar to the ones used in digital radiography, these are

being investigated for use in CT imaging

-Prototypes are being evaluated for use inbreast CT

DetectorElectronics Function - Data acquisition system (DAS)

refers to the detector electronicspositioned between the detector array and the computer

DetectorElectronics Function - Three major performing functions

1. Measures the transmitted radiation beam

2. Encodes these measurements into binary data

3. Transmits binary data to the computer

OptoelectronicData Transmission Schemes - Optoelectronics refers to the use of

lens and light diodes to facilitate datatransmission

OptoelectronicData Transmission Schemes - used in CT due to the

vast amount of data generated from thecontinuous rotation of the tube and detector arc

OptoelectronicData Transmission Schemes - capable of very high

rates of data transmission

-50 million bits per second is common

DetectorElectronic Design Innovations - Integratedmicroelectronic circuitry

A new innovation that has overcome theflawed design of long, power-consuming wires coupling the detectors to the analog-to-digitalconverter (ADC)

Detector Electronic Design Innovations - Integrated microelectronic circuitry - contains the

photodiodes and the ADCs, hencedecreasing the distance through which electrical signals must travel

Detector Electronic Design Innovations - Integrated microelectronic circuitry - composed of

smaller and compact circuitry

Detector Electronic Design Innovations - Integrated microelectronic circuitry - reduces electronic

noise, thus improving signal-to-noiseratio (SNR)

Detector Electronic Design Innovations - Integrated microelectronic circuitry- reduces power


DataAcquisition and Sampling - each detector samples

the beam intensity incident in it

DataAcquisition and Sampling - artifacts may result if

not enough samples are obtained

Sampling - Factors that affect sampling information

1. Slice thickness

2. Closely packed detectors

3. Quarter-shift detector arc

4. Multifan measurement technique

5. Double-dynamic focus


Z-flyingfocal spot technique

Z-SharpTechnology - provides doubt the

sampling, thus creating sharper images