Chapter 4 - Data Acquisition Concepts

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Data acquisition refers to the method by which the

Back 1

patient is scanned to obtain enough datafor image reconstruction

Front 2

Scanning is defined by the

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

Front 3

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

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1. beam geometry

2. components - comprising the scheme

Front 4

Beam geometry

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

Front 5

Components

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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

Front 6

Ray

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The part of the beam that falls on onedetector

Front 7

View

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A collection of rays from one translationacross the object

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Profile

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

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Data sample

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A collection of views transmitted formeasurement

Front 10

Scanning also includes the movement of the patient

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through the gantry to scan the next slice

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

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several rays

Front 12

Projection data are collected by the

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

Front 13

Three primary types of acquisition geometries

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1. parallel beam geometry

2. fan beam geometry

3. spiral/helical geometry - most recently developed

Front 14

A simple categorization of CT equipment evolved based on

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1. scanning geometry

2. scanning motion

3. # of detectors

^ this sort of categorization is known as generations

Front 15

Scanner generations

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1-7

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First generation scanners

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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

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Second generation scanners

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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

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Second generation scanners- xray tube traces a

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semicircular path during scanning

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Second generation scanners - the time decrease is

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inversely proportional to the # of detectors

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Second generation scanners - the more detectors, the

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shorter is the total scan time

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Third generation scanners

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1.Fan beam geometry

2. 360-degree rotation

3. Curved detector array

4. Continuouslyrotating fan beam scanning

5.Scan time = seconds

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Curved detector array

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

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Continuously rotating fan beam scanning

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

Front 24

Third generation- path traced by tube

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circle rather than semicirccle

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Third generation scan time

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

Front 26

Fourth generation scanners

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Two types of beam geometries

1. Rotating fan beam within a circulardetector array

2.Rotating fan beam outside a nutatingdetector ring

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Rotating fan beam within a circular detector array

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stationary ring of detectors

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Rotating fan beam outside a nutating detector ring

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

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RotatingFan Beam Within a Circular Detector Array

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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

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RotatingFan Beam Within a Circular Detector Array- xray tube traces

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circular path

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

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rays strike a detector

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RotatingFan Beam Outside a NutatingDetector Ring

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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

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

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tilting action of the detector ring during data collection

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

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

Front 35

MSCT

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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

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Spiral/Helical geometry scanners

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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

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Fifth generation scanners - Classified as high-speed CT scanners

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Scan time is in the milliseconds

Front 38

Fifth generation scanners

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Two types of scanners in this generation:

1. Electron beam CT scanner (EBCT)

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

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EBCT scanner - developed for

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high-speed CT scanning of heart andcirculation

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EBCT scanner- overall goal is to

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

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EBCT distributed by

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Siemens Medical Systems under the nameEvolution

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EBCT scanners - its design enables it to

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acquire CT data 10x faster than conventional CT scanners

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EBCTversus Conventional Scanner -Design configuration differences

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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

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EBCT acquires CT data

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10 times faster than conventional CT scanners

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Silicon photodiodes

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convert light into current

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The output from detectors is sent to the

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data acquisition system

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data acquisition system

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

Front 48

Sixth generation scanners

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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

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Seventh generation scanners

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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

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7th generation scanners -also used in

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angiography to image blood vessels

Front 51

Slip ring technology allows

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

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Slip ring technology is an

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

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Spiral/helical CT made possible through

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use of slip ring technology

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Slip ring design

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1. disk

2. cylinder

3. contactless slip ring

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Disk

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Conductive rings form concentric circlesin the plane of rotation

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Cylinder

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Conductive rings positioned along theaxis of rotation to form a cylinder

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Contactless slip ring

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Electrical energy transferred acrossrotating interface without use of electrical contact

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Slip ring brush design

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1. wire

2. composite

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Wire

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Uses conductive wire as a sliding contact

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Composite

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Uses a block of some conductive materialas a sliding contact

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Power supply

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1. low voltage slip ring

2. high voltage slip ring

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Low-voltage slip ring

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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

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High-voltage slip ring

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

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

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Slip ring advantages

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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

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In initial experiments, Hounsfield used

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low energy monochromatic gamma ray radiation

Front 66

Xray system - components include

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1. x-ray generator

2. xray tube

3. xray beam filter

4. collimators

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X-ray generator - Ct scanners use

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high frequency generators

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X-ray generator located

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within the gantry

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X-ray generator has

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High-frequencyinverter circuit

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X-ray generator

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

Front 71

X-ray generator- power ratings range from

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20 to 120 kW

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X-ray tubes

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

2. Used in first- and second-generationscanners

3. Rotating anode x-ray tubes

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Rotating anode x-ray tubes - produced

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

Front 74

Rotating anode x-ray tubes - made of

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rhenium,tungsten, and molybdenum (RTM) alloy

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

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graphite base body for high thermalcapacity

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Rotating anode x-ray tubes - angle?

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Small target angle (12 degrees)

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

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

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

Front 78

X-RayTubes in Spiral/Helical CT Scanners:

Helical/spiral scanners have additionaltube requirements and concerns

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1. Higher power levels

2. Additional heat generation

3. Heat storage

4. Heat dissipation

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CTX-Ray Tube Glass Envelope - ensures

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vacuum

Front 80

CTX-Ray Tube Glass Envelope - provides structural

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support of anode and cathode structures

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CTX-Ray Tube Glass Envelope - provides high

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voltage insulation between the anode andcathode

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CTX-Ray Tube Glass Envelope - consists of

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tubes with metal envelopes which

1. Prevents arcing

2. Allows higher tube currents

3. Increases storage capacity

4. Heat dissipation rate improvement

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Anode assembly consists of

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1.Disk

2. Rotor stud and hub

3. Bearing assembly

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Anode assembly - three basic designs

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1.All-metal disk

2. Brazed graphite disk

3. Chemical vapor deposition (CVD) graphitedisk

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ConventionalAll-Metal Disk - consists of a

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

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Conventional All-Metal Disk - transfers

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heat from the focal track very quickly

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

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spiral/helical CT imaging due to weight

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BrazedGraphite Disk - consists of a

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tungsten-rhenium focal track brazed to agraphite base body

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BrazedGraphite Disk - provides

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increased heat storage capacity due tohigh thermal capacity

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BrazedGraphite Disk - tubes for

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

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CVDGraphite Disk - consists of a

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

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CVDGraphite Disk - accommodates

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

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CVDGraphite Disk - intended for use in

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spiral/helical CT x-ray tubes

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Anode assembly - Bearing assembly is to

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provide and ensuresmooth rotation of the anode disk

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Anode assembly - rotation speeds of

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10,000 rpm arepossible

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Anode assembly- liquid bearing method

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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

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Rotor

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

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Rotor- the hub and stud

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prevent the transmission of heat from thedisk to the bearings

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Straton X-ray tube - useful for

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multislice CT (MSCT) scanners

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Straton X-ray tube - anode is

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immersed in oil which:

1. Provides higher cooling rates

2. Tolerates higher mA

3. Allows long exposure times for increasinganatomical coverage

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Straton X-ray tube - smaller size

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ensures faster gantry rotation

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Straton X-ray tube - cathode consists of an

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

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

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1. iMRC 800 x-ray tube

2. Vectron x-ray tube

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iMRC 800 x-ray tube - facilitates very high

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heat loading capacity and rapid heatdissipation

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iMRC 800 x-ray tube - features a dynamic

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

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Vectron x-ray tube - tube based on the

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Stratonx-ray tube direct anode cooling technology

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Vectron x-ray tube - operates with tube

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

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Vectron x-ray tube - electron beam that can

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

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Filtration -removes

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

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Filtration produces a

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“harder” beam

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Filtration reduces

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patient dose

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Filtration shapes the

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energy distribution across the radiationbeam to produce uniform beam hardening

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Bowtie filters - applies to a

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

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Bowtie filters - allows a more

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uniform beam to be delivered to thedetector

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Collimation - purpose is to

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

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In CT, collimation is

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equally important because it affectspatient dose and image quality

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Basic CT collimation scheme includes

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1. Prepatient collimators

2. Postpatient collimators

3. Predetector collimators

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AdaptiveSection Collimation - MSCT collimation scheme issues that result inan

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increase of patient dose

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Overscanning

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

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Overbeaming

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

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Adaptive section collimation

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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

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CTDetector Technology - CT detector functions

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3

Front 123

Capture the

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radiation beam from the patient

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Convert radiation

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beam into an electrical signal

Front 125

Convert electrical signal

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into binary coded information

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Detector characteristics - detectors exhibit several characteristics essential for CT

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image production affecting good image quality

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Capture efficiency

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

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Absorption efficiency

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

Front 129

Stability

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the steadiness of the detector response

Front 130

Response times

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

Front 131

Dynamic range

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

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Afterglow

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

Front 133

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

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1. Scintillationdetectors

2. Gas ionization detectors

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ScintillationDetectors - convert x-ray energy into

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

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ScintillationDetectors - solid-state detectors that

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consistof a scintillation crystal coupled to a photodiode tube

Front 136

ScintillationDetectors - Excellent conversion

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efficiency and photon capture efficiency

Front 137

ScintillationDetectors - exhibit

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afterglowproblems

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Gas Ionization Detectors - consists of a series of

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

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Gas Ionization Detectors - x-ray falls on

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

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Gas Ionization Detectors - promote excellent

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stability, fast response times, andexhibit no afterglow problems

Front 141

Gas Ionization Detectors - poor efficiency

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and unable to be used in MSCT

Front 142

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

Back 142

detectors that are more efficient in these respects

Front 143

Recent detector improvementclassifications

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1. Conventional energy integration (EI) detector

2. Dual-layer detector

3. Direct conversion detector

Front 144

The Stellar Detector - falls in the

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category of the firstthird-generation CT detectors

Front 145

The Stellar Detector - its innovation is that

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the electronics are totally integrated inthe photodiode

Front 146

The Stellar Detector - can measure

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

Front 147

The NanoPanel Prism Detector

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A dual-layerdetector

Front 148

The NanoPanel Prism Detector - used in

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spectralCT imaging

Front 149

The NanoPanel Prism Detector - the configuration allows for

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

Front 150

The NanoPanel Prism Detector - ensures optimum

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performancein all characteristics that influence CT image quality

Front 151

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

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through the patient

Front 152

Two approaches

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1. Energyweighting

2. Materialdecomposition

Front 153

Two dual-energymethods areused to extract spectral information from the

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x-rays transmitted through the patient during imaging

1. Dual-sourceCT

2. Fast-kVswitching CT

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Dual-Row/Dual-SliceDetectors

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Twin beam technology results in thesimultaneous scan of two contiguous slices

Front 155

Dual-Row/Dual-SliceDetectors - results in faster

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volume coverage compared with single-rowCT systems

Front 156

Dual-Row/Dual-SliceDetectors - doubling the

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sampling density and total number ofmeasurements results in excellent resolution

Front 157

Multirow/MultisliceDetectors - introduced to

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

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Multirow/MultisliceDetectors - array consists of

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multiple separate detector rows

Front 159

Multirow/MultisliceDetectors - can acquire

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4 to 320 slices per 360-degree rotation

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Multirow/MultisliceDetectors - fall into two categories

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1. Matrixarray detectors

2. Adaptivearray detectors

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MatrixArray Detector - often referred to as a

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fixed array detector

Front 162

MatrixArray Detector - design?

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isotropic

Front 163

MatrixArray Detector - Isotropic design

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1. All cells equal in all dimensions

2. Perfect cubes

3. Produces improved spatial resolution inboth longitudinal and transverse planes

Front 164

AdaptiveArray Detectors - design?

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Anisotropic in design

- cells are not equal but rather they have different sizes

Front 165

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

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determine two things

1. Number of slices

2. Thickness of each slice

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AreaDetectors - Two CT detector prototypes currentlyundergoing clinical testing

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1. 256-slice CT scanner prototype

2. Flat-panel CT scanner prototype

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256-SliceCT Prototype Detector - promotes a

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

Front 168

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

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scan larger volumes such as the entireheart in a single rotation

Front 169

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

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being investigated for use in CT imaging

-Prototypes are being evaluated for use inbreast CT

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DetectorElectronics Function - Data acquisition system (DAS)

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

Front 171

DetectorElectronics Function - Three major performing functions

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1. Measures the transmitted radiation beam

2. Encodes these measurements into binary data

3. Transmits binary data to the computer

Front 172

OptoelectronicData Transmission Schemes - Optoelectronics refers to the use of

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lens and light diodes to facilitate datatransmission

Front 173

OptoelectronicData Transmission Schemes - used in CT due to the

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

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OptoelectronicData Transmission Schemes - capable of very high

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rates of data transmission

-50 million bits per second is common

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DetectorElectronic Design Innovations - Integratedmicroelectronic circuitry

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

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Detector Electronic Design Innovations - Integrated microelectronic circuitry - contains the

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

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Detector Electronic Design Innovations - Integrated microelectronic circuitry - composed of

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smaller and compact circuitry

Front 178

Detector Electronic Design Innovations - Integrated microelectronic circuitry - reduces electronic

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

Front 179

Detector Electronic Design Innovations - Integrated microelectronic circuitry- reduces power

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consumption

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DataAcquisition and Sampling - each detector samples

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the beam intensity incident in it

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DataAcquisition and Sampling - artifacts may result if

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not enough samples are obtained

Front 182

Sampling - Factors that affect sampling information

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1. Slice thickness

2. Closely packed detectors

3. Quarter-shift detector arc

4. Multifan measurement technique

5. Double-dynamic focus

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Z-SharpTechnology

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Z-flyingfocal spot technique

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Z-SharpTechnology - provides doubt the

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sampling, thus creating sharper images