Module HModule H

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

and Imagingand Imaging

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Conventional CTConventional CT

Slice-by-slice Slice-by-slice One row of detectors One row of detectors Patient moves through gantry according to Patient moves through gantry according to slice parameters established by the slice parameters established by the Technologist.Technologist.10 x 10 means the couch will travel 10 mm 10 x 10 means the couch will travel 10 mm after each 10 mm slice is imaged.after each 10 mm slice is imaged.Conventional CT is defined as Conventional CT is defined as contiguouscontiguous acquisitionacquisition

Conventional CTConventional CT

10 x 5 would be a 10 mm slice would be 10 x 5 would be a 10 mm slice would be imaged every 5mm’s.imaged every 5mm’s.

With the above protocol With the above protocol Overlapping Slices cause the Overlapping Slices cause the

tissue within the overlapped area tissue within the overlapped area to be irradiated TWICE.to be irradiated TWICE.

Conventional CTConventional CT

Images acquired slice-by-sliceImages acquired slice-by-slice

X-ray tube rotates around the patientX-ray tube rotates around the patient

CT detectors (a single-row) capture the x-CT detectors (a single-row) capture the x-ray attenuation measurements.ray attenuation measurements.

CT couch increments to the location of the CT couch increments to the location of the next slice locationnext slice location

Filtered-back-projection used a Filtered-back-projection used a reconstruction algorithmreconstruction algorithm

Spiral CTSpiral CT

Spiral CT developed in 1989Spiral CT developed in 1989

Also called Also called Helical CTHelical CT

Helical scanningHelical scanning acquires data in volume acquires data in volume rather than slice-by-slicerather than slice-by-slice

X-ray tube travels in a continuous X-ray tube travels in a continuous rotational scanning methodrotational scanning method

Slip-ring design Slip-ring design

Slip Ring designSlip Ring design

Electromechanical devices that have Electromechanical devices that have circular electrical conductive rings circular electrical conductive rings

Conductive rings transmit energy across a Conductive rings transmit energy across a rotating surface.rotating surface.

Slip-ring eliminated the cable wrap-round Slip-ring eliminated the cable wrap-round process of conventional CTprocess of conventional CT

Generator is located inside the gantryGenerator is located inside the gantry

Slip Ring designsSlip Ring designs

Slip Ring DiskSlip Ring Disk

Conductive rings are Conductive rings are positioned in positioned in concentric circles to concentric circles to lie within the lie within the rotational planerotational plane

Slip Ring CylinderSlip Ring Cylinder

Conductive rings are Conductive rings are placed along the axis placed along the axis of rotation, forming a of rotation, forming a cylindercylinder

BrushesBrushes

Brushes are used to transmit electrical Brushes are used to transmit electrical power to the CT scanner componentspower to the CT scanner components

Brushes glide in contact groves along the Brushes glide in contact groves along the slip-ringslip-ring

Two types of BrushesTwo types of Brushes– WireWire– compositecomposite

Wire / Composite BrushesWire / Composite Brushes

Wire BrushWire Brush

Conductive wireConductive wire

One or more wires One or more wires arranged so as to arranged so as to function as a cantilever function as a cantilever springspring

2 brushes per ring are 2 brushes per ring are often used to increase often used to increase communication reliability communication reliability or current-carrying or current-carrying capacitycapacity

Composite BrushComposite Brush

A block of conductive A block of conductive material used as a sliding material used as a sliding contact contact – Silver-graphite alloySilver-graphite alloy

spring designs include:spring designs include:– cantilever cantilever – constant- forceconstant- force– compressioncompression

Two brushes per ring are Two brushes per ring are often used often used

Low-Voltage / High-VoltageLow-Voltage / High-Voltage

Low-voltage slip ring Low-voltage slip ring scannersscanners480 AC power 480 AC power Low-voltage brushesLow-voltage brushesThe slip ring provides The slip ring provides power to the high voltage power to the high voltage transformertransformerGenerator, tube, and Generator, tube, and other controls rotate other controls rotate continuouslycontinuously

High-voltage slip ring High-voltage slip ring scannersscannersAC delivers power to the AC delivers power to the high-voltage generator, high-voltage generator, located in the gantrylocated in the gantryHigh-voltage generator High-voltage generator supplies voltage to the supplies voltage to the slip-ringslip-ringHigh-voltage form the slip High-voltage form the slip ring is transferred to the ring is transferred to the x-ray tube.x-ray tube.In High-voltage slip ring In High-voltage slip ring scanners the generator scanners the generator does notdoes not rotate rotate

Types of Spiral CT ScannersTypes of Spiral CT Scanners

Single-Slice Volume CT introduced 1990Single-Slice Volume CT introduced 1990– Also called Also called Single detector-row spiral CTSingle detector-row spiral CT – Patient is translated at a constant speed Patient is translated at a constant speed

through the gantrythrough the gantry– Tube makes a continuous exposure (HU)Tube makes a continuous exposure (HU)– Faster than conventional CTFaster than conventional CT– Data from patient received by the detectorsData from patient received by the detectors– Detectors send the data to array processors Detectors send the data to array processors

– Volume dataVolume data (Not single-image data like in (Not single-image data like in conventional CT)conventional CT)

Volume DataVolume Data

Spiral CT scanners send Spiral CT scanners send volume volume datadata to the array processor to the array processor

Standard algorithms Standard algorithms can notcan not be be used for image reconstructionused for image reconstruction

Single slice volume CTSingle slice volume CT

Single slice volume CT have a 1-D Single slice volume CT have a 1-D detector array because of the single-row of detector array because of the single-row of detectors.detectors.Fan-beam profile is used in both Fan-beam profile is used in both conventional CT and single slice volume conventional CT and single slice volume CT’s.CT’s.There are pre patient and post patient There are pre patient and post patient collimators to deliver a constant beam-collimators to deliver a constant beam-width at the detectors.width at the detectors.

Single slice volume CTSingle slice volume CT

Single-slice volume CT scanners:Single-slice volume CT scanners:– Analytic reconstruction algorithms were Analytic reconstruction algorithms were

developed to improve the imaging limitations developed to improve the imaging limitations of back projection. of back projection.

Filtered back projection reconstruction algorithms Filtered back projection reconstruction algorithms (also called (also called Convolution methodConvolution method))

Convolution removes the blurring (see Seeram page Convolution removes the blurring (see Seeram page 105-108, Fig.6-11, 6-12)105-108, Fig.6-11, 6-12)

– Single-slice volume scanning uses 360-Single-slice volume scanning uses 360-degree linear interpolation algorithm (LIA)degree linear interpolation algorithm (LIA)

Single slice volumeSingle slice volume

In order to improve poor image quality and In order to improve poor image quality and artifact production, artifact production, created by the 360-created by the 360-degree LIAdegree LIA, the 180-degree LIA is applied., the 180-degree LIA is applied.

The 180-degree LIA The 180-degree LIA – Maintains the detail (slice sensitivity and Maintains the detail (slice sensitivity and

spatial resolution)spatial resolution)– Creates more noise in the imageCreates more noise in the image

Single slice volume CTSingle slice volume CT

In CT NO slices are produced by the In CT NO slices are produced by the scanning process!scanning process!

Images are produced by the Computer.Images are produced by the Computer.

CT is a digital process CT is a digital process

Single slice volume CTSingle slice volume CT

AdvantagesAdvantagesImprovements over Improvements over

conventional CT conventional CT include speed and include speed and volume coveragevolume coverage

DisadvantagesDisadvantagesLong exposures create Long exposures create

high heat units (HU)high heat units (HU)High heat units limited High heat units limited

the volume of scan the volume of scan area (z-axis)area (z-axis)

High pitch for volume High pitch for volume coverage degraded coverage degraded slice sensitivity profile slice sensitivity profile (detail).(detail).

Single slice volume CT Single slice volume CT

Single slice volume CT have a 1-D Single slice volume CT have a 1-D detector array because of the single-row of detector array because of the single-row of detectors.detectors.Fan-beam profile is used in both Fan-beam profile is used in both conventional CT and single slice volume conventional CT and single slice volume CT’s.CT’s.There are There are prepre patient and patient and postpost patient patient collimators to deliver a constant beam-collimators to deliver a constant beam-width at the detectors.width at the detectors.

PitchPitch

…….defined as the distance of couch top .defined as the distance of couch top travel, per one revolution of the x-ray tube, travel, per one revolution of the x-ray tube, divided by the slice thickness or the beam divided by the slice thickness or the beam collimation.collimation.

Normal pitch is 1:1 Normal pitch is 1:1 – The table travels 1cm with a 1cm slice The table travels 1cm with a 1cm slice

thickness thickness oror 5mm with a 5mm slice thickness 5mm with a 5mm slice thickness etc…….etc…….

Multi-slice CT scannersMulti-slice CT scanners

Developed late 1990’sDeveloped late 1990’s

Also called Also called Multi-detector-row CTMulti-detector-row CT

Had 4 detector rows (quad)Had 4 detector rows (quad)

Cone-beam profile is used in these Cone-beam profile is used in these scannersscanners

Detector rows keep increasing…..Detector rows keep increasing…..8, 16, 32, and 648, 16, 32, and 64

Recently developed 128 detector-row CT scannerRecently developed 128 detector-row CT scanner

Multi-slice or Multi-detector CT Multi-slice or Multi-detector CT scannersscanners

Because more detectors are available at Because more detectors are available at any given point in the scanning area, more any given point in the scanning area, more data can be collected.data can be collected.

Therefore, more data can be derived from Therefore, more data can be derived from the anatomy coveredthe anatomy covered

Multi-slice or Multi-detector CT Multi-slice or Multi-detector CT scanners / Pitchscanners / Pitch

The definition of pitch for multi-slice The definition of pitch for multi-slice scanners differs from that in single slice scanners differs from that in single slice CT. CT.

Definition varies according to Definition varies according to manufacturer. MDCT Multi-Detector manufacturer. MDCT Multi-Detector Computer Tomography can be:Computer Tomography can be:

Slice thickness when using single collimation, orSlice thickness when using single collimation, or

Detector-row collimation, orDetector-row collimation, or

Beam-width at the center of rotationBeam-width at the center of rotation

Multi-slice or Multi-detector CT Multi-slice or Multi-detector CT scanners / Slice Thicknessscanners / Slice Thickness

There is a difference in slice-thickness There is a difference in slice-thickness determination for MDCT.determination for MDCT.

The slice thickness is determined by the:The slice thickness is determined by the:Beam-width (BW)Beam-width (BW)

PitchPitch

Shape and width of the reconstruction filter in the Shape and width of the reconstruction filter in the Z-axisZ-axis

Detector row and beam collimation Detector row and beam collimation relationshiprelationship

d = detector-row collimationd = detector-row collimation

D = beam collimationD = beam collimation

N = number of detector rowsN = number of detector rows

d(mm) = D(mm)/Nd(mm) = D(mm)/N

Image reconstruction for MDCTImage reconstruction for MDCT

Several choices based on one of the Several choices based on one of the following:following:

Interlaced SamplingInterlaced Sampling

Longitudinal Interpolation by z-axis filteringLongitudinal Interpolation by z-axis filtering

Fan beam reconstructionFan beam reconstruction

Multi-slice/Multi-detector Multi-slice/Multi-detector advantagesadvantages

Greater volume coverage (more detectors collecting data Greater volume coverage (more detectors collecting data at one time)at one time)Allows sub-second scanning (anatomy is over each Allows sub-second scanning (anatomy is over each detector row for less than a second)detector row for less than a second)Greater detail (increased spatial resolution)Greater detail (increased spatial resolution)More efficient use of the beam profileMore efficient use of the beam profileThe acquisition of sub-millimeter slices (currently .5mm)The acquisition of sub-millimeter slices (currently .5mm)Reduction in radiation dose to the patientReduction in radiation dose to the patientGreater accuracy in needle localizations for CT guided Greater accuracy in needle localizations for CT guided biopsies)biopsies)More effective use of IV contrast mediaMore effective use of IV contrast media

Current applicationsCurrent applications

Pulmonary embolus protocols (.5mm to 1.25mm slices)Pulmonary embolus protocols (.5mm to 1.25mm slices)Trauma protocols with increased volume coverageTrauma protocols with increased volume coverageMore detailed and accurate 3-D reformations, surface shading and More detailed and accurate 3-D reformations, surface shading and volume renderingvolume renderingMultiplanar reconstructionsMultiplanar reconstructionsComputed Tomography Angiography (CTA)Computed Tomography Angiography (CTA)CT FluoroscopyCT FluoroscopyCT Interactive CineCT Interactive CineCT ColonographyCT ColonographyVirtual Reality CT (colonography, cystography, and endography)Virtual Reality CT (colonography, cystography, and endography)Calcium ScoringCalcium ScoringECG/EKG Gated Cardiac StudiesECG/EKG Gated Cardiac StudiesCoronary Artery CTCoronary Artery CT