Spiral CT Dr.shravani

7/30/2019 Spiral CT Dr.shravani

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By

Dr. Shravani


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Basic principle

The internal structure of an object can bereconstructed from multiple projections of the same.

The ray projections are formed by thin cross sections

of the scanning part with a narrow x-ray beam

The transmitted radiation from this beam is

measured by using a sensitive radiation detector.

This measured radiation is fed into a computer and

analysed by using mathematical algorithms andreconstructed as a tomographic image.


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X-ray goes through

collimator therefore

penetrate only an

axial

layer of the object,

called

"slice"

How does CT Work?


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Patient is placed in the centerof the measurement field

X-ray is passed through thepatients slice from many

direction along a 360 path

The transmitted beams arecaptured by the detectorswhich digitizes these signals

These digitized signals calledraw data are sent to acomputer which create the CTimage


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The object slice is divided

into small volume

elements called voxels.

Each voxel is assigned a

value which is dependent

on the average amount of

attenuation

How is CT Image generated?


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The attenuation values aretransferred to the computer where

they are coded & used to create aslice image


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CT Data Acquisition Components


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DATA ACQUISITIONThe scanning process begins with

data acquisition.

Data Acquisition refers to amethod by which the patient issystematically scanned by the X

ray tube and detectors to collectenough information for imagereconstruction.

A basic data acquisition schemeconsists of

X ray tube

Filters

Collimators

Detectors


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Image reconstruction

Substance HU

Air -1000

Fat -120

Water 0Muscle +40

Contrast +130

Bone +400 or more

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The pixel itself is displayed according to the mean attenuation

of the tissue(s) that it corresponds to on a scale from +3071

(most attenuating) to -1024 (least attenuating) on the

Hounsfield scale

Voxel is a volumetric pixel

The HU of common

substances:


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CT Gantry


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CT gantry internal components

1.X-ray tube &

collimator

2.Detector assembly

3.Tube controller

4.High freq. generator

5.Onboard computer6.Stationary computer


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X-RAY TUBE

Rotating anode type

More heat loading and

heat dissipationcapabilities

Small focal spot size

(0.6mm) to improvespatial resolution


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FILTERS

Compensation filter is being used

To absorb low energy x rays

To reduce patient dose

To provide a more uniform beam


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COLLIMATORS

To decrease scatter radiation

To reduce patient dose

To improve image quality Collimator width determines

the slice thickness


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DETECTORS The detectors gather information by measuring the x-ray

transmission through the patient.

Two types:

Scintillation crystal detector

(Cadmium tungstate+ Si Photodiode)

Can be used in third and fourth generation scanners

Xenon gas ionisation chamber

Can be used in third generation scanners only


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Scintillation crystal detector used in I & II gen. CT scanners

When X ray Photons are absorbed by these materials the

convert them into light photons and again converted into

electric signals by photo multiplier diode.


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Scintillation crystal detector used in III and IV gen. CT

scanners


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First Generation Scanners

Translate - rotate one detector.

Total Scan time : 25-30 minutes (5 minutes per section).

Uses pencil beam to scan the body.Arc of rotation of the gantry by 1 degree.

d


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Second Generation Scanners

Translate rotate multiple detector (30).

Uses fan beam.

Multiple detectors are used.Scan time for single section : 10 -90 seconds.

hi d i


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Third Generation Scanners

Rotate rotate type.

No linear motion of the Gantry.

Multiple detectors are used.Single section Scan time : 4 9 seconds.

F h G i S


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Fourth Generation Scanners

Rotate fixed type.

Detectors are arranged in fixed circles around the

patient and they do not move.Single section Scan time : less then 1 second.


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Scan Modes

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Conventional (incremental) CT(short pause , several minutes)

Spiral (helical) CT

(helical motion of X-ray tube,reducing time and radiation

exposure)


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What is Spiral Scan? -- just 4C

Continuously rotating tube/detector system

Continuously generating X-ray

Continuously table feed

Continuously data acquisition


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Newer method

X-ray tube moves continuously with slip-ring

technique, the outer fixed detector ring and

inner rotating x-ray tube ring having brushes thatremain in electrical contact with the inside of

outer ring.

X-ray tube moves continuously with patientmoving cephalad with constant rate producing a

spiral or helix around the patient.

Spiral CT


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CT SCANNING IN SPIRAL-HELICAL

GEOMETRY BASED ON SLIP RING

TECHNOLOGY

Sli i T h l


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Slip-ring Technology

Power is transmitted through parallel sets of

conductive ringsinstead of electrical cables.

Continuous Gantry RotationPrerequisite for Spiral CT

Non Slip-ring Scanner Slip-ring Scanner


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SPIRAL CT MAJOR STEPS

DATA ACQUSITION-ENTIRE TISSUE IS BEING

SCANNED DURING ONE BREATH HOLD

IMAGERECONSTRUCTION-INTERPOLATION USED TO

GENERATE SLICES. FILTERED BACK PROJECTION USED

TO REDUCE BLURR.


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DATA ACQUSITION

Z-AXIS


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Reconstruction of arbitrary

slices (either contiguous or

overlapping) within the

scanned volume

Distance between the

slices is called Increment.

A BVolume Data

Continuous dataacquisition

During the scanning procedure, the tube is

rotating & emitting x-rays at all time.

The patient is advanced at a constant speed

through the rotating gantryThis produced a volume data set from which

multiple slices can be reconstructed either in a

contiguous or overlapping manner.

The images can be produced at any position

within the scanned volume A & B


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Increment

Slice Thickness

Increment = Slice Thickness No Overlap

No Gaps

Contiguous Image

Reconstruction


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Increment

Overlap

Slice Thickness

Overlapping Image

Reconstruction

Increment < Slice Thickness Overlap of slices

Closer image interval

More images created


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Increment > Slice Thickness Gaps between slices

Images are further apart

Less images created

Image Reconstruction with

Gaps

Increment

Slice Thickness


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Deep Inspiration Shallow Inspiration

Misregistration due to differentrespiratory levels between slices

Partial Volume EffectStandard CT / Slice Imaging

Unable to reconstruct images atarbitrary position

Slice imaging is slow

Conventional CT scanning is slow because it

requires multiple slice acquisition to cover the

volume of interest

Despite the fact that scanning are performed

with consecutive slices, it is easy to miss small

lesion when the patient varies his depth of

inspiration from scan to scan.It is also difficult to reduce partial volume effect.

In order to do so, the lesion must be captured

fully, not partially within the slice. Several

overlapping scans may be repeated, with the

hope that the patient hold his breath at the

same depth each time.


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Why do we need spiral/helical CT scan

It eliminates respiratory misregistration and motionartifacts by rapid acquisition in a single breath-hold.

Produces overlapping images without additionalradiation. From single helical CT scan data, multiple,overlapping axial sections can be retrospectivelyreconstructed.

Multidimensional imaging is possible 2D / 3Dreformations with maximum longitudinal resolutionoptimizing image quality.


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SPIRAL CT MODES

SINGLE SLICE


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SPIRAL CT MODES

MULTI - SLICE


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Conventional CT Alternate translation & x-

ray exposure is done.

Each rotation of x-ray tubegenerates data of atransaxial image

Prospective selection of

slice positioning andspacing

Spiral CT Both are done

simultaneously

Each rotation yields data

Specific to an angledplane of section. Fromthis, by interpolation,transaxialimage is reconstructed.

Done retrospectively .


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Protocol factors of spiral CT scan

Slice thickness / Collimation:

5 to 8mm for abdomen scan.7 to 10 mm for chest scan.

5 mm for neck scan.

2 to 3 mm to scan small structures like lung nodules, smaller

blood vessels.It affects effective slice thickness of reconstructed transaxial

scan and its longitudinal resolution.


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Advantages of spiral overconventional CT

Shorter scanning timeReconstruction along z axis with overlapping

of successive images leading to detection of

small lesionsFewer motion artifacts

3D reconstruction of image


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Applications:

CT angiography.

Cardiac CT with gating to freeze cardiac

motion. Coronary artery imaging including calcium

scoring.

Tumor permeability studies.

Functional and perfusion imaging. In Emergency acute abdomen cases.


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Despite high heart rate of 90 bmp,

motion free visualization of the

coronary arteries.

.


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Hepatic Laceration & Adrenal hematoma due to abdominal trauma

Fig: A Fig: B


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Hepatic focal fibrosis misdiagnosed as hepatocellular


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carcinoma. Axial helical CT images obtained during

hepatic arterial and portal venous phases reveal multiple

round, hypo attenuating areas (arrows) in right and left

liver lobes. This finding simulated hypo vascular massand was originally misdiagnosed as possible tumour.

ARTIFACTS IN COMPUTED


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ARTIFACTS IN COMPUTED

TOMOGRAPHY


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ARTIFACT

Any discrepancy between the CT numbers

represented in the image and the

expected CT number based on the linear

attenuation coefficient


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PATIENT INDUCED ARTIFACTS

Motion

Beam Hardening Metal Artifact

Out Of Field Artifact


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MOTION ARTIFACT CAUSED BY:

Voluntary motion.

Involuntary motion.

It produces ghosting effect object

Ct image appears as if composed ofSuperimposed images

MOTION


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MOTION

MOTION
http://radiographics.rsnajnls.org/content/vol24/issue6/images/large/g04nv25g16x.jpeg


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MOTION

MOTION


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MOTION

MOTION


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MOTION


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REMEDY:

VOLUNTARY MOTION: Explanation Of The

Procedure And Good Communication With A

Patient.

INVOLUNTARY MOTION: Short Scan Time.


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BEAM HARDENING

It occurs when the average energy of an x-raybeam passing through the patient increases.

Beam is hardened high energy photons are

attenuated less by the tissue. As a result, they

are pass through the patient and reach thedetectors.

This artifact is also called cupping artifactbecause the hardening is most pronounced in

the center and less at the periphery. It resembles a cup.


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COMMON AREAS OF MANIFESTATION:

Skull Petrous Pyramids

Upper Chest And Shoulders Hips

BEAM HARDENING


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BEAM HARDENING


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BEAM HARDENING - CONTRAST
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BEAM HARDENING


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REMEDY

INCREASE kvp

Decrease slice thickness

Increase filtration bowtie filter


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METAL ARTIFACT

Manifest itself as star

streaking artifact. Its

caused by presence of

metallic objects inside oroutside the patient.

Metallic object absorbs

the photons causing anincomplete profile

METAL ARTIFACT


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METAL ARTIFACT

REMEDY

Removal of external metallic objects


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OUT OF FIELD ARTIFACT

Patient is not entirely enclosed in the scanningfield of view. Patients body can obstruct

detectors. In addition, patient tissue outsidethe sfov will further harden the x-ray beam.Artifact appears as streaks and shading.


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REMEDY:

Selection of larger sfov

Taping patient tissue

Raising patients arms above their headon the scan of chest and abdomen

RING


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RING

RING
http://radiographics.rsnajnls.org/content/vol24/issue6/images/large/g04nv25c19x.jpeg


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REMEDY

Detector calibration

Detector replacement


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TUBE ARCING

Tungsten vapor from anode and

cathode intercepts the projectile

electrons intended for collisions withthe target. Crackling sound!!!!

TUBE ARCING


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TUBE ARCING


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REMEDY

GAS BURNOFF

TUBE REPLACEMENT

LINE IN TOPOGRAM


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LINE IN TOPOGRAM

BAD DETECTOR CAUSESCONTINUOUS LINE ONTHE TOPOGRAM

REMEDY:

DETECTOR REPLACEMENT

STAIRCASE


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STAIRCASE

Improper selection of slice thicknessand slice incrementation when

generation mpr and 3-d images


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REMEDY

THIN SLICE USE

50% OVERLAP ON RECON SLICE

INCREMENTATION


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Thank You

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Spiral CT Dr.shravani