CT Scan

By: Asadinezhad, MohsenPhD in Medical Physics

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Contents Early History Tomography CT History CT Generations Spiral CT MultiSlice CT Digital Image CT Number Windowing

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Contents Major Componemts

X-Ray Tube Beam Filtration Collimators Detectors

Image Reconstruction Image Quality

Spatial Resolution Contrast Resolution Noise

CT Radiation Dose Artifacts 3

References1. Euclid Seeram, Computed Tomography:

Principles, clinical applications and quality control, W.B.SANDERS Company

ني و كنترل كامپيوتري اصول فيزيكي، موارد استفاده بالي توموگرافيبهزادنيا، تانازياميرحسين قاسمي مهر و مترجمين، سيرام اوكليدكيفي،

جهانتابانتشارات 2. Thomas S Curry, James E Dowdey, Robert C

Murrey, Christensen's physics of diagnostic radiology

3. Matthias Hofer, CT teaching manual, THIEME4

Limitations of Radiography

Structures superimposed on film

Must view structure of interest through underlying / overlying structures

Patient

X-rayBeam

Film

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Limitations of Radiography

Multiple views often required to adequately visualize a structure.

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Limitations of Radiography

Optical density dictated by total attenuation encountered by beam

Thin highly-attenuating objects appear to be same density as thicker low-attenuating object.

Patient

X-rayBeam

Film

Thin denseobject

Thick lessdense object

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Early Solution: Conventional Tomography

Sectional imaging methods first developed in 1920’s

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Tomography

Body section radiography Planigraphy Stratigraphy Laminography Tomography Tomography (by ICRU in 1962)

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Early History:Conventional Tomography

First used in 1935 Image produced on film Image plane oriented parallel to film Anatomy in plane of fulcrum stays in focus Anatomy outside of fulcrum plane mechanically blurred

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Conventional Tomography BlurringConventional Tomography Blurring

Image produced on film

Objects above or below fulcrum plane change position on film & thus blur

Conventional Tomography Blurring

b c

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Tomographic LayerTomographic Layer

⇒

S

≪ ,

2 2

2 2

S

U

2tt

b

a

V

θ

2tθ°

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623

10

912

7

1 cm4

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2

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Tube trajectories in Tomography

Linear Elliptical Circular Spiral Hypocycloidal Figure 8

Limitations of Conventional Tomography

Overlying / underlying structures blurred, not removed

5-10% subject contrast difference required for objects to appear different many anatomic systems do not have this subject

contrast

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fracture of the base of the odontoid process with anterior displacement of the atlas

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AP Projection Tomogram

TMJ

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CT Advantages View anatomy without looking

through underlying / overlying structures improves contrast

Uses tightly collimated beam minimizes scattered radiation improves contrast

Demonstrates very small contrast differences reliable & repeatedly

CT X-rayBeam

ConventionalX-ray Beam

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Film as a Radiation Detector

Analog not quantitative

Not sensitive enough to distinguish small differences in incident radiation

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CT Detectors

electronic / quantitative extremely sensitive

small radiation input differences reliably & repeatedly measured & discerned

output digitized & sent to computer

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How Did We Go From…

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Conventional vs Axial Tomography

Conventional Cut

CT Axial Cut

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Radiography vs. CT Imaging Limits of radiography / fluoroscopy

3D structures are collapsed into 2D image (obscuring of details, loss of one dimension)

Low soft-tissue contrast Not quantitative

Features of x-ray CT X-ray imaging modality (same principles of

generation, interaction, detection) Generation of a sliced view of body interior (“T”,

Tomography from Greek tomos = slice) Computational intensive image reconstruction (“C”)

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CT Image Not produced on film Mathematically reconstructed from many

projection measurements of radiation intensity Digital Image calculated

Compu-ter

Digital Image

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Basic principles

Mathematical principles of CT were first developed in 1917 by the Austrian mathematician Johann Radon (1887-1956)

Proved that an image of an unknown object could be produced if one had an infinite number of projections through the object

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Basic principles

Idea popularized by a physicist (Allan Cormack) at Tufts Univ. (1963)

Allan MacLeod Cormack (1924–1998) shortly after the official announcement of the Nobel Prizes for medicine in 1979

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CT HistoryCT History

First test images in 1967 First clinical images ~ 1971 First commercial scanner 1972

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CT HistoryCT History CT made possible by high speed minicomputer

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CT Computers Old mainframe computers too expensive & bulky

to be dedicated to CT

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Godfrey Hounsfield, the English engineer developed the first CT scanner (1972) received the Nobel Prize in medicine in 1979 together with the physicist A.M. Cormack

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Computerized Axial Tomography (CAT) Computerized Trans Axial Tomography (CTAT) Computerized Reconstruction Tomography (CRT) Digital Axial Tomography (DAT) Computed Tomography (CT)

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Before Hounsfield and Cormack ....projection radiography, skull

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Hounsfield: brain scan produced in 1974 with an 80 x 80 image matrix (a) and sagittal reconstruction generated from single scans taken with a spacing of 13 mm

Whole Body Scans

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Topogram, Scout View, Scanogramor Pilot View.

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CT Generations

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First Generation (1970), Translation/Rotation, Pencil Beam

EMI Mark I (Hounsfield), pencil beam scanner (highly collimated source) excellent scatter rejection, now outdated

2 detectors 160 measurements during translation/ 180 - 240 rotation angle in steps of ~1 Used for the head (water bag fit tightly around head, Original computer

software couldn’t deal with transition from skull to air) 5-min scan time, 20-min reconstruction Original resolution: 80 80 pixels (ea. 3 3 mm2), 13-mm slice

X-ray Tube

Detector

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Second Generation (1972), Translation/Rotation, Narrow Fan Beam

Narrow Fan beam (10˚) Linear detector array (~5-30 detectors) 180 ˚ rotation angle in steps of 5-10˚ Reduced number of view angles scan time ~20-30 s Slightly more complicated reconstruction algorithms because

of fan-beam projection

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3rd Generation Geometry

Patient

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Third Generation (1976), Rotation/Rotation, Wide Fan Beam

Wide fan beam (30-60˚) covers entire object 30-900 detectors (ionization chamber or scintillation detector) No translation required scan time ~seconds (reduced dose,

motion artifacts) Reconstruction time ~seconds Pulsed source (reduces heat load, radiation dose) Ring Artifact

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4th Generation CT4th Generation CT

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Fourth Generation (1978), Rotation/Stationary, Wide Fan Beam

Wide fan beam (30-60˚) covers entire object Stationary detector ring (600 – 4800 scintillation detectors) Rotating x-ray tube (inside or outside detector ring) Scan time, reconstruction time ~1 second Source either inside detector ring or outside (rocking, nutating

detectors)

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Comparison of 3rd and 4th Generation

Both designs currently employed, neither can be considered superior

3rd Generation (GE, Siemens):Fewer detectors (better match, cheaper)Good scatter rejection with focused septa

4th Generation (Picker, Toshiba):Less moving partsDetectors calibrated twice per rotation

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3rd & 4th Generation (Non-spiral) CT

Tube rotates once around patient Table stationary data for one slice collected

Table increments one slice thickness Repeat

Tube rotates opposite direction

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Spiral CT

Patient

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Spiral CTSpiral CT Continuous rotation of gantry & linear motion of patient

table Patient moves slowly through gantry Cables of old scanners allowed only 360o rotation (or

just a little more) Tube had to stop and reverse direction No imaging done during this time

No delay between slices Dynamic studies now limited only by tube heating

considerations Increased coverage volume / rotation

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Slip rings - spiral CT

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Spiral CT

table increment during one 360° rotation Pitch factor = -------------------------------------------------

slice thickness

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Pitch factor = 1

table motion during one 360° rotation Slice Pitch = ---------------------------------------------

slice thickness

Pitch factor = 1 means slices touch each other

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Pitch factor >1

table motion during one 360° rotation Slice Pitch = ---------------------------------------------

slice thickness

Pitch factor > 1 means gap in slices

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Pitch factor <1

table motion during one 360° rotation Slice Pitch = ---------------------------------------------

slice thickness

Pitch factor < 1 means overlap in slices Can improve visualization of objects

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Pitch factor = 1

equivalent dose to non-spiral

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Pitch factor >1

lower dose for spiral if table increment per rotation > one slice thickness

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Pitch factor <1

higher dose for spiral if table increment per rotation < one slice thickness

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5th Generation: Scanners for CV Imaging – “Imatron”

No moving parts Electromagnetically swept electron beam 50 ms scan time imaging of beating heart Developed 1983 Multi slice capability

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Cine CT (Imatron)Cine CT (Imatron) four tungsten target rings that makes a 210° arc around the patient

replaces conventional x-ray tube electron beam sweeps over each annular target ring

can be done at electronic speeds 2 detector rings with arcs of 216°

One arc with 432 detector, another with 864 detector (higher resoution) Cadmium tungstate crystal (CdWO4)

maximum scan rate 24 frames per second

Electron-beam CT, also known as fifth-generation CT

Wolbarst A B , Hendee W R Radiology 2006;238:16-39

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