Computed Tomography

1. COMPUTED TOMOGRAPHY
BY HEENA PARVEEN

2. CONTENTS-
•CT generations
•Electron Beam CT
•Sequential Mode of Acquisition
•Spiral (Helical)Mode of Acquisition
•Single Detector CT
•Multidetector CT
•Pitch in SDCT & MDCT
•Imaging System Design
1. Gantry
2. X-ray Tube
3. Filter
4. Collimater
5. Patient Couch
6. Generator
7. Detectors

3. CT GENERATIONS
CT scanners have gone through a no. of
design changes since the technology was
first introduced.
MAIN AIM-
Faster scanning (decreases scanning time)
 Simplification of mechanical motion.

4. FIRST GENERATION
Translate-Rotate Type
1 degree rotation between successive translation.
Therefore 180 translations required.
Pencil Beam (Parallel beam geometry)
Single/Double Detector
5 minute Imaging time
Also called EMI Scanner.
Only Head was scanned at that time.
Based on water bath technique.

5. ADVANTAGES DISADVANTAGES
1.Pencil beam geometry
allowed very efficient scatter
reduction.
1.High scanning time.
2.Complex mechanical motion
of rotate translate system.
3.Poor spatial resolution.
4.Shorter rotational angle.

6. SECOND GENERATION
Translate-Rotate type
5-10 degree rotation between successive translation.
Detector no increased by 30. (Linear Detector array)
Therefore 18 projections were enough
as more anatomy imaged at one time.
Narrow fan beam.
Scan time- approx 30sec
(as no of detectors increases & rotational increment)

7. ADVANTAGES DISADVANTAGES
1.Shorter scan time
compared
To 1st generation.
1.Increased radiation
intensity
towards edgs of beam.
2.Speed increased due to
increased no. of detectors.
2.Compensated with the
use of “bow-tie” filter.
3.Larger rotational
increment.
3.Scatter radiation
increased due to narrow fan
beam.
4.Complex mechanical
motion
of translate-rotate.

8. THIRD GENERATION
Rotate-Rotate type.
Tube & detector rotates 369 degree around patient.
Wide fan beam, width of fan beam (30-60 degree)
Upto 750 detectors were used ( Curved detector array)
Detectors alligned as an arc, results in constant source
to detector path length.(Advantage of good image constrast)
Scan time less than 100ms.

9. ADVANTAGES DISADVANTAGES
1.Elimination of complexity
of translate-rotate motion.
1.Expensive.
2.Shorter scan time. 2.Occurrence of Ring
Artifact.
3.Better image resolution.
4.Motion artifacts
decreases.

10. RING ARTIFACT-
•If any single detector or bank of detector
malfunctions or due to miscaliberation of
detector
the acquired signal image results in
artifact.
•If one of the detector is out of caliberation,
the detectors will give a consistent
erroneous
Reading at each angular position,resulting
in circular artifact knowns as RING
ARTIFACT.
•Most susceptible to solid state detectors
than
Xenon gas filled detector.

11. REMEDY-
•Recaliberation of the scanner will usually rectify the artifact.
•Ocassionally detectors elements need replacing which can
be costly.
•Residual ring artifacts in images are then removed by
image-processing algorithms.

13. FOURTH GENERATION
Rotate-Stationery
Fan beam.
Radiation detection is acomplished through
a fixed circular array of detectors,which contains approx 4000 detectors.
4th Generation was developed to overcome Ring Artifact.
However,most scanners in use today are based on 3rd Generation.
ADVANTAGES DISADVANTAG
ES
1.Elimination of
Ring Artifact.
2. Expensive
due to more
detector.

14. FIFTHTH GENERATION
Electron Beam CT (no rotation of tube & detector)
Introduced especifically for cardiac Imaging.
Reduces scan time to as little as 50ms,fast enough to image beating heart.
Also known as Ultrafast CT.
No xray tube; instead uses a beam of e- generated outside gantry.
Inside gantry, there are 180 degree rows of fixed detector on one side & 180
degree
of tungsten arcs opposite.
Consist of waveguide to accelerate an e- beam onto the W target through a
bending
magnet.
Electron beam rapidly move to bombard the tungsten arcs,producing an xray
beam.
Xrays then pass through the patient & transmission information is collected by
detector.

15. These units are very fast because they’ve no moving parts.
EBCT was the only fast scanner for cardiac imaging for several no. of years.
However, two additional developements resulted in significant scan time reduction
minimized use of EBCTs.

16. Sequential Mode Acquisition
Also called Step & Shoot acquition or Slice by slice mode acquisition.
Traditional method of scanning, before introduction of Slip ring technology.
Prior to late 1980s, CT table moves to desired location and remain stationery
while the xray tube & detector rotates within gantry.
There will be a pause in scanning between data acquisition (interscan delay)
During pause, table moves forward & tube and detector moves to its original position
& the whole process repeated for next slice.

17. ADVANTAGES
Can be acquired in contiguous fashion or in non-contiguous fashion.
Can be used for dynamic/cine protocols. (No table incrementation)
 Gapped images are taken when survey of area is needed. (Radiation
dose less)
Gapped slices are often used for High resolution chest imaging. (HRCT)
DISADVANTAGES
 Increases examination time- cumulative effect of pause between
scanning.
(Small delay is also significant when doing angiography or breath-hold
examination)
Motion artifact increases.
Data is limited, can’t be reconstructed, therefore Uniplaner imaging.
Misregistration.

18. ADDITIONAL ADVANCEMENTS
Spiral(helical) and Multislice(multidetector) CT are more recent CT
developments that
lead to dramatic effect on reducing scan time & improve image quality.
SPIRAL (HELICAL) CT
Some may consider this as 6th Generation.
In the late 1980s,CT scanners with slip ring technology were introduced.
SLIP RING TECHNOLOGY-Slip rings are electromechanical devices that conduct electricity
and electrical signals through rings from stationery to rotating surface.
Located inside the gantry, the slip -ring technology allowed the tube to continue to rotat
without the need to rewind.
Continuous motion of tube(and detectors) & continuous movement of table and patient
through gantry are basic components of spiral(helical) CT.
Instead of collecting data from one slice at a time,spiral CT collects data for an entire vo
Of tissue at one time.

20. ADVANTAGES
Ability to optimize iodinated contrast agent administration.
Reduction in respiratory misregistration.
Reduction of motion artifact.
Fast Acquisition(Temporal resolution is high)
Scan time decreases.
 3D reconstruction.
Multiplanar reconstruction.
The major improvement leading to its development were-
1.Slip ring design
2.More efficient tube cooling
3.Higher Xray output.
4.Improved raw data management
5.Smoother table movement
6.More-efficient detector

21. Single Detector CT
Until 1990s, all scanners contains multiple detectors alligned in single row.
Initially in 3rd & 4th generation,all 700 & 4000+ detectors were alligned in single row.
In SDCT, single detector element is wide in z-direction.
Therefore, opening/closing of collimater controls slice thickness.
 Slice thickness is limited (ST< Detector width)
 Fan beam
Each gantry rotation results in single slice.
Calculation of anatomy covered in SDCT- No of slices x slice increment
In SDCT, different slice thickness
are obtained by means of adjusting
prepatient collimation of xray beam.
A.Collimaters open maxiumum-
widest slice width available
B.Collimaters are partially closed,
results in thinner slice.

22. Multi detector CT/ Multislice CT
Newer CT system continue to use many detector element s situated in a row.
They may contain from 4-64 parallel rows. Nowadays it is extended upto 256 rows.
Instead of collecting the transmission data for one slice each timethe tube rotates around
MSCT collects 4-64 slices per revolution.
Therefore, provides longer & faster z-axis coverage per gantry rotation.
Acomplished by opening up the xray beam along z-axis.
As the no of slices imaged per revolution is opened, fan beam is now CONE BEAM.
In MDCT, ST is controlled by comination of xray beam width(collimater) & detector config
ADVANTAGES
 Spiral MSCT allows overall
improved image quality with
faster scan time.
 Acquire multiple slices in single
rotation.
 Ability to perform additional
procedure such as CT
Angiography & cardiac imaging.

23. Like in 3rd SDCT scanners, MDCT
scanners
Situated in an array.
However they have many parallel arrays
That are capable of collecting data for
multiple slices with single gantry
rotation.

25. MDCT design were responsible for demise of 4th
generation scanner.
In 4th Generation- complete ring, 4000 detector in
single row
therefore in 64 slices MDCT, 307,200 detector
required.
-Very expensive, 4th generation design unfeasible
for MDCT.
Todays scanner is based on 3rd generation
MDCT.

26. PITCH-
Distance travel by CT table per 360 degree of x-ray tube/ X-ray beam collimation width
SDCT-
(Width of collimater opening is sole determinant of slice thickness.
•Pitch =1, x-ray beam are contiguous for adjacent rotations.
•Pitch> 1, x-ray beam non-contiguos for adjacent rotations; gaps in helix but full form still ir
only fewer projections per rotation (undersampling)
• Results in decreased patient dose but also decreased image quality (low SNR)
•Pitch<1, x-ray beam overlaps results in overlapping slices.
i.e. volume of tissue irradiated more than once per scan.
-Decreases the amount of anatomy per unit time & increses radiation dose.
-Therefore,not used in SDCT.
In SDCT- can be defined as relationship between table movt and slice thickness.

27. CLINICAL APPROACH-
•Pitch is increased slightly to allow an entire area to be covered in single
breath-hold.
•Most prevelant- CT Angiography.
•PEADS CT- FLASH MODE (ULTRAHIGH pitch CT)- Possible on DSCT

28. PITCH in MDCT- (Beam Pitch)
•Pitch = Table mpvement per 360 degree rotation/ beam width
(beam width=No of slicesx slice thickness)
•Changing the pitch will affect the MDCT scan acquisition in the same way it affects
SDCT.
Cardiac CT Imaging- require high temporal resolution.
•In an effect to increase temporal resolution, two dedicated cardiac CT scanners were
introduced.
1. Dual Source CT
2. Dynamic Volume CT [Aquilion One (TOSHIBA)]

29. IMAGING SYSTEM DESIGN
1.Operating console
2.Gantry/Gantry Aperture
3.X-ray Tube
3.Filtration
4.Collimater
5.Generator
6.Patient Couch
7.Detectors

30. 1.CONSOLE-
•CT imaging system console are can be equipped with 2-3 moniters.
•Moniter 1- used by CT technologist to operate the system & do scan.
•Moniter 2-used by technologist to view and for post-processing of acquired data
before transferring them to radiologist viewing console.
•Usually linked with PACS.
•Moniter 3-specifically for radiologist for reporting purpose.

31. 2.Gantry-
•Donut shaped part of CT scanner.
•It houses many of the component necessary to produce & detect xrays
i.e. Xray tube,detector array,high voltage generator,patient support couch & mechanic
support for each.
•These subsystem receive e- commands from operating console & transmit data to com
for image production & post processing.
•Gantry aperture diameter may vary. (70-90cm)
•Can be tilt-either forward or backward.
•Laser light for positioning.
•Control panel on either side.
•Microphone embedded to allow commuication between patient & technologist.

33. 3.Xray Tube/Source-
•Rotating anode tube is used.
•Target made up of Tungsten often used.
•Focal spot- not more than 0.5mm-1mm.
•Small focal spot is more favourable.
•High speed rotors are used for best heat dissipation
•Heat storage capacity- 8 million heat unit (8MHU)
•Anode cooling rate approx 1MHU per minute
•Cooling mechanism are included in gantry.
•They can take many forms such as- oil cooled & water cooled systems.

34. ADVANCEMENT-
•STRATON TUBE- Direct cooling tube
-Heat storage capacity less
-Cooling rate very high (5MHU/minute)

35. 4.FILTRATION-
•Primary purpose- reduces low energy x-rays.
•Increasing filtration- increases quality of xray beam
-decreases quantity
-improves image contrast
•Most preferable- Aluminium
TYPES-
1.INHERENT- Present in X-ray Tube Housing
• 0.1mm Al equivalent in general purpose x-ray tube.
•In CT, it is 3mm Al equivalent
•With age, inherent filtration increases,due to vaporization of W.
2.ADDED –Thin sheet of Al positioned between protective x-ray tube housing & x-ray beam
collimater.
•1mm Al equivalent.

37. •In ct, filtration serves dual purpose-
-Removal of beam-hardening artifact.
-Shaping of filter to make beam more uniform.

38. 3.Compensating filter-
•Compensates for difference in radiopacity.
•Comes in various shapes & size.
•Eg-Wedge filter,Trough filter, Step wedge, bow-tie filter
Wedge filter for foot examination
Step wedge filter for serial radiography

39. Compensating Filter-
A.Trough Filter
B.Wedge Filter
C.”Bow-tie” filter

40. Beam shaping filters-
attenuate
beam to make beam more
monochromatic

41. b.Pre-detector collimater-
•In MSCT, S.T is also influenced by detector electronic configuration.
•Located below the patient & above the detector array.
•Restricts x-ray beam viewed by the detector array thus reduces scatter radiation.
•Thus pre-detector collimater improves image contrast.
•Also determine sensitivity profile (define slice thickness) when properly coupled with
pre-patient collimater.

42. 6.GENERATOR-
•Earlier, highly stable three-phase generator use.
•MSCT operates on High-frequency generator. (based on high voltage step-up transformer.
7.PATIENT COUCH-
•Made up of low Z material such as carbon fiber so that it does not interfere
with
xray beam transmission & patient imaging.

43. DETECTORS-
Xray beam passes through patient
Attenuation
Collection of data according to the extent of attenuation
Conversion of x-ray photons into electronic signal
Detector (single element)
Detector Array (arc of detectors)- entire collection of data
DETECTOR CHARACTERISTICS-
1.Efficiency (Capture, Absorption & Conversion )
2.Stability
3.Response Time
4.Cross talk
5.Dynamic Range

44. 1.Efficiency- refers to the ability to capture,absorb & convert x-ray photons to electric signal
-Must posses high Capture,absorption & conversion efficiency.
Capture Efficiency-ability with which the detectors obtain photons that have passed
through the patient.
-Depends on size of detector area facing the beam & distance between the two detector
Absorption Efficiency-Number of photons absorbed by detector.
-Depends on physical property of detector.
(atomic number,thickness,material , physical density)
Conversion Efficiency (Detector Quantum efficiency- Ability of detector to change
photons into electronic signal.

45. 2.Stability- refers to steadiness of detector response.
-High stability,allows a system to be used without interruption of frequent
caliberation.
3.Response Time- refers to the speed with which the detector can detect any x-ray
event & recover to detect another event.
-Time required for signal from detector to return to zero.
-It should be very short to avoid afterglow & detector “pile-up”.
4.Cross talk-amount of signal from one detector element that may leak over into
adjacent detector.
5.Dynamic Range- Maximum signal measured
Minimum signal measured

46. DETECTOR TYPES-
Based on the conversion of x-rays into electrical energy , detector are of two types-
1.Scintillation Detectors
2.Gas-filled Detectors
A. Indirect Conversion
B. Direct Conversion

47. 1.Scintillation Detector-
•Also called solid-state or cystal detectors.
Scintillation detector based on
photomultiplier tube
A.Basic structure of a photodiode
B.Electronic symbol of photodiode

48. Materials used as scintillation crystals-
•Earlier- NaI
-Hygroscopic
-Long after glow
-Limited dynamic range
•Cadmium fluoride & Bismuth Germinate
•Currently Cadmium tungstate & ceramic crystals are used.

49. 2.Gas Ionisation Detector-
•Based on principal of ionisation.
•Introduced in 3rd Generation.
•Consist of series of individual gas chambers, seperated from tungsten plates.
•Xrays falls on detector ionisation of gas(usually Xenon) occurs.
•As a result of ionisation-- + & - ions produced.
•Migration of ions to charge plate occurs.
•Migration of ions causes a small signal current that varies directly with no. of photons abs
•Enclosed by thick ceramic materials ,
Because Xe gas presurized to
about 30 atm to increase
no. of gas molecule available for
ionisation.

50. ADVANTAGES DISADVANTAGES
1. Wide dynamic range 1.Low quantum efficiency
(60-70%)
2.Rapid Response 2.Used in 3rd generation
only.
3.No afterglow
SOLID STATE CRYSTAL XENON GAS
1. High photon absorption 1.Moderate photon absorption
2.High quantum efficiency 2.Low quantum efficiency
3.Sensitive to temperature
& moisture
3.Highly stable
4.Solid material 4.Low density material(gas)
5.Can exhibit afterglow 5.No after glow
ADVANTAGES & DISADVANTAGES OF GAS FILLED DETECTORS-
CHARACTERISTICS OF DETECTORS-