1. BASICS IN
COMPUTED TOMOGRAPHY

2. INTRODUCTION
Designed by Godfrey N. Hounsfield to overcome the visual representation challenges in radiography and conventional tomography by collimating the X-ray beam and transmitting it only through small cross-sections of the body

3. G.N.HOUNSFIELD ALLAN M. CORMACK
In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology & Medicine with Allan MacLeod Cormack, Physics Professor who developed solutions to mathematical problems involved in CT

4. Important events YEAR EVENTS 1969
G.N. Hounsfield developed first clinically useful CT head scanner
1971
First clinically useful CT head scanner was installed at Atkinson-Morley Hospital (England)
1972
First paper on CT presented to British Institute of Radiology by Hounsfield and Dr. Ambrose
1974
Dr. Ledley introduced the whole body CT scanner (ACTA scanner)
1979
G.N. Hounsfield shared the Nobel Prize with Allan MacLeod Cormack

5. C.T. scan Computed tomography scan machines uses Xray
CT combines X radiation and radiation detectors coupled with a computer to create cross sectional image of any part of the body
Think like looking into a loaf of bread by cutting it into thin slices and then viewing the slices individually

6. BASIC PRINCIPLE
The internal structure of an object can be reconstructed from multiple projections of the object
CT scanning is a systematic collection and representation of projection data

7. Comparison of CT with Conventional Radiography
Conventional radiography suffers from the collapsing of 3D structures onto a 2D image
CT gives accurate diagnostic information about the distribution of structures inside the body

8. Comparison of CT with Conventional Radiography
Imagine you are standing in front of a wall, holding a pineapple against your chest with your right hand and a banana out to your side with your left hand. Your friend is looking only at the wall, not at you. If there's a lamp in front of you, your friend will see the outline of you holding the banana, but not the pineapple -- the shadow of your torso blocks the pineapple. If the lamp is to your left, your friend will see the outline of the pineapple, but not the banana. In order to know that you are holding a pineapple and a banana, your friend would have to see your shadow in both positions and form a complete mental image. This is the basic idea of computer aided tomography
In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles

9. Comparison of CT with Conventional Radiography

10. Comparison of CT with Conventional Tomography
Limitations of Conventional tomography
Image blurring
Problems with Film/screen combination

11. Image Formation and CT Numbers
Shades of gray that make up a CT image are determined by the density of a structure and the amount of x-ray energy that passes through it. This phenomenon is referred to as the attenuation of the x-ray
The degree of beam attenuation on a CT image is quantified and expressed in Hounsfield units (HUs), which are also referred to as CT numbers
air : –1000 HU water: 0HU cortical bone: HU
MATTER
LINEAR ATTENUATION COEFFICIENT
( µ )
FAT
0.194
WATER
0.222
CSF
0.227
PLASMA
0.230
RED BLOOD CELLS
0.247

12. Terminology
Hounsfield Unit (HU)- mean attenuation of x-rays by different tissues
Data Acquisition- method by which the patient is scanned to obtain enough data for image reconstruction
Beam Geometry- the size, shape, and motion of the x-ray beam and its path
Ray- part of the beam that falls on the detector.
Scan plane- region where the x-ray tube and detectors rotate

13. GENERATIONS
Data gathering techniques have developed in stages termed Generations
relate the configuration of the x-ray tube to the detectors
Scan time reduction is the predominant reason for introducing new configurations

14. FIRST GENERATION Narrow pencil beam Single detector per slice
Translate –Rotate movements of Tube- detector combination
Scan time 5min
Designed only for evaluation of brain, axis of rotation passed through the centre of the patient’s head
Over night image reconstruction

15. 1st Generation CT Scanner
Head kept enclosed in a water bath
Two side-by-side detectors
A reference detector

16. SECOND GENERATION Narrow fan beam(30-100) Linear detector array(30)
Translate-Rotate movements of Tube-Detector combination
Fewer linear movements are needed as there are more detectors to gather the data
Between linear movements, the gantry rotated 30o
Only 6 times the linear movements got repeated
Scan time~20secs

17. THIRD GENERATION
Rotate(tube)-Rotate(detectors) Translatory motion is completely eliminated
Pulsed wide fan beam(500)
Arc of detectors( )
Detectors are perfectly aligned with the X-Ray tube
Both Xenon and scintillation crystal detectors can be used
Scan time< 5secs

18. FOURTH GENERATION Continuous wide fan beam(500-550)
Ring of detectors(> 2000)
Rotate(tube)-Fixed(detector)
X-ray tube rotates in a circle inside the detector ring
When the tube is at predescribed angles, the exposed detectors are read
Scan time< 2 secs

19. IIIrd Vs IVth Gen. CT scanners

20. CT Data Acquisition Components

21. DATA ACQUISITION The scanning process begins with data acquisition.
Data Acquisition refers to a method by which the patient is systematically scanned by the X ray tube and detectors to collect enough information for image reconstruction.
A basic data acquisition scheme consists of
X ray tube
Filters
Collimators
Detectors

22. CT Gantry

23. CT gantry internal components
1.X-ray tube & collimator
2.Detector assembly
3.Tube controller
4.High freq. generator
5.Onboard computer
6.Stationary computer

24. CT Patient Couch

25. X-RAY TUBE Rotating anode type
More heat loading and heat dissipation capabilities
Small focal spot size (0.6mm) to improve spatial resolution

26. FILTERS Compensation filter is being used To absorb low energy x rays
To reduce patient dose
To provide a more uniform beam

27. COLLIMATORS To decrease scatter radiation To reduce patient dose
To improve image quality
Collimator width determines the slice thickness

28. DETECTORS Scintillation crystal detector
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

29. Detector Cross-talk
Detector cross talk occurs when a photon strikes a detector, is partially absorbed and then enters the adjacent detector and is detected again
Crosstalk produces two weak and signals coming from two different detectors
Crosstalk is bad because it decreases resolution
Crosstalk is minimized by using a crystal that is highly efficient in absorbing X-rays (high stopping power)

30. Gas filled detector’s efficiency
Gas filled detectors are less efficient than solid state detectors.
The problem can be partially overcome by the following 3 ways.
By using Xenon (z=54), the heaviest of the inert gases
By compressing the Xenon 8 to 11 atmospheres to increase its density
By using a long chamber to increase the number of atoms along the
path of the beam.

31. Disadvantage of Xenon gas detector
Efficiency ~60%
This low efficiency is caused by two factors
Low density of the absorbing material
Absorption of X-rays by the front window, which is needed to contain the high pressure gas

32. OTHER SCAN CONFIGURATIONS
Interest in faster scan times evolves from a desire to image moving structures such as the wall of the heart and contrast material in blood vessel and heart chambers and to overcome motion artifacts due to cardiac rhythm and patient breathing
Electron beam computed tomography
Dynamic Spatial Reconstructor(DSR)

33. ELECTRON BEAM COMPUTED TOMOGRAPHY
Electron gun
Large Arcs of tungsten targets
Detector ring
17 slices per second

34. What is the radiation dose with EBT?
EBT scanning is usually 1/5th to 1/10th the radiation exposure as Spiral CT scanning.

35. Radiation dose from EBT scans compared to other sources of radiation
EBT Coronary Calcium Scan to 70 mrem
Non-invasive Coronary Angiogram to 120 mrem
EBT Low-dose Lung Scan to 150 mrem
EBT Abdominal/Pelvis Scan to 300 mrem
Background Sunshine Radiation in 1 year mrem
Cross country airplane trip mrem
Standard Chest X-ray mrem
Standard Abdominal X-ray mrem
Standard Spine X-ray series mrem
Standard Coronary Angiogram to 1000 mrem
Standard G.I. X-ray series mrem
Spiral CT Whole body scan to 1000 mrem
Recommended safety limits for radiation exposure is <5,000 mrem per year.

36. Electron beam computed tomography (EBCT)
Electron beam computed tomography scanning is a new test that can be used to detect calcium buildup in the lining of arteries. It may be used as a screening tool to detect hardening of the arteries in people who are at high risk of developing atheroscerosis.
Electron beam CT scanning is much faster than standard CT scanning. Electron beam CT scanning can produce an image in a fraction of a second and can take an accurate picture of an artery even while the heart is beating
Standard CT scanning is not fast enough to take pictures of a pumping heart. In standard CT, several pictures, or "slices," of the heart are taken from different angles. These pictures are then analyzed using a computer to create a three-dimensional view of the heart.

37. Electron beam computed tomography (EBCT)
Results
Fat and calcium buildup may be seen in the arteries during an electron beam CT scan. Aggressive treatment for atherosclerosis may be appropriate.
If electron beam CT scanning does not show the presence of calcium buildup in the arteries, then the chances of having CAD are low.
A high calcium score on an electron beam CT scan indicates a greater risk of having cardiovascular problems within the next 2 to 5 years, especially when a person also has multiple risk factors for developing coronary artery disease. However, the scan does have a fairly high rate of false +ve results.

38. DYNAMIC SPATIAL RECONSTRUCTOR
is an experimental apparatus
28 X-ray tubes
X-ray tubes are aligned with 28 light amplifiers and TV cameras that are placed behind a single curved fluorescent screen
The gantry rotates about the patient at a rate of 50 RPM
Data for an image acquired in about 16 ms.
Reconstruct 250 C.S. images from each scan data

39. DSR

40. Disadvantages of DSR
High Cost
Mechanical motion is not eliminated

41. Dynamic Spatial Reconstructor
The Dynamic Spatial Reconstructor (DSR) is a high-temporal resolution, three-dimensional (3-D) X-ray scanning device based on computed tomography (CT) principles
designed for investigation of quantitative studies of cardiovascular structure and function
Initial results show that 3-D dynamic images can be obtained from patients with minimal invasiveness and that these images may provide useful diagnostic information

42. Image Quality in CT Image quality is the visibility of diagnostically
important structures in the CT image.
The factors that affect CT image quality are
Quantum mottle (noise)
Resolution : Spatial and contrast
Patient exposure
The factors are all interrelated

43. Quantum mottle (Noise)
Quantum mottle is the statistical fluctuations of X-photons absorbed by the detector
The only way to decrease noise is to increase the number of photons absorbed by the detector
The way to increase the number of photons absorbed is to increase x-ray dose to the patient
Mottle becomes more visible as the accuracy of the reconstruction improves

44. RADIATION DOSE
Even distribution of radiation dose to the tissues as exposures are from almost all angles
No overlapping of scan fields takes place
Exposure factors used are higher to improve spatial and contrast resolutions and to reduce noise

45. CT ARTIFACTS
Artifacts are distortions or errors in the image that are unrelated to the object scanned
Most common artifacts in CT are
Motion artifacts
Streak artifacts
Beam hardening artifacts
Partial volume averaging artifacts
Ring artifacts

46. EFFECTS OF ARTIFACTS DETERIORATE IMAGE QUALITY
SUBJECT INFORMATION IS LOST
PATHOLOGICAL DETAILS ARE LOST

47. MOTION ARTIFACTS Cause : Patient movement
Appearance: Blurred / streaks / ghost images
Rectification:
reduction in scan time
Clear and concise instruction to the patient
proper patient immobilization
if needed,administration of sedatives/antiperistaltic drugs

48. Motion artifact

49. STREAK ARTIFACTS
Cause: Presence and movements of objects of very high density(contrast media, metallic implants,surgical clips)
Appearance: Streaks
Remove the offending object if possible.

50. DENTURES PRODUCING STREAK ARTIFACT
SURGICAL
CLIP IN HEART
PRODUCING
STREAK ARTIFACT

54. OUT OF FIELD ARTIFACT CAUSE:- Scan FOV not covering the entire anatomy
APPEARANCE:-
Shading/streaks
REMEDY:- Ensure that scan field of view is larger than the object to be scanned

55. RING ARTIFACTS
CAUSE : Detector failure or miscalibration of a detector
APPEARANCE:- Ring
Rectification : regular quality assurance checks

56. RING APPEARANCE