Computed tomography (CT) is a medical imaging technique that uses computer-processed combinations of multiple X-ray measurements taken from different angles to produce cross-sectional images of bones, blood vessels and soft tissues inside the body. CT was introduced in the 1970s and revolutionized medical imaging. A CT scan provides more detailed images than plain X-rays and can show the different soft tissues and blood vessels throughout the body. CT scans can be used to diagnose many different conditions including cancers, infections, blood clots and injuries. While CT imaging provides very detailed images, it also exposes patients to a small amount of ionizing radiation.

1. COMPUTED TOMOGRAPHY
FARAKATH KHAN
GUIDED BY- TATU JOY

2. Introduction
History
Components of CT
Detectors
Generations
Scanning parameters
Patient preparation
Indications
Advantages
limitations
Artifacts

3. TOMOGRAPHY
It is a generic term formed from the Greek words tomos meaning
“slice” or “section” and graphia meaning “picture” or “describing” that
was adopted in 1962 by the international commission on radiological
units and measurements to describe all forms of body section
radiography.
INTRODUCTION

4. CONVENTIONAL TOMOGRAPHY :
It is a special x ray technique that enables visualization of
a section of the patients anatomy by blurring regions of the
patients anatomy above and below the section of interest.
INTRODUCTION

5. Linear tomogram of TMJ
PARASITE LINES
INTRODUCTION

6. COMPUTED TOMOGRAPHY (computerized axial
transverse scanning)
It is a radiographic technique that blends the concept of thin layer
radiography (tomography) with computer synthesis. Alan Cormack in
1960s – image
reconstruction
Godfrey Hounsfield
in 1972 – imaging
technique
120 – 140 kVp
200 – 800 mA
INTRODUCTION

7. CT HISTORY
First test images 1967
First clinical image 1971
First commercial scanner 1972

8. Sir Godfrey Newbold Hounsfield
CT brain 1972-73
Noble prize 1979
HISTORY

9. HISTORY

10. HISTORY

11. HISTORY

12. HISTORY
Siretom scanner

13. CT COMPUTERS
Old Mainframe Computers Too Expensive And Bulky
HISTORY

14. Components of CT system
• The scanning unit, i.e. the gantry, with tube
and detector system
• The patient table
• The image processor for image reconstruction
• The console

15. Scanning unit (gantry)
GANTRY DIMENSIONS
Height-2-2.5 m
Width-2-3 m
Depth-0.5-1 m
Weight-2000 kg

16. GANTRY

18. X RAY TUBE
Focal spot size 0.6*1.2 sq.mm
(small focal spot – high resolution images with thin slice)
SHIELDING
Grids, collimators and filters.
Aperture in gantry-70 cm diameter
Table-for patient movements
Patient alignment lights-high intensity halogens, low intensity
lasers.

20. CONSOLE

21. DETECTORS
Incident x ray
Electric signals
Digital pulses

22. IMAGE RECEPTORS
DIRECT EXPOSURE
FILM
SCREEN FILM
ELECTRONIC
SENSORS
IOPA
BITEWING
OCCLUSSAL
EXTRAORAL
PANORAMIC
CCD
CMOS
FLAT
PANEL
PSP
PLATE
A,B,C,D,E
SPEED

23. • Types
Sodium iodide
Calcium fluoride
Bismuth germinate
Scintillation detectors
(sodium iodide, caesium
iodide)
DETECTORS
Gas ionization
(xenon gas 20-30 atm)
Multi row detector
Adaptive array detector

24. Detector

25. • Sequential CT
• Spiral CT

26. CT scanner consists of a radiographic tube that emits a finely
collimated, fan shaped X ray beam directed to a series of
scintillation detectors or ionization chambers

27. GENERATIONS

30. SCANNER PARAMETERS
• COLLIMATION
• INCREMENT
• PITCH
• ROTATION TIME
• mAs

31. COLLIMATION
Source collimator
Detector collimator
Collimation determine the quality of slice profile
From the data volume of the multislice scanner images
can be reconstructed with slice thickness equal to or
larger than detector collimation.
SCANNER PARAMETER

32. INCREMENT
SCANNER PARAMETER
The distance between images reconstructed from a data volume .
If appropriate increment is used overlapping images can be reconstructed
Clinically useful overlap is 30-50%

33. PITCH
PITCH = TABLE FEED PER ROTATION / IMAGE THICKNESS
SCANNER PARAMETER
Larger table feed, faster scanning (few rotation) but image quality impaired

34. ROTATION TIME
Is the time interval needed for a complete 360 degree
rotation of tube detector system around the patient.
Ultra modern Ct system-0.4 seconds for one rotation
mAs
mAs value is the product of tube current and rotation
time.
The selected mAs and tube voltage determine the
dose.
SCANNER PARAMETER

35. Patient preparation – CT Head and Neck
Fasting 4 hrs prior to scan, remove outdoor
coats.

36. Radiation dose
INHERENT DESIGN
• Type Of X Ray
Tube
• Collimation
• Beam Splitting
Device
• Tube Patient –
Detector Distance
RADIOGRAPHIC TECHNIQUE
• KVp
• Patient positioning
• Slice thickness
• Slice spacing
• Specialized examination
CARE

37. Swennen and Schutyser. American Journal of Orthodontics and Dentofacial Orthopedics
Volume 130, Number 3

38. REFERRENCE BASE LINES :
1. Radiographic base line
2. Reid’s base line

39. IMAGE GENERATION
ATTENUATION
PROFILE
KERNEL
CONVOLUTION
SHARP IMAGE
Mathematical high-pass filter prior to the backprojection
The mathematical operation is ”convolution”.

40. CT IMAGES
High x ray attenuated areas –white
Matrix of individual blocks-VOXEL( volume elements)
Array of individual points-PIXELS
Each pixel is assigned a CT number or Hounsfield unit between + 1000 to -1000
CT number or PIXEL represents attenuation or density

41. –1024 HU to +3071 HU.

42. Windowing- the term used for the method of varying density and contrast.
Window width-range of CT numbers we select for display
Window level-is usually but not always, the central CT number about which the window is chosen

44. Soft tissue filter
Bone filter
WL – 50
WW - 200
WL – 50
WW - 400
WL – 600
WW - 1700
WL – 60
WW - 1700

45. TYPES
• HELICAL OR SPIRAL CT
• MULTISLICE SPIRAL CT
• 256 SLICE CT
• DUAL SOURCE CT
• INVERSE GEOMETRY CT

46. THREE DIMENSIONAL CT
MULTIPLANAR REFORMATTING (MPR)
Rectangular voxel - multiple cuboidal voxel – interpolation
(Creation of these new cuboidal voxels allows the image to be reconstructed in
any plane without loss of resolution by locating their position in space relative to
one another)
SHADED SURFACE DISPLAY (SSD)
MAXIMUM INTENSITY PROJECTION (MIP)
VOLUME RENDERING

47. Cuboid voxels can be created from the
original rectangular voxel by computer
interpolation. This allows the formation of
multiplanar and three-dimensional images

48. – CT ANGIOGRAPHY
– DENTASCAN IMAGING

53. ARTIFACTS IN CT:
1. SYSTEM RELATED
2. PATIENT RELATED
Motion Metal

54. INDICATIONS
1.Investigations of intracranial diseases
2. Investigations of suspected intracranial and spinal cord
damage
3. Assessment of fractures
4. Tumour staging assessment.
5. Investigations of tumors and Tumour like discrete swellings
intrinsic and extrinsic to the salivary glands.
6. Investigation of TMJ
7. Preoperative assessment of maxillary alveolar bone height
and thickness before inserting implants

55. ADVANTAGES OF CT:
1. Structural relationship of hard and soft tissue
2. Ability to rotate image and add or subtract structural
components.
3. Hidden surfaces can be examined
4. Accurate linear and volumetric measurements
5. Sequential scans – changes in linear or volumetric
measures
6. Eliminates superimposition of images of structures
7. Multiple contiguous or one helical image in all planes

56. LIMITATIONS OF CT:
1. Effect of blurring is greater than in conventional
radiographs
2. Resolution is limited ( size of pixel > size of silver
specks )
3. Not fine detail as conventional
4. Application in longitudinal monitoring of implant
prosthesis is limited & contraindicated
5. Metallic objects produce artifacts
6. Very expensive equipment

57. REFERENCES
 Stuart C. White, Michael J. Pharoah. Oral Radiology Principles and
Interpretation. 6th Edition pg 207 – 243.
 Freny R Karjodkar, Text Book of Dental & Maxillofacial Radiology. 2nd
Edition, Pg 256 – 335 and 381 – 405.
 David MACDonald. Oral & Maxillofacial Radiology. A Diagnostic
Approach. Pg 47 – 90
 David A. Lisle. Imaging for Students, 2nd Edition. Pg 8 -32

58. Thank you