single slice vs multislice CT

1. SINGLE SLICE ct
vs
MULTI SLICE CT
Shashi Bhushan
B Sc MIT

2. BEFORE CT
•Entire areas of body inaccessible to
radiography (brain, retroperitoneum,etc.)
•Some useful diagnostic procedures were
either potentially harmful or considerably
uncomfortable (exploratory laparotomy,
peumoencephalography)

3. WHY CT?
•Better contrast resolution
•No superimposition of tissues.
•3D imaging

4. PRINCIPLES OF CT
• Radiographic tube emits x-rays while rotating axially around
patient.
• Array of detectors on opposite side of patient detects x-rays
transmitted through patient.
• Computer algorithms use digitized data from detectors to create
axial tomographic images of body.
• CT = tomography + algorithms + high
speed digital computer

6. Development of Workable CT
Scanner
• 1963- Cormack in S. Africa develops algorithm for
accurate reconstruction of images from
radiographic projections.
• 1971- Hounsfield, a computer engineer in
England produces first working CT scanner used
clinically on patients.
Cormack and Hounsfield awarded Nobel
Prize in medicine and physiology in 1979.

7. Sir Godfrey Hounsfield with a
prototype CT scanner in 1974

8. Helical (spiral) CT

9. Helical (spiral) CT
• Simultaneous patient translation and x-ray scanning generates
volume of data.
• X-ray beam traces a helix of raw data from which axial images
must be generated
• Each rotation generates data specific to an angled plane of
section.
• To create true axial image, data points above and below
desired section must be interpolated to estimate value in axial
plane.
• Thus, interval between
reconstructed transaxial images can be
chosen retrospectively.

10. • Transverse images can
be reconstructed at
any z-axis position.

11. • Movement of x-ray
tube is not a spiral.
• Appears so because of
translation movement
of the patient.

12. INTERPOLATION

13. Technological considerations of
helical CT
• Slip-ring technology (no electrical cables
connecting gantry to ground) allows source
detector assembly to rotate continuously.
• Previously, frequent, abrupt changes between
scans were necessary to permit winding and
unwinding of cables.

14. SLIP RING
Introduced in 1990
allowed continuous
rotation of the tube.
Power and signals
transmitted to gantry using
Brushes on Static Rings.
No need to start and stop
the rotation.
Scan time as fast as 0.3
sec.Brushes
Static Rings.

15. Type of spiral CT
Single slice spiral CT
Multi slice Spiral CT
(MSCT)

16. Single Slice CT Scan
1.) Acquires one slice at a
time. The table moves to the
start the acquisition of next
slice.
2.) Long acquisition time
3.) Creates artifacts in images
(partial volume artifact).
A) Single Slice CT Scan

17. Single Slice Helical CT Scan
1.) Tube rotates and Couch moves
continuously
2.) X-ray tube is always on and rotating
3.) Instead of one slice it scans a volume
4.) Covers larger area of the patient in
one single breath hold
5.) Less artifact compared to single slice
CT Scan
Note:
Development of Slip ring Technology
helped the development of Helical scan
Systems.
Single Slice Helical CT Scan

18. B) Multi Detector CT
• Introduced in 1998
• It had the ability to acquire four or more
images with each rotation of the gantry, which
has made the significant impact in patient
throughput.
• They are combined with high heat capacity x-
ray tubes.

20. Multi Detector System
Multi Slice Helical CT Scan

21. Multi slice Ct Scan
1.) Uses the principle of Helical scanner
2.)Acquires Multiple slices per tube rotation
3.) Covers more volume of the patient in one rotation and less time.

23. MSCT: Concepts and Issues
• Detector Thickness: the Z-axis
size of each basic detector
element
• Detector Collimation: total
combined size of elements
“linked” to acquire individual
slices.
• Beam Collimation: total Z-axis
x-ray beam width.
• Slice Thickness: determined by
detector collimation and not
by x-ray beam collimation.

24. What Helped the development of Multi Slice CT Scan
1.) High heat capacity X-ray tubes (upto 8 MHU).
2.) Multi Detector Technology.
3.) Dual focal spot Technology.
4.) Fast Reconstruction Algorithm maintaining good image quality.

25. Tubes with High Heat capacity were developed

26. Development of Dual focal Spot Technology

29. COMPARISON OF SINGLE SLICE ct AND
MULTI-SLICE CT
Detector configuration
Reconstructions
Pitch
image quality

30. 1. DETECTORS CONFIGURATION
MS- detector array segmented in z axis, a
mosaic
SS- long, narrow array with length of single detector
aligned in z axis.

31. Mosaic Detector (for 16 slice CT)
• 16 cells in Z direction --each cell 1.25 mm
(in Z axis).
• 16 cells (Z) x 912 cells (transverse) = 14592
total cells.

32. 2. RECONSTRUCTIONS
SS- reconstruct images of SAME thickness with different
image indexing (table increment intervals)
 MS- acquire 3D raw data that are contiguous in
space. Therefore can reconstruct images at various
thicknesses AND at different intervals.
–If image index < image thickness, results in
overlapping slices
–Must have raw data available for any type of
reconstruction

33. RECONSTRUCTIONS algorithms
SS and MS are similar
First step done by machine, z axis interpolator
works on raw data to weight projections nearest
the slice location most heavily.
Second step selected by user: for soft tissue
images, want to suppress noise and increase low
contrast sensitivity. For bone want higher
contrast.

34. 3D RECONSTRUCTIONS
• Multiplanar reconstruction (MPR)
• Volume rendering (VR)
• Maximum intensity projection(MIP)
• Surface Shaded Display (SSD)
.

35. • A volume is built by
stacking the axial slices.
The software then cuts
slices through the volume
in a different plane
1. Multiplanar reconstruction

36. • Volume rendering is a
set of techniques used
to display a 2D
projection of a 3D
discretely sampled dat
a set
2. Volume rendering

37. • voxels with
maximum intensity
traced from the
viewpoint to the
plane of projection.
3. Maximum intensity projection

38. 4. Surface Shaded Display(SSD)

39. 3. PITCH
• Beam pitch:
relates patient translation per 360 degree revolution to the
width of the x-ray beam
• Slice pitch:
has the same form as beam pitch except the denominator is the
slice thickness rather than the beam width.

40. Pitch
• Pitch= ratio of the table increment during a 360
degree rotation to the collimated beam
thickness.
• Pitch = Table Movement per Rotation
Slice Collimation
40

41. For single slice CT, pitch is defined as the patient couch
movement per 360 rotation divided by slice thickness .
41

42. For MSCT, there are two definitions for pitch
a) Beam pitch
b) Slice pitch
42

43. 43

44. Data Acquisition
Pitch = Table Movement per Rotation
Slice Collimation
• Contiguous Spiral
Pitch = 1 (10 mm / 10 mm)
• Extended (Non-Contiguous) Spiral
Pitch = 2 (20 mm/ 10 mm)
• Overlapping Spiral
Pitch = 1/2 ( 5 mm / 10 mm)

46. Pitch and Noise
• To reconstruct image, projections must be collected over
180degree gantry rotation and fan angle of x-ray beam
(45 degree), about 2/3 of spiral.
• Since reconstruction algorithms need fixed number of
projections to make image and since pitch only affects
how these projections are distributed in spiral, not the
number of projections, pitch does not affect noise
• No difference between SS and MS

47. 4. image quality
Comparison b/n ssct & msct
SSCT
• Resolution.
• Noise.
• Motion blur.
• Partial volume artifact.
MSCT
• Resolution.
• Noise.
• Motion blur.
• Partial volume
artifact.

48. Advantages
of
Multislice CT Scan
48

49. Advantages of multi-slice over single-slice
•Same acquisition in shorter time
•Thin slices give better z-axis resolution
•Scan larger volumes in the same time

50. 50

51. 51

52. 52

53. applications
Of Msct
•significant reduction in scan time.
• four to eight times faster than with single slice spiral
CT.
•The speed of acquisition allows greater coverage with
thinner slice thickness- enabling high quality
reconstructions and CT angiograms.
•allows better image quality for other post-processing
techniques like virtual endoscopy.

54. IsotropicImaging

55. IsotropicImaging

56. Cardiac Imaging
1.) Noninvasive CT angiography can be used to rule out significant disease in
major vessels and their branches.
2.) CT Angiography can pinpoint blockages,stenoses,aneurysms, or other kinds
of lesions
56

57. 1.) Multiplanar Visualization is highly helpful for examination of musculoskeletal
system
2.) Fastly replacing X-ray myelogram for spine and nervous system
57

58. Virtual endoscopy
This is useful in acute stroke to assess cerebral ischaemia and
areas at risk and results from this can be used to prevent
stroke progression.

59.  Multi-slice scanning has done a lot to transform
the way we use CT in the last four years
• Improvements in existing techniques
• New techniques
 Power and speed of machines increasing all the
time
• 16 slice scanners now available
• Reconstruction and data handling / viewing have
to keep pace
 Dose from CT becoming increasingly important
• With power comes responsibility to use it
carefully
Conclusions