Acquiring an image part 1

This section covers the role of the physical equipment in acquiring an image i.e. the gantry and detectors.

Axial vs spiral scanning


Axial scanning


Axial scanning


"Step and shoot"

  1. Gantry stops and rotates to acquire data from single slice
  2. X-rays switched off
  3. Patient moves to next slice
  4. Rotates to acquire data from next slice


Spiral scanning


Spiral scanning


  • Aka helical
  • Gantry keeps rotating continuously releasing x-ray beams.
  • The couch simultaneously moves.
  • This results in a continuous spiral scanning pattern.


  • Avoids respiratory misregistration as scan performed during one breath
  • More effective use of contrast agent as faster scanning enables scanning during multiple phases in one contrast injection e.g. portal venous, angiographic, delayed
  • Overlapping slices allows better reconstruction and helps in showing smaller lesions
  • Pitch > 1 can be used to reduce scan time and / or radiation dose and still cover the same volume

All images are now acquired in this way.



The pitch is the measure of overlap during scanning.

Pitch = distance couch travels / width of slice

CT scanner pitch of 2


Pitch = 20/10 = 2


CT scanner pitch of 1


Pitch = 10/10 = 1


CT scanner pitch of 0.5


Pitch = 5/10 = 0.5


  • A pitch number > 1 = couch travels more than the width of the beam i.e. there are gaps
  • A pitch number < 1 = couch travels less than the width of the beam i.e. there is overlap

For higher pitch numbers:

  • Advantages:
    • Lower radiation dose
    • Quicker scan
  • Disadvantages:
    • More sparsely sampled


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Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.


FRCR Physics Notes: Medical imaging physics for the First FRCR examination

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Multislice scanning

Rather than just have one row of detectors, we now have multiple parallel rows of detectors. Certain rows of detectors can then be selected to change the slice thickness along with the collimator.


Multislice scanning



  • Faster scanning due to wider total active detector width
  • Better dynamic imaging due to faster scanning times
  • Thinner slices
  • 3D imaging is enabled by thin slices
  • Simultaneous acquisition of multiple slices


Detector arrays

Types of Multislice Detector Types:

  1. Linear
  2. Adaptive
  3. Hybrid arrays


1. Linear array

Linear detector array

  • All the rows of the detectors are the same width


2. Adaptive array

Adaptive detector array

  • The elements within the central detector rows are the thinnest and they get wider towards the outside.
  • Advantages:
    • As few detector elements as possible activated to still give a large range of detector slices
    • Fewer detector rows activated means fewer septae dividing up the rows. This improves the dose efficiency.
  • Disadvantage:
    • Upgrading to more data channels requires an expensive detector replacement.


3. Hybrid array

Hybrid detector array

  • Similar to linear arrays in that the elements within the detector rows are the same width across. However, the central group of detector rows are narrower than the outer rows.
  • These are the main detector arrays used for 16-slice scanners and above.

Multislice pitch

There are two methods to calculate the pitch in a multislice scanner. The first (pitchd) is analogous to the single slice pitch and only takes into account the width of the x-ray beam.

Pitchd = couch travel per rotation / width of x-ray beam

However, this does not fully represent the overlapping of the x-ray beam and, instead, pitchx is now used.

Pitchx = couch travel per rotation / total width of simultaneously acquired slices

This is comparable to the definition of pitch for single slice spiral scanning as the total collimated width is analogous to the detector subgroup width in single slice spiral scanning.


  Key points

  • Pitch
    • Single slice pitch = detector pitch = couch travel per rotation / detector width
    • Multislice pitch = beam pitch = couch travel per rotation / total width of simultaneously acquired slices
  • Slice thickness
    • Single slice CT = determined by collimation. Limited by detector row width. 
    • Multisclice CT = determined by width of detector rows

Σ  Summary
  • Spiral scanning now used instead of axial scanning
  • Pitch = distance couch travels / width of beam
    • Pitch > 1 means there are gaps between slices
    • Pitch = 1 means there is no beam overlap
    • Pitch < 1 means the beam overlaps

Multislice scanning

Multislice scanning uses lots of rows and each row consists of equal-sized detectors

  • Rows combined to give different number of slices. Number of slices limited by number of data channels.
  • Older scanners may use one of the following types of detector array:
    • Linear array: all detector rows are of equal width
    • Adaptive array: detector rows are of different widths
    • Hybrid array: central rows narrower than outer rows. Most commonly used array today.

Multislice pitch

  • Pitchx = distance couch travels / total width of slices
  • Pitchd = distance couch travels / detector subgroup width

Next page: Acquiring an image part 2

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