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
"Step and shoot"
- Gantry stops and rotates to acquire data from single slice
- X-rays switched off
- Patient moves to next slice
- Rotates to acquire data from next slice
- 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 from different breaths in 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
Pitch = 20/10 = 2
Pitch = 10/10 = 1
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:
- Lower radiation dose
- Quicker scan
- More sparsely sampled
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.
- 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
Types of Multislice Detector Types:
- Hybrid arrays
1. Linear array
- All the rows of the detectors are the same width
2. Adaptive array
- The elements within the central detector rows are the thinnest and they get wider towards the outside.
- 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.
- Upgrading to more data channels requires an expensive detector replacement.
3. Hybrid 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.
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.
- 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
- Spiral scanning now used instead of axial scanning
- Pitch = distance couch travels / width of beam
- Pitch = 1 means there is no beam overlap
- Pitch < 1 means the beam overlaps on subsequent rotations
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.
- Pitchx = distance couch travels / total width of slices
- Pitchd = distance couch travels / detector subgroup width