The image quality depends on:
- Resolution
- Matrix
- Field of view (FOV)
- Slice thickness
- Signal-to-Noise Ratio (SNR)
- Contrast
- Artefacts
Artefacts are covered later and contrast depends on the scan parameter.
Resolution
The resolution is the size of the individual pixel (2D) or voxel (3D). The smaller the pixel or voxel the greater the resolution. It is intimately related to the field of view, the matrix size and the slice thickness as shown in the equation below.
Pixel size = field of view / matrix
Voxel volume = (field of view / matrix) x slice thickness
Matrix
The matrix size is the number of pixels in the images. Increasing the matrix size will increase the number of pixels in the image but, as they are still within the same field of view, they will be smaller.
Increasing the matrix size:
- Increases spatial resolution – smaller pixels/voxels means better detail
- Decreases signal – there are fewer photons per voxel so the signal is less
- Increases scan time – more voxels need to be acquired (note this is only in the phase encoding direction as each voxel requires a new signal) i.e. more voxels means more signals need to be created.
Field of view
The field of view (FOV) determines the size of the area to be imaged; a larger field of view means a larger area imaged. The matrix size remains the same and so, to fill up a larger area, the voxel becomes larger.
Increasing the FOV:
- Increases the signal – a larger voxel means more signal received per voxel
- Lower resolution – the voxels become larger
- Increased viewing area
Slice thickness
Increasing the slice thickness:
- Increases the signal
- Decreases the resolution
- Increases the partial volume effect
- Gives larger object coverage
Slice Gap
The slice gap is the amount of space between slices. It is measured as a percentage of the slice thickness. In the real world, slices are not perfect and the signals form a bell-shaped curve. The slice gap is the gap between the peaks of these curves. We want to minimise the amount of space between each slice to prevent sections being missed. However, when slices overlap an area of cross-talk results which causes artefacts (the overlapping area contains signal from both slices and the protons become saturated resulting in no signal). Usually, a gap of 10-20% is used to minimise the cross talk.
Increased slice gap:
- Less cross-talk
- Increased coverage – slices placed further apart and, therefore, cover a larger area.
Another way to avoid cross-talk artefact is to image non-contiguous slices (e.g. slices 1, 3 and 5 in one sequence and then 2, 4 and 6).
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Signal-to-noise ratio
The signal-to-noise ratio (SNR) is a useful concept in every modality of radiology. There is always background noise in x-ray, CT and MRI. To get a useful picture, the amount of signal from the thing being imaged should be greater than the noise. A higher SNR means a better and more useful image (more signal than there is noise).
The greater the size of the voxel / pixel the more signal there is per point in the image, improving the SNR. However, a greater voxel / pixel means each point in the image is larger and the resolution is lower.
Higher resolution = lower SNR (assuming all other factors remain equal)
Number of Acquisitions
Another way to improve the signal is to scan the same area several times. This is determined by the number of acquisitions (Number of Signal Averages (NEX/NSA)). Each acquisition fills k-space. We can repeat the number of acquisitions and then average the signals to create the image, thus collecting more signal per slice imaged.
Increasing NEX:
- Increases signal – however, the signal is only increased by √NEX (doubling the NEX only increases the SNR by √2 i.e. 1.4)
- Less noise
- Fewer artefacts due to signal averaging
- Increased scan time – doubling the NEX doubles the scan time
Σ Summary
- Pixel area = field of view / matrix
- Voxel volume = (field of view / matrix) x slice thickness
- Increasing the matrix size:
- Increases spatial resolution
- Decreases signal
- Increases scan time
- Increasing the FOV:
- Increases the signal
- Lower resolution
- Increased viewing area
- Increasing the slice thickness:
- Increases the signal
- Decreases the resolution
- Increases the partial volume effect
- Gives larger object coverage
- Increased slice gap:
- Less cross-talk
- Increased coverage
- Increasing NEX:
- Increases signal
- Less noise
- Fewer artefacts due to signal averaging
- Increased scan time
- To improve the SNR:
- Increase NEX
- Lower resolution
- Thicker slices
- Larger FOV
- Use surface coils
- To improve the resolution:
- Increase the matrix
- Decrease the FOV
- Decrease the slice thickness
