As with other modalities the three major factors that determine image quality are:
- Contrast
- Noise
- Spatial resolution
Contrast
In radionuclide imaging contrast is created by the differential uptake of a radiopharmaceutical agent. Lesions can have negative (smaller lesion activity than surrounding tissue) or positive (larger lesion activity) contrast.
Subject contrast
The subject contrast is a property of the imaged object i.e. the radioactivity level in a lesion relative to healthy tissue. It is calculated as:
CS = subject contrast
AL = activity per unit volume of the lesion
AT = activity per unit mass of the healthy tissue
Image contrast
Image contrast is the difference in the display between the lesion and surrounding healthy tissue. It is represented as the counts per unit area and also called the count density or information density. It can be expressed as counts per pixel.
CI = image contrast
SL = counts per unit area of the lesion
ST = counts per unit area of the health tissue
Factors reducing image contrast
- Smaller subject contrast
- Greater background gamma radiation (for positive contrast radiopharmaceuticals)
- Background radiation gives a count density that is overlayed on to the whole image.
- Sources of background radiation include radioactivity in tissue above and below the lesion and other radioactive sources (in vicinity of patient or in environment)
- Using a collimator: photons penetrate septae and count density decreases
- Scattered radiation from patient
- Attenuation of the gamma radiation from a deep lesion which is much greater than for surrounding healthy tissue
- Greater patient movement
- Low spatial resolution of the gamma camera
Noise
Radionuclide imaging is an inherently noisy investigation. Too much noise will impair detectability of an object, especially if it is a low contrast object.
Structured noise:
- Non-random count density that interferes with object of interest due to:
- Uptake in structure that is not of interest e.g. muscle uptake in PET after exercise, bowel uptake of gallium-67 citrate
- Imaging system artefacts e.g. non-uniformity of the gamma camera
Random noise:
- Aka statistical noise or quantum mottle
- Due to random variations in count density as a result of random activity of radioactive decay
- More significant contributor of noise
Calculating noise
Relative noise (noise contrast) decreases as the count number (signal) increases.
σ = √N
↓
SNR = N/σ = N/√N = √N
↓
CN = σ/N = √N/N = 1/√N
↓
CN = 1/√AS
σ = random noise (a standard deviation)
N = counts
SNR = signal to noise
CN = noise contrast
A = area
S = count density
Factors reducing noise
- Longer acquisition time
- Increased activity of radiopharmaceutical (for given acquisition time)
- More sensitive gamma camera (however, increasing the sensitivity to decrease the relative noise also decreases the spatial resolution and contrast)
Spatial resolution
Spatial resolution is quantified as the full width at half maximum (FWHM) measurement on a graph of counts or count rates vs distance. This is either measured when a radioactive point source (point source function, PSF) or when a line source (line source function, LSF) is imaged. The LSF is more commonly used. A Fourier transform of the LSF gives the modulation transfer function (MTF) which quantifies how accurately the image represents the object.
Intrinsic spatial resolution (RI)
The intrinsic spatial resolution is the maximum resolution achievable by the detector and electronics and depends upon many factors:
- Energy and linearity correction of scintillation
- Range of light in scintillation crystal. A thicker crystal means more spread and variation in the depth of the signal which reduces spatial resolution
- Higher gamma photon energy = more scintillation photons = smaller statistical variation which improves spatial resolution
- Optimised collection and detection of scintillation photons: good optical coupling and photomultiplier tube (PMT) shape (square or hexagonal better than circular), more PMTs
- Only PMTs above certain voltage contribute to signal (eliminates noise)
Intrinsic spatial resolution at 140 keV is between 2.5 mm FWHM (0.4 lp/mm) and 4 mm FWHM (0.25 lp/mm).
Collimator spatial resolution (RC)
For a parallel hole collimator the collimator spatial resolution is:
RC = collimator spatial resolution
d = hole diameter
b = distance from radiation source to collimator
h = hole length
From this equation you can see that resolution is improved by using a collimator with long holes of small diameter positioned as close to the patient as possible. However, there is still rapid degradation of spatial resolution the deeper the imaged object lies. Taking images from different orientations helps to minimise this.
System spatial resolution (RS)
The RS takes into account the intrinsic and the collimator spatial resolution to give the spatial resolution of the whole system.
RS = system spatial resolution
RI = intrinsic spatial resolution
RC = collimator spatial resolution
Factors reducing resolution
- Low intrinsic spatial resolution of the gamma camera
- Thick scintillation crystal
- Small number of PMTs
- Low threshold for PMT voltage to contribute to signal
- Low spatial resolution of the collimator
- Large diameter holes
- Short holes
- Far from patient
- Increased patient motion
- Imaging deeper structures
- Large display pixels
- Increased scattered radiation
- Improved with narrower energy acceptance window
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
PET
Contrast
- Tomographic technique overcomes reduced contrast caused by radiation in front of and behind the lesion
- Random and scatter coincidences reduce contrast
Noise
Noise reduced by increasing sensitivity of the system which is determined by:
- Intrinsic detector efficiency
- Scintillation crystal with higher LAC and more depth = better absorption of gamma photons = greater sensitivity
- Geometric detection efficiency
- Higher number of gamma photons that reach detector = greater sensitivity
- Better in 3D than 2D acquisition
- Width of photopeak acceptance window
- Wider photopeak acceptance window = greater sensitivity
- However, also increases scatter coincidence detection rate which reduces contrast
Resolution
- Positron range
- Distance from site of disintegration to annihilation
- Longer range = poorer spatial resolution
- 15O is 2 mm, 18F is better at 0.6 mm
- Non-colinearity of the annihilation photons
- If positron or electron have residual momentum at time of annihilation the angle between the paths of the two gamma photons produced will not be exactly 180°
- The greater the deviation the poorer the spatial resolution
- Detector element size
- Smaller elements = better spatial resolution
- Thickness of crystal
- Thicker crystal = poorer resolution
- Resolution better through centre than periphery of detector ring
- Reconstruction filter
- PET has much higher count rate sensitivity than SPECT and so noise is less of a problem
- PET images can be reconstructed with much higher spatial frequency
Σ Summary
Nuclear imaging
Contrast
- Subject contrast = property of imaged object
- Image contrast = property of the displayed image
- Reduced by:
- Small subject contrast
- High background radiation
- Collimator
- Scatter
- Deep lesions being more attenuated
- Patient movement
- Low gamma camera spatial resolution
Noise
- Structured: non-random due to uptake in structures not of interest or imaging system artefacts
- Random
- Reduced by:
- Long acquisition time
- Increased radiopharmaceutical activity
- More sensitive gamma camera
Spatial resolution
- Measured as full width at half maximum (FWHM)
- Intrinsic spatial resolution: maximum resolution achievable by detector and electronics
- Collimator spatial resolution: improved by using collimator with long holes of small diameter positioned as close to the patient as possible
- System spatial resolution: takes into account intrinsic and collimator spatial resolution
- Reduced by:
- Low intrinsic spatial resolution of gamma camera
- Low spatial resolution of collimator
- Patient movement
- Deeper structures
- Large display pixels
- Increased scatter
PET
Contrast
- Tomographic technique overcomes reduced contrast from overlying structures
Noise
- Reduced by increased sensitivity of system:
- Higher intrinsic detector efficiency
- Higher geometric detection efficiency
- Wider photopeak acceptance window
Resolution
- Reduced by:
- Longer positron distance from disintegration to annihilation
- Non-colinear annihilation photons
- Larger detector elements
- Thicker crystals
- Different reconstruction filter
Next page: NM Artefacts
- Sarah Abdulla
- Last updated: 10 October 2021
