Nuclear Medicine Image quality & Artifacts

NM Image quality

As with other modalities the three major factors that determine image quality are:

Contrast, Noise, Spatial resolution

Contrast

• 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.

1) Subject contrast

Property of the imaged object i.e. radioactivity level in a lesion relative to healthy tissue.

Calculated as:

Where:

CS = subject contrast

AL = activity per unit volume of the lesion

AT = activity per unit volume of the healthy tissue

2) Image contrast

Difference in the display between the lesion and surrounding healthy tissue.

• Represented as counts per unit area
• Also called count density or information density.
• Expressed as counts per pixel.

Where:

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 (because of the inherently small signal from a limited amount of radioactivity).

- The activity is distributed through the body and typically only 20% concentrated in the organ of interest.

- The gamma rays are emitted isotopically and only a small fraction passes through the collimator holes.

• Too much noise will impair detectability of an object, especially low contrast object.
• Noise is the principal factor in determining the quality of gamma images. Gamma imaging is therefore said to be noise limited or dose limited.

Types:

Random noise:

• Aka statistical noise or quantum mottle
• Due to statistical variation in the number of photons being detected (random variations in count density as a result of random activity of radioactive decay)
• More significant contributor of noise

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

Electronic noise:

Caused by instability in the circuits between the receptor and viewer.

Calculation:

Relative noise (noise contrast) decreases as the count number (signal) increases

Where:

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 also decreases the spatial resolution and contrast)

Spatial Resolution

Resolution of gamma cameras is approximately 5–10 mm

Calculation:

• Quantified as: Full width at half maximum (FWHM) measurement on a graph of counts (graph is called the Line Spread Function) 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.

Types:

1) Intrinsic spatial resolution (RI)

The maximum resolution achievable by the detector and electronics

Depends upon many factors:

• Energy of gamma photon (i.e. Higher = more scintillation photons & smaller statistical variation - Improves spatial resolution)
• Energy and linearity correction of scintillation
• Thickness of scintillation crystal (range of light spread) (i.e. thicker crystal - more spread & variation of the signal with depth - Reduces spatial resolution)
• Optical coupling & shape of photomultiplier tube (PMT): good optical coupling & square or hexagonal PMT (better than circular) - optimised collection and detection of photons

• Only PMTs above certain voltage contribute to signal (eliminates noise)
• Range at 140 keV: between 2.5 (0.4) - 4 mm FWHM (0.25 lp/mm).

2) Collimator spatial resolution (RC) (opposite collimator sensitivity / efficiency)

For a parallel hole collimator the collimator spatial resolution is:

Where:

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:

• Narrow holes (Small diameter)
• Long holes (Thicker collimator)
• Positioned as close to the patient as possible (Reduce distance from patient).

However, there is still rapid degradation of spatial resolution the deeper the imaged object lies. Taking images from different orientations helps to minimise this.

NB:

The better the resolution, the less the sensitivity

↑ Sensitivity → ↓ radionuclide needed to be administered → ↓ patient dose.

3) System spatial resolution (RS)

Spatial resolution of the whole system

Takes into account the intrinsic & the collimator spatial resolution to give the.

Where:

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

1. Large diameter holes
2. Short holes
3. Far from patient
4. Increased patient motion
5. Imaging deeper structures (↑body mass, obese patients) →↑attenuation & scatter
6. Large display pixels
7. Increased scattered radiation
8. Improved with narrower energy acceptance window

PET image quality

Contrast

• Tomographic technique overcomes reduced contrast caused by radiation in front of and behind the lesion
• Random and scatter coincidences reduce contrast

Noise

Reduced by increasing the system sensitivity, 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

Whole-body PET systems achieve a spatial resolution slightly better than 5 mm FWHM in the center of the detector ring

Factors affecting:

1. Range of positron

- Distance from site of disintegration to annihilation

- Determined by maximal positron energy of the radionuclide & density of the tissue

- Longer range = poorer spatial resolution

- 15O is 2 mm, 18F is better at 0.6 mm

2. Location of annihilation event:

- Resolution better at center (FWHM is 4 mm) than periphery (8 mm) of detector ring

- This occurs because of detector thickness and inability to determine the depth where an annihilation occurs

3. 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

4. Compton scattering and random events

5. Intrinsic spatial resolution of detectors

- The most significant factor affecting resolution

6. Size of detector element

- Smaller elements = better spatial resolution

7. Width of angle of acceptance of detectors

- The more is the angle of acceptance , the less is the resolution

8. Thickness of crystal

- Thicker crystal = poorer resolution

9. 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

NM Artifacts

1) Technical issues

- Site of injection causes high radiotracer activity

- Extravasation of injection causes uptake in lymph nodes

2) Equipment malfunction

Malfunction of gamma camera system

- Photomultiplier tube (PMT) failure ? Cold spot (reduced counts)

- Correction matrix failure

- Cracked or broken scintillation crystal ? Linear defect

- Differences in detector sensitivity

Incorrect setting of the pulse height analyser window … detection of Compton Tail

3) Patient related

• Attenuation can be caused by:

- Objects worn by the patient e.g. belt buckles

- Breast attenuation: especially breast prosthesis

- Diaphragmatic attenuation: especially in obese patients, having ascites or on dialysis

• Patient motion causes misalignment of reconstructed images
• Urinary contamination

4) Physiological uptake

Head and neck

Brain cortex, Waldeyer’s ring, Salivary glands, Extra-ocular muscles, Larynx in excessive talking

Muscles

Stress-induced tension (trapezius & paraspinal muscles), Hyperventilation (diaphragm), Insulin (skeletal muscle), Vigorous exercise,

GIT / GUT

Caecum / right colon more glucose avid, Renal collecting system, ureters and bladder, Uterine uptake in menstruation

Miscellaneous

Lactating breasts, Myocardial uptake post-prandially, Brown fat, Thymus in children

5) SPECT and PET/CT specific

• Center-of-rotation error

- Mismatch between actual axis of rotation & center-of-rotation in SPECT ? back projection will be affected

- Misregistration between radionuclide and CT images

• All CT related artifacts
• Truncation

- SPECT field of view is larger than CT field of view

- No CT data available for attenuation correction of the SPECT images