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Nuclear Medicine:
Image Quality, Sources of
Artefacts and the
Nuclear Medicine
“What… ?!” Quiz
Katrina Cockburn
Nuclear Medicine Physicist
Image Quality in NM
 Image Quality is largely subjective
 Beware of believing pretty = better!
 Can measure physical properties:
 Resolution
 Noise (inc. SNR)
 Contrast
 Can qualitatively score “aesthetic”
properties
Physical Measures of Image
Quality
 Spatial Resolution
 Smallest separation between two point
sources which will permit them to be
distinguished as two distinct sources
 Noise
 Statistical uncertainty in the number of counts
recorded
 Contrast
 Differences in intensity in parts of the image
corresponding to different concentrations of
activity within the patient
Spatial Resolution
Full Width Half Maximum (FWHM)
Full Maximum
Half Maximum
FWHM
Significance of FWHM
FWHM and Resolution
 Two sources separated by the FWHM will
be resolved
 Easy to measure using modern processing
computers
 Typical values:




LEHR at 0mm;
LEHR at 100mm:
LEGP at 0mm;
LEGP at 100mm;
4.6mm
8.3mm
4.7mm
10.2mm
Image Quality: Collimator
 High Sensitivity, General Purpose, High
Resolution
 Trade off between spatial resolution and sensitivity
 Distance Dependence
LEHS
20
1.2
18
LEGP
14
12
10
8
Relative Sensitivity
FWHM (mm)
LEHS
1
16
0.8
LEGP
0.6
0.4
0.2
6
0
4
0
50
100
150
Source-Collimator Distance (mm)
0
50
100
150
Source-Collimator Distance (mm)
Noise
 All stages in imaging system subject to
statistical variation
 Radioactive decay
 Number of scintillation photons in crystal
 Number of electrons at photocathode and
dynodes…
 SD of Noise = √(Average Pixel Count)
 More counts = better S/N ratio
Noise

Avg Pix SD
Count
Noise
100
10
10%
1000
32
3%
10,000
100
1%
Increased Counts → Reduced Noise
Image Quality: Recorded
Counts
 Administered Activity
 Diagnostic Reference Levels - ARSAC
 Uptake
 Radiopharmaceutical Properties
 Time to Imaging
 Attenuation
 Patient Size
 Acquisition Time
 Typical Imaging Times 3-60 minutes
Contrast

Contrast = (R1 - R2)
R2
R2: Background
R1: Lesion
Image Quality: Background Activity
 Non-specific radiopharmaceutical uptake
 Choice of pharmaceutical
 Pathology
 Scatter
 Limited energy resolution
 Septal Penetration
 Photon energy
 Collimator choice
Image Quality: Patient Motion
 Long Imaging Times
 Limit to time patient can remain still
 ~60% of Cardiac scans need correction
 Positioning and immobilisation devices can
help but still limit to 30mins
 Physiological Motion
 Cardiac Gating
 Respiratory Gating
Image Quality Comparison
Thallium-201
Tc99m-tetrofosmin
SAME PATIENT
MYO97C33
TET97036
Image Artefacts
 Pharmaceutical
 Labelling problems
 Patient
 Attenuation
 Movement
 Contamination
 Equipment
 Image non-uniformity
 Centre of Rotation
errors
 Operator
 External attenuation
 Acquisition errors
The Nuclear Medicine “What…?!”
Quiz
 Normal Images
 Abnormal images
 Images with artefacts caused by:
 Patient movement,
 Co-morbidities
 Pharmaceutical problems
 Contamination
 Incorrect processing
Can you tell which is which?
(Sadly no prize for the winner!)
Normal Bone Scan
 Symmetry
 Kidneys and
bladder
 Soft Tissue
“Superscan”
 Axial skeleton and




pelvis almost
complete metastases
Retains symmetry
Cannot visualise
urinary system
Cannot visualise soft
tissue
Limb bones poorly
visulised
Contamination
 Urinary
contamination
common
 Often find traces in
department
 Patient hands?!
Urinary Catheter
and Bag
 Extremely common
in Ca Prostate
patients
 Image with emptied
bag moved out of
field of view
 If only find out later,
re-image legs
separately
Free
Pertechnetate
 Improper labelling
of the HDP
 Can see stomach,
heart and thyroid
 Usually results in
increase in dose
A little bit unfair…
Extravasation
 Can obscure joints
 Always administer on
opposing side to
suspect joints
 Always use a venflon
or butterfly
 Radiation necrosis in
therapy doses
Ventilation scan
 Use radioactive
aerosol although can
use gasses or
particles
 Normally used with
perfusion scan for PE
 Can be used for
volume and function
estimation
Attenuation
 PE is normally wedge shaped, this is
round
 Chest x-rays routinely performed as part of
the VQ procedure
Planar Myocardial
Perfusion Study
 Very old study
 Performed with
Tl-201
 Modern images are
done as SPECT
Myocardial
Perfusion
Study
 Where is the
heart?
 Carefully
examine
outline of
patient
 Breast
attenuation
Breast Attenuation
Breast
Attenuation
 Classic breast
attenuation pattern
 “Defects” in anteroseptal region
 Defects are fixed
 Walls move normally
DMSA Kidney Scan
 Looks for scarred areas
of kidneys
 Can be used to
determine the divided
function of the kidneys
 Can be useful post UTI
DMSA Scan with
patient motion
 Patient has moved
position midway through
the scan
 Has effect of smearing
the counts and making
the kidney look big and
underperfused
 Repeat imaging shows
normal perfusion
Thyroid
 Many
radiopharmaceuticals
are taken up by
thyroid
 Thyroid imaging used
in parathyroid
localisation scans
Gastric Emptying
Study
 Used to examine
gastric emptying
problems
 Now also used in
gastric pacing
studies
DATScan
 Binds to pre-synaptic
dopamine
transporters
 Diagnosis of
Parkinsonian
disorders
 Normal appearance is
comma shaped
putamen
 Abnormal is “full stop”
shape of one or both
putamen
 Normal shaped Putamen
 What’s making it look “odd”
 Change the windowing of the images…
 “Missing” section of brain?!
 Patient brought back for CT scan
 CT showed large arachnoid cyst
Post ablation
thyroid scan
 Taken 7-10 days
after ablation
 Still large amount of
I-131 in the patient’s
system
 Star artefact due to
 poor windowing
 hexagonal collimator
holes
 High Activity in thyroid
 Micturating renogram
 Kidneys get hotter suggesting reflux
 But, background changes intensity and analysis
suggests no increase in kidney counts
Incorrect display
Lymphoscintigram
 Administration of
radioactive colloid
 Colloid moved through
the lymphatic system
 Allows assessment of
the cause of
lymphoedema
Radionuclide
Ventriculogram
 Red cells are labelled
with pertechnetate
 The image is acquired
gated
 Allows precise,
repeatable
measurement of LVEF
Another unfair
one…
 Oesophagogastrectomy
 Stomach pulled into thorax
 One minute before the bone scan the patient
drank his radioactive urine
Uriposia
 DMSA kidney images
with apparent uptake
in the gut
 Originally suspected
to be improper
labelling or
contamination of
pharmaceutical
 Later found to be
caused by the patient
drinking their own
urine
 Just shows that
Uriposia is not that
uncommon…