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Nuclear Medicine: Image Quality, Sources of Artefacts and the Nuclear Medicine “What… ?!” Quiz

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

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Nuclear Medicine: Image Quality, Sources of Artefacts and the Nuclear Medicine “What… ?!” Quiz

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  1. Nuclear Medicine: Image Quality, Sources of Artefacts and the Nuclear Medicine “What… ?!” Quiz Katrina Cockburn Nuclear Medicine Physicist

  2. 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

  3. 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

  4. Spatial ResolutionFull Width Half Maximum (FWHM) Full Maximum Half Maximum FWHM

  5. Significance of FWHM

  6. FWHM and Resolution • Two sources separated by the FWHM will be resolved • Easy to measure using modern processing computers • Typical values: • LEHR at 0mm; 4.6mm • LEHR at 100mm: 8.3mm • LEGP at 0mm; 4.7mm • LEGP at 100mm; 10.2mm

  7. Image Quality: Collimator • High Sensitivity, General Purpose, High Resolution • Trade off between spatial resolution and sensitivity • Distance Dependence LEHS LEHS LEGP LEGP

  8. 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

  9. Noise • Increased Counts → Reduced Noise

  10. 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

  11. Contrast • Contrast = (R1 - R2) R2 R2: Background R1: Lesion

  12. Image Quality: Background Activity • Non-specific radiopharmaceutical uptake • Choice of pharmaceutical • Pathology • Scatter • Limited energy resolution • Septal Penetration • Photon energy • Collimator choice

  13. 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

  14. Image Quality Comparison Thallium-201 Tc99m-tetrofosmin SAME PATIENT MYO97C33 TET97036

  15. Pharmaceutical Labelling problems Patient Attenuation Movement Contamination Equipment Image non-uniformity Centre of Rotation errors Operator External attenuation Acquisition errors Image Artefacts

  16. 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!)

  17. Normal Bone Scan • Symmetry • Kidneys and bladder • Soft Tissue

  18. “Superscan” Axial skeleton and pelvis almost complete metastases Retains symmetry Cannot visualise urinary system Cannot visualise soft tissue Limb bones poorly visulised

  19. Contamination • Urinary contamination common • Often find traces in department • Patient hands?!

  20. 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

  21. Free Pertechnetate • Improper labelling of the HDP • Can see stomach, heart and thyroid • Usually results in increase in dose

  22. 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

  23. 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

  24. Attenuation • PE is normally wedge shaped, this is round • Chest x-rays routinely performed as part of the VQ procedure

  25. Planar Myocardial Perfusion Study • Very old study • Performed with Tl-201 • Modern images are done as SPECT

  26. Myocardial Perfusion Study • Where is the heart? • Carefully examine outline of patient • Breast attenuation

  27. Breast Attenuation

  28. Breast Attenuation • Classic breast attenuation pattern • “Defects” in antero-septal region • Defects are fixed • Walls move normally

  29. 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

  30. 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

  31. Thyroid • Many radiopharmaceuticals are taken up by thyroid • Thyroid imaging used in parathyroid localisation scans

  32. Gastric Emptying Study • Used to examine gastric emptying problems • Now also used in gastric pacing studies

  33. 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

  34. Normal shaped Putamen • What’s making it look “odd” • Change the windowing of the images…

  35. “Missing” section of brain?! • Patient brought back for CT scan • CT showed large arachnoid cyst

  36. 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

  37. Micturating renogram • Kidneys get hotter suggesting reflux • But, background changes intensity and analysis suggests no increase in kidney counts Incorrect display

  38. Lymphoscintigram • Administration of radioactive colloid • Colloid moved through the lymphatic system • Allows assessment of the cause of lymphoedema

  39. Radionuclide Ventriculogram • Red cells are labelled with pertechnetate • The image is acquired gated • Allows precise, repeatable measurement of LVEF

  40. Oesophagogastrectomy Stomach pulled into thorax One minute before the bone scan the patient drank his radioactive urine Uriposia Another unfair one…

  41. 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…

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