Medical Imaging A review of medical imaging technologies

Medical Imaging A review of medical imaging technologies with some opportunities for detector development Nick Cook, Medical Physics, Christchurch Hospital

Medical Imaging • Intrinsically Digital - CT, MRI, Angiography/Fluoroscopy, Ultrasound, Gamma Camera, PET, SPECT • Analogue - Radiography (X-rays) - accounts for 70% of all clinical images • Digital X-ray - computed radiography, direct digital radiography (a: Si, a: Se, CCD, CMOS) • Radionuclide imaging: gamma camera, PET, SPECT • Digital advantages - CAD, digital processing, teleradiology Nick Cook, Medical Physics, Christchurch Hospital

Picture Archiving and Communications Systems (PACS) • Most large hospitals in developed world are installing PACS • Digital capture and storage of medical images • Standardised communications and image format • Large archive accessible from Dr’s clinics, wards, theatres, etc • Teleradiology gives access to specialists in other hospital/country • Do away with film library, waiting times and lost films Nick Cook, Medical Physics, Christchurch Hospital

Radiology Images Chest CT Pelvic Angiograph Head MRI Pancreatic Fluoro Renal US Nick Cook, Medical Physics, Christchurch Hospital Chest X-ray

Nuclear Medicine Images PET Gamma Camera Nick Cook, Medical Physics, Christchurch Hospital SPECT

Screen/Film Imaging The first X-ray image dates from 1895, and plane film technology has continued to dominate medical imaging for over 100 years Mrs Wilhelm Roentgen’s hand Nick Cook, Medical Physics, Christchurch Hospital

Film / Digital Response to Exposure Film provides an inherent logarithmic compression of exposure onto the available optical densities of the film but has a limited dynamic range Digital detectors give a linear response to exposure so must have logarithmic processing, also gives a much larger dynamic range Nick Cook, Medical Physics, Christchurch Hospital

Computed and Direct Radiography Direct TFT, CCD, CMOS Computed Radiography • Photostimulable phosphor plates • TFT - Amorphous Si, Se • Cheap • More sensitive than CR • Flexibility of plates for mobile X-rays • Faster imaging process/workflow • Plates can be used on existing X-ray machines • Enables real-time processing • Dual energy imaging • Video fluoroscopy - if frame rates permit Nick Cook, Medical Physics, Christchurch Hospital

Computed Radiography Workflow is similar to screen/film process: 1. Latent image formation on photostimulable phosphor CR plate 2. Plate is read by scanning laser 3. Digital image is processed and archived Conventional screen/film Computed Radiography Nick Cook, Medical Physics, Christchurch Hospital

Advantages of CR Flexibility: • Plates are portable and ‘cheap’ • Plates work with existing X-ray equipment • CR can be used for most exams - mammography being one exception • Hospitals can translate to a digital environment without buying new X-ray equipment • Archives and viewing workstations are established • Other digital modalities can now be considered Nick Cook, Medical Physics, Christchurch Hospital

Flat Panel TFT Detectors Indirect: a-Si Direct: s-Se Nick Cook, Medical Physics, Christchurch Hospital

Flat Panel Detector Expensive Not portable Pixel size ~ 100 m Efficient - lower dose Improves throughput in dedicated rooms Nick Cook, Medical Physics, Christchurch Hospital

Optically Coupled CCD chip is small ~ 2 x 2 cm Pixels are small ~ 12 m Large field must be demagnified using fibre-optic taper or lenses - inefficient Nick Cook, Medical Physics, Christchurch Hospital

CMOS - Medipix 2 55 x 55 m pixels 1. 5 x 1. 5 cm chip size Si, Ga. As, Cd. Te, Cd. Zn. Te sensor chip Photon-counting rather than integrating Thresholding Fast. Nick Cook, Medical Physics, Christchurch Hospital

Dose and Energy ALARA - As Low As Reasonably Achievable Diagnostic Energy Range: Lower energies give greater absorbed dose 120 k. V for a chest X-ray, to maximise latitude in bone and soft lung-tissue imaging Different tissues are more susceptible to damage, eg glandular breast tissue 25 k. V for mammography to maximise contrast in the soft tissues of the breast Nick Cook, Medical Physics, Christchurch Hospital

Digital Mammography • Small calcifications require 20 line pairs/mm • Glandular tissue more sensitive to radiation • Higher doses needed, repeated imaging also increases dose • Due to coupling inefficiencies, higher dose than film • 11 lp/mm not as good as film Nick Cook, Medical Physics, Christchurch Hospital

Stereotactic Biopsy Mammography • Biopsy of suspect lesion in the breast to determine malignancy • The breast is clamped and imaged from two angles to determine location of tumour • Computer calculates needle insertion point and path • Dramatically decreases the time of the procedure with increased patient comfort and cost effectiveness • Smaller imaging field - 5 x 5 cm Nick Cook, Medical Physics, Christchurch Hospital

Fluoroscopy • Video plane radiography, gives live view of internal motion and function • Photocathode/anode acceleration gives amplification of image • Video needs frame rate of ~ 30 fps • a: Si flat panels have been used - reads 30 million pixels/s (a: Se too slow) Nick Cook, Medical Physics, Christchurch Hospital

Nuclear Medicine Imaging Cd. Zn. Te detectors have found applications in gamma cameras, where they replace the scintillator and PMT Surgical gamma probe - 140 ke. V from 99 Tc Medipix (Bertolucci et al, 2002) - 2 mm think Cd. Zn. Te for high efficiency Sentinal Lymph Node Biopsy - to establish progression of breast cancer - intraoperative probe provides 1 mm resolution of the node location for biopsy Nick Cook, Medical Physics, Christchurch Hospital

PET Scan • Detectors are scintillation crystals coupled to PMTs • Decay constant of BGO ~ 300 nsec • Faster detectors improve discrimination of coincidence • Very fast detectors time of flight calculations and positron imaging Emitted positron travels short distance from nucleus before annihilation with electron, producing two 511 ke. V photons Nick Cook, Medical Physics, Christchurch Hospital

Dual Energy Imaging Bone Image Soft Tissue Image Nick Cook, Medical Physics, Christchurch Hospital

Digital Tomosynthesis Image Stitching Nick Cook, Medical Physics, Christchurch Hospital

Medipix Wish List • Large area tiled detector - 45 x 30 cm • Fast readout - 107 pixels in 1/30 sec • Sensitive from 20 to 400 ke. V • Efficient - reduce dose, enable fluoroscopy • Feedback circuit to control X-ray tube and cut off when sufficient image info is captured? • Research projects for Medical Physics MSc students Nick Cook, Medical Physics, Christchurch Hospital