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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
CCD chip is small ~ 2x2cm
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
55x55 m pixels
1.5x1.5 cm chip size
Si, GaAs, CdTe, CdZnTe sensor chip
Photon-counting rather than integrating
Thresholding
FastNick Cook, Medical Physics, Christchurch Hospital
Dose and Energy
ALARA - As Low As
Reasonably Achievable
Diagnostic Energy Range:
Lower energies give
greater absorbed dose
120kV 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
25kV 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 - 5x5cm
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 ~ 30fps
• 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
CdZnTe detectors have found applications in
gamma cameras, where they replace the
scintillator and PMT
Surgical gamma probe - 140keV from 99Tc
Medipix (Bertolucci et al, 2002)
- 2mm think CdZnTe for high efficiency
Sentinal Lymph Node Biopsy
- to establish progression of breast cancer
- intraoperative probe provides 1mm
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 ~ 300nsec
•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 keV 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 - 45x30cm
• Fast readout - 107 pixels in 1/30 sec
• Sensitive from 20 to 400keV
• 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