Nuclear medicine

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16 Nuclear medicine

The introduction of a specific pharmaceutical (depending on which part of the body is to be targeted), which has been labelled with a radioisotope, into a patient. The gamma rays emitted by the radioisotope are scanned by a detector and the diagnostic image is produced showing the concentrations of the radiopharmaceutical (e.g. a bone scan) or an indication of function (e.g. the glomerular filtration rate of the kidneys)

Terminology

Charge Collection	The pooling of electrons across a crystal
Gamma Camera	A large, stationary, scintillation counter, which records the activity over the whole field at the same time. Used to detect pathologies where the physiology of the structure is changed
Gantry	A structure or support, in which the X-ray tube, detectors and associated electronics are housed
Half-life	The amount of time taken for the radioactivity of a radioactive substance to decay by half the initial value. The half-life is a constant for each radioactive isotope
Image Fusion	When a PET (or SPECT) image and a CT image are viewed together by one being superimposed on the other
Pharmaceutical	A drug used in medicine
Photodiode	A semiconductor used to detect light and then generate electricity in proportion to the quantity of light detected
Photomultiplier	Equipment that produces an amplified current when exposed to electromagnetic radiation (light). Photons hitting the cathode produce electrons which in turn hit other surfaces thus producing more electrons, forming a pulse of electricity which forms the subsequent image
Planar	A two dimensional image
Pulse Height Analyser	Receives the signal from the photomultiplier and only produces an electrical signal if the input pulse lies in a predetermined range
Radioisotope	Any isotope that is radioactive. Forms of an element which have the same atomic number but different mass numbers, exhibiting the property of spontaneous nuclear disintegration
Radiopharmaceutical	A drug consisting of a radioactive compound
Scintillation Counter	A number of scintillator crystals in containers, one surface of the crystal is attached to a transparent glass window and the other surfaces are coated with magnesium oxide to reflect light back into the crystal; the back of the crystal is attached to a photomultiplier tube. If a gamma ray hits the crystal, light is produced and some reaches the photocathode of the photomultiplier
Scintillator	A sodium iodide (or caesium iodide) crystal with a thallium activator
Segmented	Divided into sections
Solid State Detector Material	Semi conductor – Cadmium zinc telluride (CdZnTe)
Spatial Resolution	The smallest distance between two objects that can be visually seen on an imaging system

Equipment

Pharmaceutical	A drug which is absorbed by a specific (targeted) area of the body
Radioisotope	Technetium 99m (⁹⁹Tc^m) most commonly used • Is a gamma emitter • Has a half-life of 6.02 hours • Is readily available • Is readily combined with pharmaceuticals • Has lower emissions than other types of radiation It is combined with a specific pharmaceutical so that a specific area of the body will take up the radioisotope
Detector – Gamma Camera	Radiation from the patient passes through: A multichannel collimator • Parallel lead columns • Absorb extraneous radiation • Only allows gamma rays that are at 90° to the crystal face to reach the crystal A scintillation counter • Gamma rays from the patient hit the glass window • Light is produced by the crystal Photodiodes or photomultipliers • Detects the light from the crystal • Converts the light to an electrical pulse Pulse height analyser • Filters the electrical pulses • Only allows pulses of a predetermined strength to be measured over time • The signal then goes to the computer monitor Fig. 16.1 The main parts of a gamma camera.
Operator Console	Where the operator can determine the settings for the scan
Display Station	For the viewing, analysis, networking and storage of the final image

Development

Rectilinear Scanners	• Original scanner • Detector was sodium iodide with a thallium activator connected to a photomultiplier • Detector moved mechanically over the patient • At the same time a light source moved over a film • The whole body can be scanned and the results recorded on a 35.5 × 43 cm film • Image recorded on X-ray film or paper • Scan time 30 minutes • Motion artefacts a problem (due to breathing) • Superseded by the gamma camera
Gamma Cameras	• The patient is given a radiopharmaceutical and then scanned • The gamma photons from the patient strike a sodium iodide crystal • Photons are converted into light • Passed through photomultiplier tubes • Light changed to electronic signals • Data collection by an analogue camera • Image photographed onto film • Application: two dimensional (planar) imaging and physiology
Mobile Gamma Cameras	• Utilise solid state digital detectors • Silicon photodiodes and segmented caesium iodide scintillators replace photomultipliers • Now possible to scan in near real time • Give two dimensional and three dimensional images
Dual Headed Gamma Cameras (GCPET)	• Utilise solid state digital detectors • Acquire two views simultaneously – 180° apart • Attenuation correction possible • Whole body scan takes 25 minutes • Therefore increased throughput
	Advantages • Cost-effective • Can be used for all gamma camera studies • In addition can be used for PET studies
	Disadvantages • Not accurate in detecting cancerous lesions less than 10 mm
Gamma Probe	Designed for use by a surgeon to detect small quantities of tissue labelled with either Technetium 99m or Indium 111 • Small hand held probe • Tip contains a caesium crystal • Contained in a stainless steel tube • The tube is shielded with tungsten and has clip-on collimators to reduce the radiation to the operator • The tip is angled to make it easier to move • Linked to an automatic analyser • Produces an auditory signal when radiation is detected Application • To detect radioactive tissue • To check if all radioactive tissue has been surgically removed • For lymph node mapping • Localisation of tumours found on PET scans

Positron Emission Tomography (PET)

Definition	When a positron is emitted it travels a few millimetres then annihilates with a free electron resulting in the emission of two 511 keV photons leaving at nearly 180° to each other. A ring of scintillation detectors are positioned so that they capture the photons and produce a computerised image. Only if two detectors opposite each other register a photon within a nanosecond of each other are the photons registered. They are used in conjunction with CT scanners where the CT scanner shows the anatomy and the PET scanner the function of an organ or tumour, the images being superimposed on each other. Planar or three dimensional images can be produced
Cyclotron	• Used to produce isotopes with a short half-life • A small, self-shielded machine used in a hospital setting
Cyclotron	Function • Uses magnets to accelerate ions towards a target material • To produce a positron-emitting isotope • Which is automatically attached to a pharmaceutical
Detectors	• Several hundred detectors rotate round the patient • Detect the simultaneous emissions of the gamma photons • Scintillation detectors include: Lutetium oxyorthosilicate (LSO) Bismuth germinate (BG) Gadolinium oxyorthosilicate (GSO) Sodium iodide Ceramic detectors • In three dimensional mode there are no metallic collimators between the detectors to allow more gamma rays to be detected
Gantry	• Can be a shared gantry with a CT scanner • The helical CT scan is performed first and then the PET scan • Whole body scan takes 20 minutes
Software	• Image acquisition • Processing • Reconstruction The CT and PET images can be viewed side by side The images can be viewed separately The anatomical and the functional images are fused together to produce the final image Window width can be adjusted The display colours can be changed Vertical, horizontal and three dimensional sections can be viewed
Radioisotope	Examples • Fluorine-18 half-life 110 minutes • Carbon-11 half-life 20.3 minutes • Nitrogen-13 half-life 10 minutes • Oxygen-15 half-life 2 minutes 2 seconds
Radiopharmaceutical	Fluorine-18 fluorodeoxyglucose (18F-FDG)
Application	• Oncology Differentiates between benign and malignant tumours Demonstrates the stage and grade of the tumour Shows the site of the tumour Demonstrates the response to treatment Can demonstrate cancerous lesions below 10 mm • Cardiology • Neurology

Single Photon Emission Computerised Tomography (SPECT)

Definition	A specialist gamma camera which rotates round the patient at 2.8 revolutions per minute and a number of two dimensional images with a slice thickness of 10 mm are taken by measuring the emission of single photons. They can be used in conjunction with CT scanners where the CT scanner shows the anatomy and the SPECT scanner the function of an organ or tumour, the images being superimposed on each other
Gamma Camera	• Collimator contains many thousand parallel, hexagonal channels • Sodium iodide scintillation detectors are used
Gantry	• Contains the gamma camera detector head • Can be a shared gantry with a CT scanner and therefore hold the X-ray tube • Whole body scan takes 15 to 20 minutes
Software	• Image acquisition Approximately every 3 to 6° of rotation • Processing Image manipulation possible to remove overlying anatomy • Reconstruction Three dimensional images possible – Lower resolution, and increased noise (than planar images) – May be prone to artefacts Resolution 128 × 128 pixels
Radioisotope	Examples • Technetium 99m • Indium 111
Radiopharmaceutical	Examples • Tetrofosmin – cardiac work • Hexamethylpropylene amine oxime (HMPAO) – brain scans • Phosphates – bone scans
Application	• Cardiac • Brain • Bone
Limitations	• Incorrect compensation for attenuated photons can cause underestimation of activity • Areas of intense activity can cause streaking and obscure other areas of the body • Can be difficult to locate increased radiotracer activity

Sources

	Dakin M 2001 Positron emission tomography in the UK. Synergy, November
	Griffiths M 2005 SPECT/CT hybrid imaging technology, techniques and clinical experience. Synergy, January
	Griffiths M, Aston A, Roberts F 2003 CT 2003 future’s bright the future’s fusion. Synergy, November
	Griffiths M, Holmes K 2002 The development of nuclear medicine equipment. Synergy, November
	Higgins R, Smith L, Vinjamuri S 2004 Pancreatic carcinoma and imaging with PET. Synergy, May
	Hogg P, Lewington G 2005 An overview of PET/CT and its place in today’s UK healthcare system. Synergy, December
	Millns M, Owens S, 2001 Unclear nuclear medicine? Not any more! Synergy, February
	Moorhouse S 2005 Gamma camera SPET imaging of solitary pulmonary nodes. Synergy, January
	Old S E, Dendy P P, Balan K K 2000 Preliminary experience in oncology of positron emission tomography with dual headed gamma camera. Radiography 6:11–17