Imaging in Nuclear Medicine

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The basic physical parameters of importance are the frequency of the wave, the speed of sound v and the density? Among many others, there are two typical artefacts in ultrasound. The first artefact is due to the coherent nature of the sound wave: the sound wave is a coherent pulse which will interfere with its reflected, refracted and transmitted components to give rise to speckling, similar to the speckling observed in laser light.

The second artefact is due to the physics of reflection: interfaces between tissues that are parallel to the wave propagation will not reflect the wave and will therefore not be seen in ultrasound. For some applications such as obstetrics or cardiology, the clinical information in the images is very high. Furthermore, the technique is safe and relatively inexpensive. Current research tends to eliminate artefacts, improve the image contrast and improve the presentation of the data.

TRA Medical Imaging Nuclear Medicine Section Protocols

The MRI technique stems from physics research carried out by Gorter, Rabi, Purcell, Bloch and many others that led to the discovery and development of nuclear magnetic resonance techniques just before and after world-war II. Medical applications and imaging were introduced in the seventies by, among others, Lauterbur, Damadian and Mansfield. Without an external magnetic field, the magnetic moment of the hydrogen nuclei will point at random in all directions.

A Snapshot of Nuclear Imaging

There will be no net magnetisation. More spins will align their spin in the direction of the field 'spin-up' than in the opposite direction 'spin-down' because the energy in spin-up direction is lower than in spin-down direction. The global energy of the spin system will, therefore, decrease while the magnetisation increases. This magnetisation implies a transfer of energy from the spin system to another system: the 'lattice', or tissue in the case of MRI. This transfer of energy is characterised by an exponential relaxation law with a time constant T 1 , also called spin-lattice relaxation time.

In typical MRI field strengths 0. Next to interacting with the lattice, the spins can also interact among each other: as one spin flips from down to up, another spin can absorb the released energy and flip from up to down. This spin-spin redistribution of energy, internal to the spin system, is also characterised by a relaxation time, called spin-spin relaxation time and noted as T 2. Typical values for T 2 are 10 — ms, again depending on the tissue type. For a typical MRI field strength of 1. By sending radio waves at resonant frequency some spins which were spin-up will absorb the energy of the wave and flip to spin-down, thereby increasing the global energy of the spin system.

The energy of the spin system will now no longer be in equilibrium with respect to the tissue temperature and hence violate the normal Boltzmann distribution in equilibrium.

Diagnostic imaging

The spin system will subsequently re-emit the extra energy as radio waves at resonant frequency. By varying local magnetic fields 'gradients' , fine-tuning the frequency, the polarisation and the duration of radio wave pulses to excite the spin system, and by modulating the delay after which the re-emitted waves the 'signal' are measured, MRI images can be reconstructed.

The contrast in the images then depends on the four following factors: N H , T1, T2 and flow any movement of nuclei during the imaging sequences. The clinical value of MRI images is recognised in a large number of pathologies. Examples are the base of the skull and articulations such as the knee. Current research tends to widen the scope of information gathered. Examples are magnetic resonance angiography MRA to visualise the vascular structure without injection of contrast media, functional MRI to visualise areas of specific brain function, and diffusion imaging.

Other ongoing efforts involve the shortening of the acquisition times that used to be tens of minutes and are now between seconds and a few minutes. MRI is a rather safe technique for both patients and staff. Obvious precautions, such as removing metallic objects that could fly into the magnet due to the very high field strength should be taken. Patients with internal metallic objects such as clips should be excluded from the imaging procedure. The same is true for patients with pacemakers.

Most other potential hazards are associated with the generation of heat due to induced currents. Radiography is imaging with an external X-ray source. X-rays were accidentally discovered but not recognised as such by Goodspeed at the University of Pennsylvania in Only weeks after the discovery, medical applications started as illustrated by figure 3.

The imaging process in radiography is based on the detection by film or other adequate detectors of the transmission of X-rays originating in a point source the X-ray tube. Along their path from source to detector, the X-rays photons with a mean energy in the range between 15 and 60 keV undergo photon-matter interactions.

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Among the four classical interactions, the photoelectric effect, Compton scattering, coherent scattering and pair formation, only the first two are relevant because of the energy range. The photoelectric effect is the main photon-matter interaction of importance in radiography; it creates the shadow image through absorption by the body structures, and allows the detection of the photons by the detector.

X-ray film is still the most widely used detector.

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However, the characteristics of film are such that it is not very sensitive to X-rays. Therefore, a phosphor screen that transforms the X-ray in visible light is put against the film - thereby drastically increasing its sensitivity and allowing a similar decrease in radiation exposure to the patient. Today, large field of view semiconductor detectors gradually replace film.

The spectrum of clinical applications of radiography is overwhelming, but inherently limited by the fact that it is a projection technique: the information along the path of the X-ray is integrated and information on changes in absorption along the path is lost in the image. This is the reason why X-ray computed tomography was developed.

Because of the ionising character of X-rays, a real health risk exists. Early radiographers paid a high toll as victims of radiation induced illnesses such as leukaemia. The loss of information due to the projection of a shadow in classical radiography limits its clinical value.

Several methods have been devised in order to overcome this loss: tomography through blurring of out-of-focus structures by moving the X-ray source and film in opposed directions, stereoscopic views etc The advent of powerful data processing allowed for new approaches and in Hounsfield introduced Computed Tomography CT following pioneering work carried out by Oldendorf and Cormack. In order to have enough data to mathematically reconstruct virtual slices, one needs projections from different angles. Two angles allow the reconstruction of objects as squares. This is of course not satisfactory.

As a rule of thumb, the quality of the reconstruction shape, intensity However, for a fixed total acquisition time, the noise in each projection increases also with this number. Some optimum has to be found between resolution and noise. Tomographic images are then reconstructed by means of analytical or iterative reconstruction algorithms.

As for projection radiography, a drawback of CT is the radiation burden to the patient, especially for young children. It is expected that the future switch from integrating detectors to counting detectors will allow a drastic reduction in patient dose for equivalent image quality, thus eliminating this burden.

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The use of radioactive tracers that are introduced in the living system to study its metabolism dates from when de Hevesy and Paneth studied the transport of radioactive lead in plants. In , de Hevesy and Chiewitz were the first to apply the method to the study of the distribution of a radiotracer P in rats.

The major development of nuclear imaging also called scintigraphic imaging started with the invention of the gamma camera by Anger in In parallel, positron imaging was developed. Both imaging modalities are now standard in the major nuclear medicine departments. The tracer principle, which forms the basis of nuclear imaging, is the following: a radioactive biologically active substance is chosen in such a way that its spatial and temporal distribution in the body reflects a particular body function or metabolism.

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In order to study the distribution without disturbing the body function, only traces of the substance are administered to the patient. The radiotracer decays by emitting gamma rays or positrons followed by annihilation gamma rays. The most often used radio-nuclides are Tcm in 'single photon' imaging and F in 'positron' imaging.

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Tcm is the decay daughter of Mo which itself is a fission product of U. The half-life of Tcm is 6h, which is optimal for most metabolic studies but too short to allow for shelf storage. Mo has a half-life of 65h. This allows a Mo generator a 'cow' to be stored and Tcm to be 'milked' when required. Tcm decays to Tc by emitting a gamma ray with an energy output of 14O keV. This energy is optimal for detection by scintillator detectors. Tc itself has a half-life of years and is therefore a negligible burden to the patient.

F is cyclotron produced and has a half-life of minutes. It decays to stable O by emitting a positron. The positron loses its kinetic energy through Coulomb interactions with surrounding nuclei. When it is nearly at rest, which in tissue occurs after an average range of less than 1 mm, the probability of a collision with an electron greatly increases and becomes one. During the collision matter-antimatter annihilation occurs in which the rest mass of the electron and the positron is transformed into two gamma rays of keV. Because the source of the rays is no longer a point source, but distributed through the object, adapted 'optics' have to be used for image formation.

There is no known material which refracts gamma rays the way that lenses do with visible light. One, therefore, has to rely on selective absorption of the rays based on geometrical criteria.

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The first, historical method but still used for particular applications, is based on the 'camera obscura' principle: a lead cone is placed over the detector and a pin-hole opening is made at top of the cone, perpendicular to the centre of the detector surface. Only those rays which pass through the pin-hole form an image on the detector. The image is inverted and enlarged or reduced with respect to the object, depending on the distances between object, pin-hole and detector. The second method is based on the multiple hole collimator: a thick lead or tungsten sheet in which thousands of parallel holes are drilled other manufacturing techniques exist.

Typical hole sizes are a couple of cm in length with a diameter of a couple of mm. The collimator structure is an inherent limitation to the ultimate camera resolution. Furthermore, its geometric efficiency is very low e. Only those rays that hit the detector through the holes in parallel contribute to the image, which then corresponds to a one to one mapping of the radioactive distribution. Nuclear Medicine, Radiology and Imaging is a scientific journal which covers high quality articles based and applicable to the broad field of applied medical sciences.

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