Medical Imaging Techniques
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Various techniques are used for the extraction of information or data from the images for the purpose of making a medical diagnosis of a patient. Generally, image data is in the form of microscopy images. Major techniques used for medical imaging are :
1.1. Radiology and Computed Tomography with x-rays
1.1.1. Diagnostic Radiology
X-rays were discovered by W. C. Röntgen in 1895 and the prospect for medical diagnosis was immediately recognised [Glasser 1934]. They penetrate most biological tissues with little attenuation, and thus provide a comparatively simple means to produce shadow, or projection, images of the human body. The radiographic image represents the distribution of x-ray photons transmitted through the patient. Hence it is a 2D projection of the attenuating properties of tissues along the path of the detected x-rays. The principal interactions causing attenuation are photoelectric absorption and (inelastic) scattering. In a simplified x-ray imaging system Photons are scattered, absorbed or transmitted without interaction. Most scattered photons are removed by an antiscatter device (e.g. a lead grid). The detector is either a screen-film system, an x-ray photographic film or an image intensifier. Commonly used photon energies range from 17-150 keV, the choice for a particular application or tissue probed being a trade-off between acceptable radiation dose and achievable image contrast. Bones cause significantly higher attenuation than soft tissues, as their photoelectric cross-section and density is higher. The resulting higher contrast means that x-ray diagnostic radiology is particularly suitable for imaging (broken) bones
1.1.2. Computed Tomography
Conventional radiography provides no depth information, as the 3D body structure is projected onto a 2D image. Another limitation is the low soft tissue contrast, which is particularly important in brain imaging, where the soft tissues are enclosed by the highly attenuating skull. In contrast, x-ray computed tomography (CT) imaging produces thin 2D sections of the body, approximately 1 mm in thickness. Sub-millimetre spatial resolution with good discrimination between tissues (better than 1% attenuation change) can be achieved.
In 1972 G. N. Hounsfield first presented a clinical CT scanner at the Annual Congress of the British Institute of Radiology, the design of which is described in [Hounsfield 1973].
Since then, the introduction of clinical x-ray computed tomography has revolutionized medical imaging and may be described as the greatest advancement in radiology since the discovery of x-rays. First generation CT systems employed a narrow pencil beam from a collimated source that scans linearly across the patient in order to obtain a parallel projection. The system is then rotated to obtain several such projections. Tomographic slice images representing attenuation values are reconstructed by inverting the measured projection data. The underlying mathematical principles were originally developed by J. Radon in 1917 ([Radon 1917]), long before the first prototypes were constructed. The method most commonly used today is called filtered backprojection and employs the following steps.
- Record projections at different angles around the object to be imaged.
- Convolve each projection with a filter function (which prevents the occurrence of the 'star artefact' in conventional backprojection).
- Finally, reconstruct the image by backprojecting the filtered projections along their original line-of-sight and summing up the attenuation values.
1.2. Diagnostic Ultrasound
In diagnostic ultrasound imaging, high frequency pulses of acoustic energy are emitted into the patients' body where they experience reflection at boundaries between tissues of different characteristic impedance. From the measurement of time delay and intensity of the reflected pulses (echoes), an image indicating tissue interfaces can be reconstructed. Ultrasound imaging is considered to involve negligible risk, provided that the incident intensities are sufficiently small. The relatively simple technology employed makes it also rather inexpensive as compared to other clinical imaging modalities. The spatial resolution depends on many factors and is typically of the order of a millimetre. Acoustic frequencies typically range from 1 to 15 MHz.
The central component of an ultrasonic imaging system is the transducer. It converts electrical signals to sound waves, and vice-versa. Ultrasound transducers consist of one or several piezoelectric crystals coupled to the tissue via an index matching gel. In order to produce a 2D image, the transducer is either moved mechanically, or an acoustic beam is steered by using interfering waves originating from an array of crystals. Ultrasonic Doppler systems are able to detect the Doppler shift in the wavelength of scattered waves. This allows the blood flow velocity in a vessel to be measured.
There are widespread clinical applications of diagnostic ultrasound systems. These include monitoring of the unborn foetus, the cardiovascular system, abdomen, breast, thyroid, eye and other parts of the body. Imaging of the adult brain is difficult because of the attenuating and refracting properties of the surrounding skull. However, in neonates the anterior fontanel provides an effective acoustic window that allows diagnosis of cerebral disorders such as haemorrhage, hydrocephalus and congenital malformations the physics and instrumentation of ultrasound imaging are covered in detail by [Hedrick, 1995].
1.3. Magnetic Resonance Imaging
In Magnetic Resonance Imaging (MRI), also referred to as Nuclear Magnetic Resonance Imaging (NMR), the patient is placed inside a strong magnetic field that is usually generated by a large bore superconducting magnet. Nuclear magnetic resonance is utilized to obtain images as a function of proton spin density and relaxation times (or spectra of 31P and 1H in NMR spectroscopy). NMR imaging principles are covered in depth by [Mansfield, 1982].
MRI is a relatively new imaging method, with much advancement yet to come. For example, only recently has it become possible to obtain functional information by using haemoglobin as a paramagnetic tracer. This method, called functional MRI (fMRI), is capable of directly measuring brain activation. In combination with high-resolution anatomical scans it is likely to at least partially replace PET scans. Figure 3-8 shows combined fMRI and standard anatomical MRI images of the author revealing the response to a visual stimulus. Among the reasons for the success of MRI as a diagnostic imaging tool are the high resolution (sub mm), complete non-invasiveness and very low risk. Disadvantages are the high costs, bulkiness of the equipment, the requirement for the patient to stay still in the magnet for up to about half an hour and the problems associated with the presence of high magnetic fields.
1.4. Radioisotope Imaging
Radioisotope imaging is fundamentally different from the previously introduced imaging modalities in that the radiation originates from inside the body. Radioisotope compound used for tracing quantities that are injected into the patient's body where they decay and produce detectable g-photons. Hence it is possible to obtain images of the distribution of the radionuclide. Through the suitable choice of a labeled agent its distribution can be made representative of physiological function, such as blood flow, blood volume and various metabolic processes. The physics of nuclear medicine is discussed in more detail by [Sorenson 1987].
1.4.1. Single Photon Emission Computed Tomography
In Single Photon Emission Computed Tomography (SPECT) a single g-ray is emitted per nuclear decay. A gamma camera, fitted with a parallel-hole collimator, rotates around the patient and records 1D projections of the radioactivity. A large number of such data sets allows the reconstruction (using a filtered backprojection similar to x-ray CT) of a 2D cross-sectional image of the radiopharmaceutical distribution in the body. Combining opposite projections helps to take into account the photon absorption within the body. SPECT provides functional images with improved contrast at the expense of spatial resolution, as compared to planar radioisotope imaging.
1.4.2. Positron Emission Tomography
Positron Emission Tomography is working on Annihilation Coincidence Detection (ACD). Decay of a radionuclide produces a positron which, after a short travel (approx. 1 mm), collides with an electron and annihilates, thus generating two antiparallel É¤-rays at 511 keV each. The patient is surrounded by a large number of scintillation detectors and coincident detection of the two g-photons defines a line through the patient along which the annihilation event occurred PET scanners have mostly been designed and used for brain imaging. Typical uses include the diagnosis and localisation of brain tumours and strokes, as well as the monitoring of blood flow changes associated with local brain function. Because of the high instrumental cost and the requirement of a cyclotron for the production of short-lifetime radioisotopes, PET systems can normally only be found in large clinical or research facilities.
1.5. Electrical Impedance Tomography
Electrical Impedance Tomography is a new method and still far from being an established clinical imaging modality. It has, however, been included in this chapter because it is in some aspects reminiscent of optical tomography. This applies particularly to the image reconstruction techniques. A recent review on the progress in the field is provided by [Boone 1997].
EIT produces an image of the resistivity of the body, which varies significantly between different types of tissues. For instance, bone has a resistivity of 150 W cm while blood is a much better conductor at only 1.6 W cm. Typically 16 electrodes are equidistantly placed around the region of interest, for example the thorax, or brain. An AC current at frequencies of the order of tens of kHz is injected via a pair of electrodes, and the resulting voltage difference measured between the remaining electrodes. While investigators initially used a simple backprojection for the reconstruction, more recently iterative reconstruction schemes have been employed that provide much more quantitatively accurate images. These methods are necessary because of the inherent non-linearity of the problem.
Ultrasound technique uses high frequency broadband sound waves in the megahertz range that are shown by tissue to varying degrees to produce (up to 3D) images. This is basically associated with imaging the fetus in pregnant women. Uses of ultrasound are numerous that is it include imaging the abdominal organs, heart, breast, muscles, tendons, arteries and veins. It provides less structural details than other techniques such as CT or MRI, but it has several benefits which make it ideal in various conditions, in particular that it studies the function of moving structures in 3D images in real-time, emits no ionizing radiation, and contains speckle that can be used in elastography.
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