Primer on Medical Imaging

During the last weeks of 1895, Wilhelm Roentgen performed numerous experiments to characterize the new kind of rays he had serendipitously discovered. One of the most striking results of these experiments, an image of the bones in his wife's hand, demonstrated how the properties of X-rays could be harnessed to create pictures of internal bony anatomy. A new era in medicine had arrived.

X-ray imaging, or radiography, quickly became an important tool for physicians. During the decades after its discovery, developments such as contrast agents enabled the imaging of structures besides the skeletal system, broadening the scope of radiography. More recent advances such as fluoroscopy and computed radiography (CR) enable digital processing of radiographs. However, the inherent 2D projection geometry of radiographic imaging remains an obstacle to the use of these images for more quantitative planning. Stereotactic techniques, whereby reference markers are rigidly fixed to the patient's bony anatomy, can provide a basis for calculation of 3D positions from an orthogonal pair of radiographs, but this process is cumbersome, requiring detailed geometric calculations, and does not provide complete 3D information.

Since its development in the early 1970s, computed tomography (CT) has evolved into a comparatively inexpensive and fast tool for acquisition of detailed scans of patient anatomy. Current multislice scanners are able to scan all or large portions of anatomy quickly and at high resolution. Fast scans with very thin slices, which return 3D volumes of image data, are opening up applications in CT angiography, cardiac scanning and trauma. CT image data are relied upon extensively for many medical and surgical purposes, including diagnosis (i.e. visual inspection of image slices), and for the creation of virtual models for planning. Examples include stereotactic and image-guided surgery, radiotherapy and calculation of tumor volumes. In addition, quantitative CT (qCT) is increasingly popular for calculation of bone density. In qCT, Hounsfield numbers are calibrated to mineral densities by scanning tubes with known concentrations of minerals in solution. The qCT technique is relevant to this discussion because it extends this imaging modality beyond the simple generation of images of internal anatomy into the realm of non-invasive testing of physiology.

Cone-beam CT (CBCT) is a technique for 3D X-ray imaging whereby a large X-ray field, emitted in a cone shape, rotates about a subject in conjunction with an opposed 2D detector, such as an image intensifier (II) or amorphous silicon detector. CBCT enables acquisition of a full 3D volume of data in a single rotation of the X-ray source and detector, compared with the slice-by-slice acquisitions in traditional CT (or the slab-by-slab acquisitions of multislice CT). CBCT requires significant computing power to process large volumes of data but has the potential to acquire significant amounts of that data rapidly while exposing subjects to a smaller dose of ionizing radiation than traditional CT (Danforth, Dus and Mah, 2003). Currently, CBCT devices are commercially available for head and neck and intraoperative imaging at very high spatial resolution, but with lower contrast resolution than traditional CT. This can be expected to improve with refinements in detector technology.

Invented in the 1980s, magnetic resonance (MR) imaging employs strong magnetic fields and radio waves to produce detailed images. MR imaging utilizes the behavior of hydrogen protons in water molecules in the body and tends to reveal more soft tissue information than CT, which measures X-ray attenuation and therefore only shows variations in tissue density. MR imaging can reveal significant soft tissue detail such as ligaments and tendons as well as the presence of tumors. Image acquisition parameters can be tuned in many different ways, so it is possible to acquire a variety of different image data while the patient is in the scanner. Examples include proton density scans and MR angiograms. A more recent variant of MR is functional MR (fMR) which detects subtle changes in magnetic fields at a molecular level that can indicate greater blood flow to certain areas of the brain. The results of fMR can indicate which areas of the brain are active while a subject is performing a particular task. This information can be quite valuable in evaluating patients with stroke or brain tumor, and its use in planning surgery and radiation therapy is increasing. Similarly, MR spectroscopy is developing into an imaging modality that can provide indications of the concentrations of different chemicals revealing pathological processes in the body non-invasively.

Positron emission tomography (PET) has become increasingly popular and is frequently used to measure functional and metabolic activity. In PET imaging a radioactive agent is tagged to a material that is biologically processed by the patient. Radioactive oxygen-15, which a patient inhales prior to a functional brain scan, would be one example. When the subject performs a particular task, like speaking, the oxygen isotope is delivered to the areas of the brain involved in that task, and its radioactive emissions are measured and reconstructed in a set of tomographic images. PET scans are also used to stage lung and liver tumors, meaning that they show which parts of a tumor are most metabolically active and the extent of the cancer. This information is used increasingly to facilitate treatment planning, so interventions can be directed specifically at the most active portions of a tumor.

Ultrasound (US) imaging plays an important role in several medical specialties, largely because it does not use harmful ionizing radiation and can be performed quickly under a variety of circumstances (i.e. in office, in OR). Newer US devices can image in 3D and in 4D, meaning that 3D images are acquired rapidly over time, providing a 3D cine view. With calibration, US image data can be correlated with CT or other imaging modalities to be used quantitatively for treatment planning and verification.

Many types of non-medical device have been reverse engineered for use in the medical sector. Scanning technology that records surface topology optically, using lasers, or physically, using articulating arms, provides a means of gathering significant amounts of geometrical information without exposing the patient to the ionizing radiation of CT, X-ray and PET. Another advantage is that image data of relevant anatomy may not be subject to digital artifacts from, for instance, metallic implants in teeth which can cause streaking in CT. Also, 3D color scanners can gather surface color data to create realistic virtual models of patients' faces. Several groups are investigating the use of this type of device in cranio-maxillofacial (CMF) surgery (Yamada et al, 2002).

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