Advanced Imaging Techniques in Oral and Maxillofacial Radiology

Advanced Imaging Techniques

Computed tomography in oral and maxillofacial radiology Computed tomography (CT) is today commonly used in imaging of the maxillofacial area. Conventional CT examinations are usually performed in medical X-ray departments. However, a relatively new technique named cone-beam computed tomography (CBCT) or digital volume tomography (DVT) has now also become available for dental purposes. The advantage with this technique is a lower radiation dose compared to conventional CT. Common examples when DVT is used are; for diagnosing the position of impacted canines and suspected root resorption of the adjacent lateral incisor, preoperative planning examination of periapical areas when intraoral radiography has given uncertain information. examination of larger areas in diagnosing e.g. facial anomalies, extensive traumata and tumors. of implant treatment and Conventional CT is used for

Computed tomography (CT) is today frequently used in imaging of the oral and maxillofacial region. All radiological examinations must be based on clinical information and relevant clinical questions that should be answered. A useful investigation is one, in which the result - positive or negative - will alter patient management or add confidence to the clinician's diagnosis . This is especially important regarding CT because the examination is expensive and might give very high radiation doses to the patient. CT has the advantage over other radiographic techniques in that it has an inherent high-contrast resolution and tissues that differ in physical density by less than 1 % can be distinguished.

CT is a digital technique providing images of thin slices with variable thickness. The technique was described in 1972 by Allan McLeod Cormack and Godfrey Newbold Hounsfield, Hounsfield constructed a machine where the X-ray tube rotated around the patient and a thin slice (8 mm) of the patient was scanned. In the first generation of CT machines the image reconstruction time was around 30 minutes per slice. Today CT machines are available that scan more than 100 mm/s with the images appearing on the monitor almost instantaneously. By simultaneously scanning several slices of the body (multislice CT), the scan time can be reduced significantly and the smallest details (resolution around 0.3 mm) can be imaged within short scan times. Multislice CT machines are common in medical radiology departments; the slice thickness is usually less than 1 mm by use of very small X-ray detectors and a fan-shaped X- ray beam transmitted through the patient.

Apart from the types of CT machines described, DVT has now become available for maxillofacial imaging. In contrast to conventional CT, where slices are scanned DVT produces an image volume from a large number of conventional x-ray images. From this volume, slices of different thicknesses can be reconstructed in any plane. One advantage with DVT over conventional CT is the lower radiation dose. The scan time is relatively short (around 20 seconds) and the geometric resolution is high for some machines (3 line pairs/mm). Most machines have the appearance of a panoramic machine and the software is usually adapted to maxillofacial imaging. In both DVT and conventional CT artifacts are produced, which can create problems in the reconstruction and interpretation of the images. The most common artifact in the maxillo-facial region is produced by metal objects and it is important to try to avoid exposing metal fillings and crowns.

Radiation dose The radiation dose for DVT can be considerably lower than that from conventional CT (3). The effective dose from DVT examinations varies substantially, depending on the device, imaging field and selected technique factors (4). When conventional CT is used low-dose protocols are now recommended whenever possible. A low-dose CT examination of the paranasal sinuses, for example, gives a dose similar to that from a conventional, digital radiographic examination but with substantially better diagnostic quality.

Cone beam computed tomography or digital volume tomography Since this method was first introduced for imaging of the jaws in the late 1990s, several machines have been constructed by many manufacturers around the world. This can be seen as reflecting a need of more accurate imaging of the often anatomically complex structures of the jaws where, in conventional imaging, structures outside the region of superimposed on the latter making diagnosis difficult. It also may be seen as a need of a less expensive and, not least, less dose conventional CT. interest often become requiring technique than

With the very rapid development of DVT machines it is hard to know exactly how many of these are currently on the market. Suffice it to say that the variation among them in many important aspects is greater than that between conventional CT machines. They vary, e.g. in regard to: the size of the volume that can be examined; the size of the elements that make up the volume (the volume elements - or voxels); the thickness of the slices that can be obtained; how the positioning of the volume is achieved; spatial and contrast resolution; image quality; radiation doses; image capturing technique and whether the patient is examined in a sitting, standing or supine position.

Some of these differences probably depend on whether manufacturers considered their machines a substitute for conventional CT for certain diagnostic tasks, a complement to it, or a combination of both. For some diagnostic tasks in the head and neck region, requiring good resolution of soft tissues, as in the diagnosis of malignant tumors and when examining the severely traumatized patient, conventional CT is still the method of choice. In a radiology department, DVT can thus be considered a complement to conventional CT. However, many tasks that previously could only be managed by CT can now be handled by means of DVT, often with better results. Other tasks, for which one rarely contemplated the use of CT, due to both higher radiation doses and costs as well as less satisfactory results, can DVT. now, be handled by

In our experience most diagnostic problems in the jaws can be solved by the use of rather small volumes. Larger volumes than 8 cm x 8 cm are rarely needed, and when they are, the examinations can be made with a medical CT unit. When the information required concerns hard tissues only, the use of a low-dose-protocol can yield doses on a par with those from a DVT examination. When detailed soft tissue information is required medical CT examinations must be used. DVT examinations are mainly used for: pre-implant planning purposes and for post-operative examinations when problems have arisen ; assessment of pain in the jaws, and presurgical assessment of endodontically treated teeth with remaining problems(Fig 1) ;determination of anatomy and position of non-erupted teeth and their influence on neighbouring tooth structures (Figs. 2-4). Other applications are assessments of dento- alveolar traumata, temporomandibular joint problems, cystic lesions, and tumors, in short, all types of problems for which three-dimensional information is of essence.

Fig. 1. In a patient with several episodes of unilateral maxillary sinuitis, a DVT examination showed an almost completely filled right maxillary antrum and soft tissue swellings in neighbouring ethmoidal cells. At the apical part of the upper right second molar, a small lesion is seen at the apex of its palatal root, which is fused with the disto-buccal root.

Fig. 2. A curve drawn along the longitudinal axis of a non-erupted lower molar provide cross-sectional images, perpendicular to this curve, yielding detailed information about the relation between the roots of the tooth and the mandibular canal cavity.

Fig. 3. A non-erupted lower molar has caused extensive resorption of the distal root of the adjacent second molar. A volume-rendered image with some of the thinner, marginal, bone digitally removed shows the position of the third molar underneath the distal part of the crown of the second molar.

Fig. 4. The axial view of a DVT examination shows non-erupted canines and premolars, the latter in less than normal positions. To fully describe the anatomy of these teeth, and their positions relative to other teeth requires that many new sections are made in different directions. In such a case the volume rendered images provide a quick understanding of the anatomical conditions.

Magnetic resonance imaging Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body. MRI provides much greater contrast between the different soft tissues of the body than computed tomography (CT) does, making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging. Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body. Radiofrequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body. MRI is a relatively new technology, which has been in use for little more than 30 years (compared with over 110 years for X-ray radiography). The first MRI Image was published in 1973 and the first study performed on a human took place on July 3, 1977.

How MRI works The body is mainly composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner these protons align with the direction of the field. A second radiofrequency electromagnetic field is then briefly turned on causing the protons to absorb some of its energy. When this field is turned off the protons release this energy at a radiofrequency which can be detected by the scanner. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.

Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue. Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint, in the case of arthrograms, MR images of joints. Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants, cochlear implants, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radiofrequency pulses. MRI is used to image every part of the body, and is particularly useful in neurological conditions, disorders of the muscles and joints, for evaluating tumors and showing abnormalities in the heart and blood vessels.

An advantage of MRI is its ability to produce images in axial, coronal, sagittal and multiple oblique planes with equal ease. MRI scans give the best soft tissue contrast of all the imaging modalities. With advances in scanning speed and spatial resolution, and improvements in computer 3D algorithms and hardware, MRI has become an essential tool in musculoskeletal radiology and neuroradiology. One disadvantage is that the patient has to hold still for long periods of time in a noisy, cramped space while the imaging is performed. Claustrophobia severe enough to terminate the MRI exam is reported in up to 5% of patients. Recent improvements in magnet design including stronger magnetic fields, shortening exam times, wider, shorter magnet bores and more open magnet designs have brought some relief for claustrophobic patients. However, in magnets of equal field strength there is often a trade-off between image quality and open design. MRI has great benefit in imaging the brain, spine, and musculoskeletal system. The modality is currently contraindicated for patients with pacemakers, cochlear implants, some indwelling medication pumps, certain types of cerebral aneurysm clips, metal fragments in the eyes and some metallic hardware due to the powerful magnetic fields and strong fluctuating radio signals the body is exposed to. Areas of potential advancement include functional imaging, cardiovascular MRI, as well as MR image guided therapy.

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Physics principles Nuclear magnetism Subatomic particles such as protons have the quantum mechanical property of spin. Certain nuclei such as1H (protons),2H,3He,23Na or31P, have a non zero spin and therefore a magnetic moment. In the case of the so-called spin-1/2 nuclei, such as1H, there are two spin states, sometimes referred to as "up" and "down". Nuclei such as12C have no unpaired neutrons or protons, and no net spin: however the isotope13C (referred to as "carbon 13") does.

Resonance and relaxation Relaxation (NMR) In the static magnetic fields commonly used in MRI, the energy difference between the nuclear spin states corresponds to a photon at radio frequency (rf) wavelengths. Resonant absorption of energy by the protons due to an external oscillating magnetic field will occur at the Larmor frequency for the particular nucleus. The net magnetization vector longitudinal magnetization is due to a tiny excess of protons in the lower energy state. This gives a net polarization parallel to the external field. Application of an rf pulse can destroy (with a so- called 90 pulse) or even reverse (with a so-called 180 pulse) this polarization vector. has two components. The

The transverse magnetization is due to coherences forming between the two proton energy states following an rf pulse typically of 90 . This gives a net polarization perpendicular to the external field in the transverse plane. The recovery of longitudinal magnetization is called longitudinal or T1relaxation and occurs exponentially with a time constant T1. The loss of phase coherence in the transverse plane is called transverse or T2relaxation. T1is thus associated with the enthalpy of the spin system (the amount of spins in parallel/anti-parallel state) while T2 is associated with its entropy (the amount of spins in phase). T1is longer than T2and times may vary depending on the fluidity of the tissues (e.g., if inflamed). Contrast agents work by altering (shortening) the relaxation parameters, especially T1.

MRI versus CT A computed tomography (CT) scanner uses X-rays, a type of ionizing radiation, to acquire its images, making it a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for non-calcified tissue, though MR images can also be acquired from bones and teeth as well as fossils. CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh such as iodine or barium. Contrast agents for MRI are those which have paramagnetic properties, e.g. gadolinium and manganese. Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features. (See Applications above.)

MRI can generate cross-sectional images in any plane (including oblique planes). In the past, CT was limited to acquiring images in the axial (or near axial) plane. The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality. For purposes of tumor detection and identification in the brain, MRI is generally superior.[32][33][34]However, in the case of solid tumors of the abdomen and chest, CT is often preferred due to less motion artifact. Furthermore, CT usually is more widely available, faster, much less expensive, and may be less likely to require the person to be sedated or anesthetized. MRI is also best suited for cases when a patient is to undergo the exam several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation.

Ultrasound (US) Medical ultrasonography uses ultrasound (high-frequency sound waves) to visualize soft tissue structures in the body in real time. No ionizing radiation is involved, but the quality of the images obtained using ultrasound is highly dependent (ultrasonographer) performing the exam. The first ultrasound images were static and two dimensional (2D), but with modern-day ultrasonography 3D reconstructions can be observed in real-time; effectively becoming 4D. Because ultrasound does not utilize ionizing radiation, unlike radiography, CT scans, and nuclear medicine imaging techniques, it is generally considered safer. For this reason, this modality plays a vital role in obstetrical imaging. US have been used to image the major salivary glands and the soft tissues. Doppler US is used to assess blood flow as the difference between the transmitted and returning frequency reflects the speed of travel of red cells. Doppler US has also been used to assess the vascularity of lesions and the patency of vessels prior to reconstruction. on the skill of the person

Sialography This is the imaging of the major salivary glands after infusion of contrast media under controlled rate and pressure using either conventional radiographic films, or CT scanning. The use of contrast media will reveal the internal architecture of the salivary glands and show up radiolucent obstructions, e.g. calculi within the ducts of the imaged glands. Particularly useful for inflammatory or obstructive conditions of the salivary glands. Patients allergic to iodine are at risk of anaphylactic reaction if an iodine-based contrast medium is used. Interventional sialography is now possible, e.g. stone retrieval. Arthrography Just as the spaces within salivary glands can be outlined using contrast media so can the upper and lower joint spaces of the TMJ. Although technically difficult, both joint compartments (usually the lower) can be injected with contrast media under fluoroscopic control and the movement of the meniscus can be visualized on video.