THE TECHNOLOGY
Magnetic resonance imaging The basis of MRI imaging modality is a magnetic field generated around a patient which aligns spinning nuclei, the majority of which are protons, electromagnetically with the field flux. A radiofrequency pulse is then applied, causing some of the nuclei to absorb energy (resonate), where protons spin in a higher energy state. When the pulse is turned off, relaxation occurs, protons return to the lower energy state and the stored energy is released, which is detected by a receiver coil in the MRI system. These signals are then used to generate the image.
Ultrasonography
This application uses ultrasonic energy (in the range of 7.5– 30 MHz) to obtain a topographical map of tissue interfaces at differing depths in the body. A transducer converts electrical energy into sonic energy using piezoelectric crystals. The transducer is held against the body part of interest. This ultrasonic beam interacts with the various tissues which all have different acoustic impedance. Some of the sonic waves will reflect (echo) back to the transducer, generating an electric signal which is used to produce the diagnostic image. Each tissue has a characteristic echo pattern, allowing detection of tissue boundaries or pathological changes within the tissue. Real-time imaging is possible, as the processing of these echoes occur at a rapid enough rate to allow perception of motion.
Nuclear medicine
A functional imaging technique, which detects abnormal metabolic processes in the body, rather than anatomical/ morphological changes, which may not be discernible in the early stages of some diseases.
• Radionuclide imaging: evaluates tissue function by utilising radioactive atoms or molecules (radionuclides) which emit gamma rays (e.g. Technetium 99m). These radionuclides are combined with a pharmaceutical to form a radiotracer which is distributed to various parts of the body based on their chemical properties. A gamma camera captures the emitted photons, converting them to light, then into a voltage signal for image reconstruction.
• Single photon emission computed tomography (SPECT): Is a method for acquiring tomographic slices through a patient, where a single or dual headed gamma camera rotates around the patient detecting emitted gamma rays. This data is processed via filtered back-projection or iterative reconstruction algorithms to form axial slices, similar to a CT image.
• Positron emission tomography (PET): Is an imaging technique using positron emitting radionuclides (e.g. 11C, 13N, 15O, 18F) which are usually combined with pharmaceuticals such as glucose or amino acids, to assess metabolic processes in the body. After a set period of time, positron emission decay occurs, and two photons are produced which travel in opposite directions. A PET camera has a ring of detectors which can map the photons that arrive at the same time and this information is used to produce a functional image of organs and tissues.
• There are now hybrid imaging systems where the nuclear medicine images are co-registered with CT or MRI images (i.e. SPECT/CT, PET/CT and PET/MRI) allowing for combined morphological and functional imaging.
ADVANTAGES
MRI
• No ionising radiation.
• Excellent soft tissue contrast compared to X-ray based techniques due to the higher water content in soft tissues. Certain anatomical and pathological structures with greater vascularity and permeability can be enhanced by intravenous paramagnetic contrast agents such as gadolinium.
Ultrasonography
• No ionising radiation.
• Good soft tissue discrimination and sensitivity for superficial mass lesions.
• Colour Doppler sonography for evaluation of blood flow is possible. • May be a useful alternative for patients who are contraindicated for MRI.
Nuclear medicine
• Evaluates physiologic alterations of tissues.
• Identify early changes of some diseases not demonstrated in other techniques.
• PET has very high spatial resolution and is able to detect very small lesions.
DISADVANTAGES
MRI
• Ferromagnetic objects may move, overheat and therefore injure the patient when in the vicinity of the magnetic field. Therefore, this modality is contraindicated for some patients with some implanted metallic objects or medical devices.
• Metals used in dentistry will not move but may distort the image in its vicinity. Titanium implants only cause minor degradation of the image.
• Longer scan times.
• May not be suitable for claustrophobic patients.
• The use of gadolinium-based contrast media must be used with caution in those with renal impairment as this has been associated with nephrogenic systemic fibrosis. Gadolinium deposition within regions of the brain has recently been discovered and is currently being investigated.
Ultrasonography
• Difficulty in imaging deeper structures and structures obscured by bone.
Nuclear medicine
• Associated with ionising radiation.
APPLICATION IN THE OROFACIAL REGION MRI
• Evaluating of soft tissue anatomy and pathology, characterisation and extent of lesions e.g. evaluating for perineural spread of tumours. • Additional characterisation of soft tissue components of bone lesions.
• Considered the gold standard in the assessment of the soft tissues of the temporomandibular joint, particularly the articular disc position. Also demonstrates joint effusions, synovitis, marrow oedema, and changes in the adjacent masticatory muscles.
• Implant dentistry: Identifying the location of inferior alveolar neurovascular bundle where multislice CT or cone beam CT is not able to demonstrate the location of the mandibular canal.
Ultrasonography
• Commonly used in the orofacial region for the evaluation of salivary gland, cervical lymph nodes and neck lumps.
• Ultrasound guided fine needle aspiration and core biopsies.
• Other applications in this region include evaluation of the thyroid glands and carotid vessels.
Nuclear medicine
• Osteoblastic metastatic neoplasms involving bone.
• SPECT has been used to assess mandibular growth in patients with asymmetry. The limited specificity of these studies must be considered. Correlation with CT and/or MRI is useful.
• The extent of medication-induced osteonecrosis of the jaw (MRONJ).
• Gallium and radio labelled white cell radionuclide and SPECT/ CT imaging is useful in diagnosing base of skull osteomyelitis.
• PET/CT imaging is useful for skeletal imaging for assessment of primary bone tumours, locating metastases in bone and detecting osteomyelitis. It is often correlated with post contrast CT or MRI scans.
• PET/CT is particularly useful for staging squamous cell carcinoma and other head and neck malignancies.
INTERPRETATION
Radiologists, neuro-radiologists, maxillofacial radiologists and nuclear medicine specialists perform the interpretation of these studies.
Published on March 15, 2021
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