The digital revolution in diagnostics imaging specialty has brought in dynamic changes in healthcare areas of medicine and dentistry. Recent developments in imaging sciences have enabled dental researchers to visualize structural and biophysical changes effectively. Various imaging techniques are used in the evaluation of patients with maxillofacial pathologies including head and neck cancer, before, during and after treatment. New techniques and technologies for intra-oral radiography allow doctors to conduct densitometric studies and assessments of dento alveolar structures. Longitudinal changes in alveolar bone can be studied by computer assisted image analysis programs. These techniques have been applied to dimensional analysis of the alveolar crest, detection of gain or loss of alveolar bone density, implant bone healing and caries detection. Dental application of computed tomography (CT) includes the detailed radiologic anatomy of alveolar processes, official soft tissues and air spaces and developmental defects. Image analysis software permits bone mass mineralization to be quantified by means of CT data. CT has also been used to study salivary gland disease, injuries of the facial skeleton and dental implant treatment planning. Magnetic Resonance Imaging or MRI has been vastly used in retrospective and prospective studies of internal derangements of the temporomandibular joint. Assessments based on MRI imaging of the salivary glands, paranasal sinuses and cerebrovascular disease has also been reported. Magnetic Resonance Spectroscopy (MRS) has been applied to the study of skeletal muscle, tumor and to monitor the healing of grafts. Nuclear imaging provides a sensitive technique for early detection of physiological changes in soft tissue and bone. It has been used in studies of Periodontitis. Oseteomyelitis, oral and maxillofacial tumors, fractures, bone healing, temporomandibular joint (TMJ) and blood flow. Specialized imaging applications are discussed below.
The use of radiopaque contrast media for visualization of soft tissue is referred to as contrast radiography. Its various implications region include.
Cystography (for soft tissue cysts)
Sialography (for salivary glands)
Arthrography (injection of dye into superior compartment of TMJ for its examination)
Arteriography (localization of one or more local arteries)
Barium swallow (important investigation in cases of dysphagia).
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI)
High resolution rapid computed tomography (CT) and Magnetic Resonance Imaging (MRI) have long since proven themselves sensitive and reliable in appropriate applications. Indeed. Imaging has become an indispensable tool in the characterization and staging of conditions involving the head and neck and clinicians have come to incorporate imaging data with physical examinations and endoscopy. Ct and MRI not only provide essential information about the deep extension of clinically detected masses, they can also delineate additional clinically unsuspected lesions. The excellent tissue characterization of MRI scans can lead to an accurate diagnosis of many benign processes as well.
Nowadays in most patients Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) is performed for pretherapeutic staging of a head and neck malignancy. Both techniques can supply the information needed by the clinician for adequate treatment planning. The most widely used technique is CT, as it has a number of important advantages over MRI. These advantages of CT scan over MRI are listed below:
Relative low cost
Easy to execute and this in a reproducible way
Short examination time resulting in less image quality degradation caused by motion such as swallowing and respiration .
Superior bone detail
High quality multiplanar imaging on multidetector CT systems
Easy extension of the study into the upper thoracic cavity or intracranial cavity if needed
Easier interpretation, especially regarding nodal involvement.
However CT also has a number of disadvantages compared to MRI. These disadvantages are as under.
Relative low soft tissue contrast resolution
Administration of iodinated contrast agent is necessary
The advantages of MRI over CT in the evaluation of head and neck pathologies are its superior soft tissue contrast resolution and the absence of radiation exposure. The disadvantages of MRI are mainly related to the long acquisition time which makes the technique sensible to motion artifacts, which cause a non diagnostic study. It is also some what more difficult with MRI to properly stage both primary tumor and neck nodal disease in a single study. The lower availability of MRI thereby resulting in a longer waiting list and its higher cost should also be taken into consideration.
Advances in Computed Tomography (CT scan)
Standard Spiral CT
CT scanning of the head and neck should be tailored for the anatomic region under consideration. Digital lateral scout radiography may assist in planning the scan ranges. Spiral (Helical) CT scanning is rapidly replacing conventional dynamic CT (slice-by-slice acquisition) in most medical centers. Multirow detector technology, which entails rotating the x-ray tube while simultaneously employing multiple parallel detector arrays rather than a single detector row has further advanced spiral CT by reducing scan time and greatly increasing anatomic coverage.
Multi Detector Spiral CT
State of the art CT of the head and neck requires the use of MDCT. The rapid acquisition results in a volumetric data set, reconstructed to a stack of thin and overlapping native images; this reduces partial volume averaging and motion artifacts. Furthermore, full advantage of the injected contrast agent is accomplished by optimal timing between injection and image acquisition. The disadvantage of this technique is the overall higher radiation exposure.
Low Radiation Three Dimensional Computerized Tomography (3-D CT)
3D CT is new technology in imaging diagnostics which gives high definition 3-D CT scans to Radiologists, of maxilla and mandible areas of patients. 3D CT is very quick in scan and takes approximately 20 seconds to scan. This technique of imaging is patient friendly as the patient is seated in upright position similar to a standard panoramic procedure. Further the level of exposure to radiation for patients is quite lower than typical standard medical CT scan or Full mouth Series X-ray (FMX).
3-D CT data is useful for many areas of medicine, surgery and dentistry. However in dentistry field is used for the following procedures and dental conditions.
Sinus and airway studies
Oral Surgery for Impacted teeth evaluations
Oral path reconstruction studies
Dental Implant Planning
Bone grafting evaluations
CT Angiography (CTA)
CTA is now challenging catheter angiography as the primary method for assessing the vessels of head and neck. In addition to evaluating the carotid arteries for evidence of atherosclerotic disease, CTA can effectively detect arterial dissection in trauma patients. Primary arterial disease such as fibro muscular dysplasia is also well demonstrated. Evaluation of vascular encasement by tumors is another application.
The advantages of CTA include its rapid data acquisition and relative noninvasiveness properties that are especially important for the critically ill patients. Moreover patients with cardiac pacemakers and ferromagnetic intracranial aneurysm clips, which are contraindications to MR Angiography (MRA), can undergo CTA. The drawbacks of CTA include venous contamination due to ill timed contrast boluses, physician time intensive data manipulation at a workstation, artifacts due to metallic foreign bodies and potentially confusing in formation caused by heavy mural calcification and adjacent bone.
Perfusion CT (CTP)
It is showing promise for evaluating masses by measuring the mean transit time, blood flow and blood volume in benign neck lesions in comparison with malignant lesions. This application could potentially overcome some of the limitations of other techniques such as Positron Emission Tomography (PET).
3D Virtual Bronchoscopy
3D Virtual Bronchoscopy is a visual inspection of the trachea and airways. The procedure of 3D Virtual Bronchoscopy is carried out by putting a rigid or flexible scope through the nose or mouth of the Patient. The procedure is carried out under anesthesia. In 3D Virtual Bronchoscopy, the data obtained from Multislice CT is examined. The Multislice CT of the neck and upper chest, targeted to the airway with a single breath hold with no pain to patient.
Advances in Magnetic Resonance Imaging (MRI)
MRI of the neck should be tailored for the anatomic region and process under evaluation. Unenhanced axial T1- weighted images display anatomic relationships and can detect lesions (e.g., lymph node lesions) embedded within the fat. TI weighted coronal images can define the false vocal cords, true vocal cords, laryngeal ventricle and the floor of mouth. TI- weighted sagittal images provide helpful information about the pre-epiglottic space and nasopharynx.
T2-weighted transaxial images characterize tissue and can detect tumor within muscle, demonstrate cysts and assists in differentiation of post therapy fibrosis from recurrent tumor. Fast Spin Echo (FSE) T2-weighted imaging has the added advantage of a relatively short acquisition time. Gradient moment nuling, flow compensation, cardiac gating and presaturation pulses minimize motion artifacts.
MR Spectroscopy (MRS)
MRS has been used to examine 31-phosphorus 19-fluorine, and 13-carbon in tissues. However, most investigations have focused on hydrogen (proton) spectra. Although intracranial application of MRS has been steadily increasing but applications in the extracranial head and neck have been disappointing. First of all MRS requires a homogeneous magnetic field. Susceptibility artifacts introduced by the paranasal sinuses airway and bone and pulsation artifacts from the carotid artery, severely degrade data. Additionally a large amount of fat within the neck produces a lipid peak that obscures the relatively small peaks of tumor markers such as choline. Finally, MRS remains rather nonspecific. Because many potentially lethal head and neck tumors begin as small nests of cells within the mucosa, the lack of sufficient spatial resolution by MRS severely limits its application. Moreover most head and neck lesions begin as mucosal processes and the mucosa abuts the airway, further contributing to inhomogeneity and artifacts. Nevertheless, work continues and newer techniques may eventually make MRS practical for head and neck application.
MRI data sets can be reconstructed in three dimensions and specifically can yield diagnostic angiographic images without the pain or risks of traditional angiography. The possible areas of imaging include MR Angiography of the vessels of the brain, neck, chest, pulmonary arteries, abdominal aorta, renal vessels, peripheral arteries and veins.
In some conditions like acute stroke the molecular movement of water is reduced. The diffusion MRI is used to evaluate such conditions. It is one of the fastest and around 100%accurate especially in the diagnosis of acute cerebral infarction. These scanning techniques help in the diagnosis of certain conditions like ischemia and helps in the treatment of the conditions with thrombolytic therapy.
This technique of functional MRI is used to find out the regions of increase cerebral blood flow which is temporary and can be there due to some surgery. By using this technique it can be assessed which patient s fit for surgery and which patient is at risk. So before surgery the functional MRI technique is very useful for planning the surgery.
Applications of CT and MRI in Head and Neck
Nuclear Imaging Techniques
The artificial manufacture of gamma ray emitting isotopes like gallium-67or Tc99m and development of gamma scintillation cameras has made the diagnostic modality of radio nucleotide a reality. This modality allows for dynamic visualization of basic uptake and secretion from many of tissues of human body. This technique can be used to detect areas of increased metabolism, termed as hot spot or areas of decreased metabolism called as cold spot. It is used in detection of metastatic tumors, salivary gland pathologies, TMJ tumors and lymphatic pathology.
Newer imaging techniques such as Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are being used more frequently in the evaluation of head and neck cancer. 18-labuoro-2-deoxy-D-glucose (FDG) as tracer, have received a lot of attention in recent years. PET can be used in pretreatment staging process, during radiotherapy planning and in the post treatment work-up. Overall, PET has a higher sensitivity for detecting tumor compared to CT and MRI. Physiological tracer uptake e.g in the thyroid gland, lymphoid tissue, salivary glands and in active muscles may cause confusion. Tracer accumulation in ammation such as observed in therapy induced tissue changes may also cause false-positive results. Other pitfalls of PET imaging are the low spatial resolution and the lack of tracer uptake in some neoplasm causing false negative results. The high cost of PET imaging is an important drawback.
Ultrasonography with Color and Power Doppler
Ultra sonography diagnostic imaging technique is based on ultrasound and is used to scan muscles and pathologies. Doppler measurements, employing Doppler effect is used to diagnose whether structures are moving to or away from ultra sound probe and its relative velocity. Its speed and direction is determined and marked by calculating the frequency shift for particular sample volume. This information is displayed graphically using spectral Doppler or as an imaging using color Doppler or Power Doppler.
Advantages of Ultrasonography with Color and Power Doppler
1. Organ structure is depicted with Ultrasonography.
2. The equipment of Ultrasonography equipment is easily available.
3. Ultrasonography with color and power Doppler is not known to give any long term or short term side effects and the patient can get it done with ease without any discomfort.
4. If it is compared with other diagnostic aids used like computed X-ra Tomography it is less expensive.
5. As compared to other imaging modalities its spatial resolution is quite good.
6. It can be used for examination at bed side as small scanners are there which are easy to carry to the bed side.
7. It gives the live images which help in choosing the most useful section for diagnosing and documentation and helps the operator during ultrasound guided biopsies or injections.
The role of Ultrasound in maxillofacial diagnosis is diverse. It is very useful for diagnosis of vascular lesions, hemangiomas, face and neck masses neurofibromas, massetric hypertrophy, salivary gland disorders as well as for knowing the nodal status. Several studies have also proved that ultrasound can be used as an alternative diagnostic therapy to MRI in diagnosis of TMJ disc derangements. Interestingly ultrasound with Color and Power Doppler can also be used to differentiate periradicular lesions.
Laser Doppler Flowmetry
Laser Doppler flowmetry is non invasive, physiologic technique used to measure the microcirculatory blood flow, in the laminar level. The biggest advantage of this technique is that it is non invasive and it is considered that ultimately the disease occurs at the microcirculatory laminar level. It can be used to measure pulpal blood flow and helpful in diagnosis of burning mouth syndrome demonstrating the micro circulating blood flow in the area of burning sensation.
It is the procedure by which the heat naturally emitted by body is detected, measured and imaged. The resultant image thermogram is a visualization of the distribution of heat pattern of body surface. The most commonly used method is Infrared Thermography.
Local metabolism, which generates heat, is often increased in areas of malignant tumor inflammation and injury. The temperature is represented in shades of gray. This technique have been used to differentiate dental pain from a typical odontalgia, spread of infection in cellulites and TMJ internal derangements.
Advances in Sialography
The advances in fiberoptic imaging and miniaturization of digital imaging cameras and videos have resulted in development of Sialoendoscopy, in which endoscope is inserted into parotid and submandibular ducts in cases of sialolithiasis or any other obstructions.
There are unimaginable advances in the imaging technology which has enabled the clinician to diagnose the problem at the earliest. To detect the recurrent cancer at the earliest great advances are there in MR imaging, PET, and SPECT. Now the manufacturers are trying to combine the potential of all the imaging techniques to get the best results. It is possible to combine CT with PET or CT with SPECT but future need is to combine MR scanning with some metabolic imagine device such as PET or SPECT. The imaging technology is not only used for diagnosis but it is not being integrated with the therapy part. The MR imaging or the CT and sonography are being used not only in operative rooms but also during the radiation therapy. These imaging systems can be used as screening device to detect the high risk patients. In such patients the early detection of any disease can help in the treatment.