INTRODUCTION OVER the last 40 years an array of imaging modalities has been developed that has enhanced the already versatile x-ray generating equipment and film used in conventional image production. computed tomography was developed in the early to mid 1970s and is a radiographic technique for producing cross-sectional tomographic images. Claimed to be 100 times more sensitive than conventional x-ray systems, it demonstrated differences between various soft tissues never before seen with x- ray imaging techniques.
HISTORY 1961-Oldendorff W.H recognized the potential of reconstruction tomography. 1963- Cornmark used a source and detector rotate around a non symmetrical phantom and a computed for processing the transmission data. 1972- Godfrey Hounsfield an engineer at EMI(Electrical musical instruments) limited, England announced the invention of a revolutionary imaging technique which he referred to as Computerized Axial Transverse Scanning..
With this technique he was able to produce an axial cross-sectional image of the head using a narrowly collimated ,moving beam of X-rays. 1979-Cornmack and Hounsfield were awarded the Noble prize in Physiology and Medicine. From 1971to 1975 ,within a span of 4 years, four generation of scanner evolved, which yielded shorter times and better control over the patient‟s motion. In fifth generation CT scanner, scanning time is reduced to 16 milliseconds. 1998- CBCT was invented
TOMOGRAPHY Tomography is a process by which an image layer of the body is produced, while the images of the structures above and below that layer are made invisible by blurring.
Tomography may be classified into many types: Conventional Tomography Computed Tomography Three - dimensional C T Spiral Computed Tomography Emission Computed Tomography
Conventional Tomography Tomography is a generic term, formed from the Greek words tomo (slice) and graph (picture) that was adopted in 1962 by the International Commission on Radiographic Units and Measurements to describe all forms of body section radiography.
This is achieved by a synchronized movement of the film and the tube in opposite directions, about a fulcrum (i.e. the plane of interest in the patients body). Objects closest to the film are seen most sharply and objects farthest away are completely blurred. The thickness of the image layer depends on the angle of rotation or the amount of movement of the tube. Some degree of image degradation also occurs within the image layer. The greatest amount of blurring is at the periphery of the image layer, and the sharpest image is at the center
Computed Tomography (CT) A computed tomographic image is a display of anatomy of a thin slice of body developed from multiple X-ray absorption measurements made around the bodys periphery.
Computed tomography (CT) permits the imaging of thin slices of tissue in a wide variety of planes. Most CT is done in the axial plane, and many CT scans also provide coronal views; sagittal slices are less commonly used.
Slice thickness is usually10 mm through the body and brain 5 mm through the head and neck, unlessthree dimensional reconstruction isanticipated. In such cases, the slice thickness is 1.0 to1.5 mm in order to provide adequate data
BASIC PRINCIPLE CT scanners use the X-rays to produce the sectional or slice images ,as in conventional tomography, but radiographic film is replaced by sensitive detectors. The detectors measure the intensity of the x-ray beam emerging from the patient and convert this into digital data which are stored and can be manipulated by a computer. This numerical information is converted into a gray scale representing different tissue densities ,thus allowing a visual image to be generated.
1. First generation (Rotate / Translate, pencil beam) The original EMI unit was the first generation scanner. It was rotate/translate pencil beam system. Only two detectors were used, which measured transmission of X-ray through the patient for two different slices. That is two tomographic sections were taken simultaneously. It was designed specifically for evaluation of brain. In this unit head was enclosed in a water bath. The linear motion was repeated 180 times and after one linear movement ,gantry rotated 1 degree. X-ray beam was on during linear motion ,while off during rotation. The transmitted radiation was 160 times during each linear movement . Total no. of transmission-160x180= 28,800 Scan time was 4.5 to 5 min. Matrix was 80x80
Second generation (Rotate / Translate, narrow fan beam) Second generation scanner were also of translate- rotate type. These units were incorporated a linear array of 30 detectors. The use of 30 detectors increased the utilization of the X-ray beam by 30 times over the single detector used per slice in first generation systems. Source detector assembly intercepting a fan shaped (a narrow fan angle of 10°) beam rather than a pencil sized X-rays beam. Instead of moving 1 degree at the end of each linear scan ,the gantry rotates through a greater arc, upto 30 degree. So linear movement have to be repeated six times to cover 180 degree. Scan time was 10-90 sec.
Third generation (Rotate/rotate wide fan beam) The translation motion of first and second generation was a major limitation because at the end of each translation, the translational inertia of X-ray tube/detector system had to be stopped; the whole system rotated and then the translation motion had to be restarted. This design could never have led to fast scanning. To overcome this limitation third generation scanners evolved. Third generation scanner uses increased number of detector (upto about 750 detector) and rotate-rotate system i.e. X-ray tube and detector array were rotated. The detector is aligned around an area of a circle whose centre is focal spot. X-ray beam is collimated into fan beam (fan angle was about 50°). Scan time was 2 to 10 sec.
Cone Beam Radiology CBCT uses a round or rectangular cone – shaped x-ray beam centered on a two – dimensional x-ray sensor to scan a 360 degree rotation about the patient‟s head. During the scan a series of 360 exposures or projections, one for each degree of rotation, is acquired, which provide raw digital data for reconstruction of the exposed volume by computer algorithm.
Depending on the equipment, scan time range from 17 sec to little more than 1 min. Multiplanar reformatting of the primary reconstruction allows for both three- dimensional and two-dimensional images of any selected plane to be made. Resolving power is four times that of CT Less expensive Radiation dose is 3-20 % that of conventional CT.
Fourth generation CT scanner (rotate /stationary) Fourth generation CT scanner were designed to overcome the problem of electronic drift between many detectors used in the system so this design eliminated ring artifact. Fourth generation CT scanner uses rotate only motion. Huge tube rotated but the detector assembly does not. The detector forms a ring that completely surrounds the patient. The X-ray tube rotates in a circle inside the detector ring and X- ray beam was collimated to form a fan beam. Was not faster in principle than third generation. Easier detector calibration.
Fifth generation systems- Developed by Dr. E Woods of Mayo Clinic. System consists of multiple rays tubes and detectors. Such a unit is primarily used to image 3D sections of the heart and reduces artifacts caused due to cardiac rhythm.
CT EQUIPMENT The equipment consist of: Gantry containing x-ray source, detectors and electronic measuring devices Motorized table- used to position the patient within gantry X-ray power supplies and controls Computer Viewing devices
X-RAY TUBES Radiation source for CT would supply monochromatic X-ray beam by which image reconstruction is simple and more accurate. Earlier models used oil-cooled, fixed – anode, relatively large (2x16mm)focal spot tubes at energies of about 120 kilovolt (constant potential) and 30mA. The beam was heavily filtered to move low energy photons and to increase the mean energy of the radiation.
Most newer fan beam units have a diagnostic –type x-ray tube with a rotating anode and a much smaller focal spot ,in some units down to 0.6mm and generate X-rays in short bursts, or pulses. These tubes are air-cooled and operate at much higher currents, upto 600mA. They are (cathode-anode ) perpendicular to the fan beam to avoid asymmetry in X-ray output because of the heel effect.
Recently, special types of X-ray tubes have been developed for CT. These tubes are designed to withstand the very high heat loads generated when multiple slices are acquired in rapid sequence.
COLLIMATORS The x-ray beam is collimated at two points ,one close to the x-ray tube and the other at the detector. The collimator at the detector is the sole means of controlling scatter radiation. The collimators also regulate the thickness of the tomographic section(voxel length)
Detectors-1.Gas filled ion chamber detectors made of high pressure xenon. Capture about 50 % photons in the beam2. Solid state detector commonly used, 80% efficient. Usually made of cadmium tungstate.
TECHNIQUE image The process by which production of CT occur is called scanning. The patient lies down with the part of the body to be examined within the circular gantry, housing the X-ray tube head and detector. The level of plane and thickness of the section to be imaged are selected and x-ray tube head rotates around the patient, scanning that section. As the tube head rotate around the patient each set of detector produces an attenuation or penetration profile of the region of the body being examined.
These detectors produce electrical impulses that are pro-portional to the intensity of the X-ray beam emerging from the body That intensity is determined by various factors; 1. the energy of the X-ray source, 2. the distance between the source and the detector 3.the attenuation of the beam by the material in the object being scanned. Penetration profile is stored in the computer, which calculates the density or absorption at points on a grid formed by the intersections of penetrating profiles. The CT image is a digital image, reconstructed by the computer, which mathematically manipulates the transmission data obtained from the multiple projections
The image consists of a matrix of individual blocks called voxels (volume element).It consist of an array of individual points or pixels. The size of the pixel is determined by: The geometry of the scan, The frequency and spacing of measurements, The number of penetration profiles and The size of the x-ray source and detector Each pixel is assigned a CT number or Hounsfield unit(HU) between +1000 to -1000, depending upon the amount of the absorption within that block of tissue.
Each number or pixel represents a calculation of the actual attenuation of the X-ray beam by materials with the body. It represents the absorption characteristics or linear attenuation coefficient of that particular volume of tissue in the patient.
IMAGE RECONSTRUCTION Images are typically 512 x 512 or 1024 x 1024 pixels. Rapid image reconstruction done by Two-dimensional Fourier Analysis Filtered back projection
CT numbers for various body tissues. Absorber CT number/HU Bone (dense) +400 to +1000 Soft tissues +40 to +80 Water 0 Fat -60 to -100 Lung -400 to -600 Air -1000
The computer can construct an image by printing the numbers or assigning different degree of greyness to each CT number. In some system ,the numerical values are translated into colours or brightness level that can be displayed on a television screen or printed on a paper.
Image display-----In two basic mode As a paper printout of CT numbers As a gray scale image on a cathode ray tube or television monitor.
WINDOW LEVEL AND WINDOW WIDTH These two variables enable the visual image to be altered by selecting the range and level of densities to be displayed. Window level-is the CT numbers selected for the centre of the range depending on weather the lesion under investigation is in the soft tissue or bone. Window width-is the range of CT numbers selected for various shades of grey.
The contrast and brightness of the image may be adjusted as necessary although the images are usually viewed in two modes: Bone windowing and soft-tissue windowing. In Bone windowing, the contrast is set so that osseous structures are visible in maximal detail. With soft-tissue windowing, the bone looks uniformly white, but various types of soft tissues can be distinguished.
DISTORTIONA signal can contain errors or distortions that arerepeatable (deterministic). For instance, if a patientmoves during the acquisition step, parts of theanatomy may be blurred or in different positionsfrom their true location. If the image reconstructionprocess requires linear and consistent data, but themeasurements for some reason are not consistent,artifacts can arise that may result in lost informationor spurious image features.
ARTIFACT Any discrepancy between the CT numbers represented in the image and the expected CT number based on the linear attenuation coefficient
IMAGE ARTIFACTS1. Partial volume artifact2. Beam hardening artifact (due to absorption of low energy photon from the beam.)3. Metal artifacts
METAL ARTIFACT MANIFEST AS “STAR STREAKING” ARTIFACT. CAUSED BY PRESENCE OF METALLIC OBJECTS INSIDE OR OUTSIDE THE PATIENT. METALLIC OBJECT ABSORBS THE PHOTONS CAUSING AN INCOMPLETE PROFILE
CONTRAST MEDIA Is used to obtain a differential change in the attenuation values of normal and pathologic tissues so that recognition of pathology is facilitated. one can expect a large variety of contrast enhancement in pathological tissues due to tissue alterations, mainly related to differences in vascular contrast distribution volume and total distribution volume. Iodine based iodine monomers- iothalmate, diatrizoate, metrizoate Non ionic monomer like iopamidol ,iotrioxol
INDICATIONS Intracranial diseases and trauma Malignancy of jaws Infection Post-irradiation Salivary gland Temporomandibular joint Implants Fracture Foreign body Imaging of unerupted and displaced teeth, bone grafting.
Advantages Cross-sectional imaging Superior contrast and resolution Geometric accuracy Images can be manipulated Axial tomographic sections are obtainable Images can be enhanced by the use of i.v contrast media, providing additional information.
Disadvantages Expensive Facilities are not widely available Very thin contiguous or overlapping slices may result in a high dose of radiation. Geometric miss Metallic objects such as fillings produce marked streak artifacts across the CT image.
Recent advances in CT 3 DIMENSIONAL CT- in this data obtained from CT scan is reformatted into 3D images. 3D CT requires that each voxel, shaped as rectangular parallel piped or rectangular solid be dimensionally altered into multiple cuboidal voxels. This process, called interpolation creates sets of evenly spaced cuboidal voxels (cuberilles) that occupy the same volume as the original voxel. The CT numbers of the cuberilles represent the average of the original voxel CT numbers surrounding each of the new voxel. Creation of these new cuboidal voxels allows the image to be reconstructed in any plane without loss of resolution by locating their position in space relative to one another.
Ultrafast CT: I matron (San Francisco) has developed a CT scanner capable of acquiring the data upto 10 times faster than conventional CT. About 50msec is able to freeze cardiac and pulmonary motion, enhancing the quality without motion artifact. Spiral CT scanners: (discovered in 1989) in this while the gantry containing the x-ray tube and detectors revolve around the patient, the patient table continuously advances through the gantry. This results in the acquisition of a continuous spiral of data as the x-ray beam moves down the patient. Advantage Improved multiplanar image reconstruction Reduced examination time(12sec vs 5 min) Reduced radiation dose(<75%)
Helical CT is now standard. In helical CT scanners, pitch refers to the amount of patient movement compared with the width of image acquired. table travel per X-ray tube rotation Pitch= image thickness
Multidetector helical CT (MDCT, multislice CT, or multirow CT) Introduced in 1998 Widely used With this method ,anywhere from four to 64 adjacent detector arrays are used in conjunction with helical CT. Time for full cycle rotation-0.35sec. The quality of axial ,reformatted, and three dimensional images ,also improved with this as compared to single-slice machines.
Electron beam CT– recent development In this machine an electron gun generates an electron beam that is focused electrostatically on a fixed tungsten target circling halfway around the patient. The X-rays that are generated expose the detector array circling the other half of the patient. Because there are no moving parts ,an image may be acquired in less than 100 microseconds. This technique is primarily used for cardiac imaging to stop heart motion.
Emission CT- is similar in principle to x- ray transmission CT .instead of section morphology ,it reflects physiological processes that concentrate the radionuclide in one or more organs or body compartments.
CBCT CTLess radiation dose MoreMore time LessImages are of lower HighercontrastSlice thickness 0.1 1-2 mmmmLess expensive More
Odontogenic infections Cellulitis- soft tissue swelling obliterating fat planes. Abscess- irregular zone of low density with a peripheral rim of contrast enhancement. Acute osteomyelitis- zone of increased contrast enhancement. Chronic osteomyelitis- destructive pattern with peripheral rim of contrast enhancement.
Carl W Hardin and RIC Harnsberger (1985)made use of CT in evaluation of infections andtumours involving the masticator spaces andfound that CT is helpful in differentiatinginflammation from frank abscesses.Alan A Schwimmer et al (1988) emphasized therole of CT in the diagnosis and management oftemporal and infratemporal space abscesses.
Sialadenitis CT is non-invasive, painless and less time-consuming. Non-contrast CT for detecting calculi. Contrast CT for abscess/cellulitis Salivary calculi seen as high density, non- contrast enhancing mass along the course of the duct.
Nick Bryan et al performed CT in 27 patientswith salivary gland neoplasm and concluded thatwhen CT is combined with the clinicalinformation and laboratory findings, the overallspecificity in identifying the tumour becomes90%.
ODONTOGENIC CYSTS Appear as localized, expansile degenerative area having a fluid density throughout the lesion. Do not show contrast enhancement in contrast aided imaging (except Aneurysmal Bone Cyst).
John W Frame and Michael JC Wake evaluatedmandibular keratocysts with CT and establishedthat CT provides better methods of accuratelydisplaying the margins of the keratocysts, the areasof bony perforation and any extension into softtissues.
ODONTOGENIC TUMOURS Appear as an expansile lesion having a soft tissue density which show moderate enhancement in contrast aided imaging (except cemental tumours). Foci of cystic degeneration are commonly seen. Show breach in cortical plates. Foci of calcifications are noticed in maturing odontogenic tumours.
Ameloblastomas- bicortical expansion, thinning and breach of bony walls, extension of tumour into adjacent soft tissue spaces. Focal cystic degenerations commonly seen in multilocular lesions. Plexiform ameloblastomas have high contrast enhancement due to high vascularity. Cystic ameloblastomas show a predominant fluid density. Malignant ameloblastomas have a grossly destructive pattern. Focal hyperdense areas suggesting calcifications maybe noticeable in Pindborg tumour.
Osborn et al (1982) made a study, on imaging ofseveral mandibular tumours and established theirosseous and soft tissue extensions. CT was foundvaluable in excluding the involvement of mandibleby primary osseous and soft tissue lesions ofadjacent areas.
MALIGNANCIES Seen as predominantly destructive lesions interspersed with focal high contrast enhancement areas. Invasive lesions show no/minimal expansion. Demarcation from surrounding soft tissue is difficult without contrast aided imaging. Reparative lesions like central giant cell granulomas also manifest as destructive, contrast enhancing lesions showing minimal or no expansile pattern.
Close LG et al (1986) found a critical factor in thepretreatment evaluation of patients with carcinoma of theoral cavity or oropharynx, the presence or absence of bonyinvasion. CT was more specific than conventional X-rayfilms in detecting bone invasion.Mark A Cohen and Yancu Hertzanu (1988) in their study onCGCG using CT, conventional tomogram and conventionalradiographs, proved CT to be superior in clearlydemonstrating the soft tissue mass of lesion, its extensioninto adjacent structures and bony destruction.
Fibro-osseous lesions CT pattern depends on the maturative stage of lesion. Cemental lesions are distinguished based on the continuity of the lesions with the roots of the tooth and the periodontal ligament space, separating the lesion from the bony alveolus.
Ariji Y et al (1994) studied cases of florid cemento-osseousdysplasia with conventional radiography and CT andobserved thin, low density areas around high density masseswith expansion of buccal and lingual cortical plates. CT wasable to give additional information by identifying the densityof these masses which ranges from 772-1582 HU. Thesevalues were suggestive of cementum or cortical bone.
MAXILLARY LESIONS Maxillary lesions share similar pictures in contrast to mandibular lesions which make this difficult to distinguish them.
Brenna Betti N, Bruno E et al (1993) For earlydiagnosis of the maxillary antrum carcinoma,besides a conventional radiographic test, also ofmore specific analysis, as the computedtomography and radio therapy.Colin P and Hodson N. Thirty-two patients withhistologically proved malignant disease involvingthe paranasal sinuses were studied by CT.Significantly greater tumor extent wasdemonstrated by CT than by conventional methods.
TMJ CT helps identify the bony changes in the TMJ like destruction of the condylar head, wearing of articular elements, traumatic lesions within and outside the capsule. Advantageous over arthrography as it is a painless procedure with superior resolution.
conclusion CT scan has made a major impact on the practice of dentistry, particularly in oral and maxillofacial diagnosis, surgery and management of a wide variety of oral lesions. Advances in computer softwares already allow 3 D visualization of anatomy and pathology, but further improvement in clinical performance is expected.
Here, radiant energy is in the form of radiofrequency wave rather than X-ray. Father of MRI- Felix Bloch
TYPES OF ATOMIC MOTION 1. The electron orbits the nucleus 2. The electron spins on its own axis 3. ***The nucleus spins on its own axis***
MRI USES THE HYDROGEN ATOM•1 electron orbits the nucleus•The nucleus contains no neutrons but contains 1proton THE HYDROGEN NUCLEUS HAS A NET POSITIVE CHARGE•Hydrogen nucleus is a spinning, positively chargedparticle
LAW OF ELECTROMAGNETISM•A charged particle in motion will create amagnetic field•The postitively charged, spinning hydrogennucleus generates a magnetic field WHY HYDROGEN?•Very abundant in the human body-H20•Has a large magnetic moment
MAGNETIC MOMENTThe tendency of an MR active nuclei to align its axis of rotation to an applied magnetic field MR ACTIVE NUCLEI odd # protons or odd # neutrons or BOTHe.g. Hydrogen1, Carbon13, Nitrogen15, Oxygen17, Fluorine19, Sodium23, Phosphorus31 STABLE ATOMS # protons = # electrons IONS # protons # electrons
When a body is placed into the bore of the scanner, the strong magnetic field will cause theindividual hydrogen nuclei to either: A) ALIGN ANTI-PARALLEL TO THE MAIN MAGNETIC FIELD (B0) OR B) ALIGN PARALLEL TO THE MAIN MAGNETIC FIELD (B0) Anti-parallel high energy B0 NMV Parallel low energy
NET MAGNETIZATIONVECTOR An excess of hydrogen nuclei will line up parallel to B0 and create the NMV of the patient
PRECESSION Due to the influence of B0, the hydrogen nucleus “wobbles” or precesses (like a spinning top as it comes to rest) The axis of the nucleus forms a path around B0 known as the “precessional path”
PRECESSION The speed at which hydrogen precesses depends on the strength of B0 and is termed the “precessional frequency” The precessional frequency of hydrogen in a 1.5 Tesla magnetic field is 63.86 MHz The precessional paths of the individual hydrogen nucleus‟ is random, or “out of phase”
The spinning protons wobble or “precess” about that axis of the external Bo field at the precessional, Larmor or resonance frequency. Magnetic resonance imaging frequency ω= Bo where is the gyromagnetic ratio The resonance frequency ω of a spin is proportional to the magnetic field, Bo.
RESONANCEOccurs when an object is exposed to an oscillatingperturbation that has a frequency close to its own natural frequency of oscillation
RADIOFREQUENCYENERGY Follows the Law of Electromagnetism (charged particles in motion will generate a magnetic field) Magnetic field known in MR as B1 Applied as a “pulse” during MR sequences The RF pulse is applied so that B1 is 90 to B0
DURING RESONANCE…1) The hydrogen atoms begin to precess “in phase” 1)
2)The hydrogen atoms align with the RF‟s magnetic field(B1) and they flip!!
AS THE NUCLEI PRECESS IN-PHASE IN THE B1 PLANE, A CHANGING MAGNETIC FIELD IS CREATEDIF YOU PLACE A RECEIVER COIL (ANTENNA) IN THE PATH OF THE CHANGING MAGNETIC FIELD, A CURRENT WILL BE INDUCED THIS IS FARADAY’S LAW OF INDUCTION
FARADAY’S LAW OF INDUCTION A changing magnetic field will induce an electrical current in any conducting medium COILS Used to:•transmit pulses of radiofrequency energy•receive induced voltage - MR SIGNAL•increase image quality by tuning in to one bodypart at a time
RELAXATIONWhen the RF pulse is turned “off”, the NMV “relaxes” back to B0 (away from B1) NMV B0 B1
DIFFERENT TYPES OF COILS Gradient Coils Radiofrequency Coils Shim Coils
GRADIENT COILS Three types as per cartesian coordinate system- for x, y and z axis; „slice selection gradient‟, „phase-encoding gradient‟ and „frequency-encoding gradient‟. Slice location is selected by frequency of the RF pulse while the thickness is selected by the bandwidth.
RADIOFREQUENCY COILS For transmitting and receiving signals at the resonant frequency of the protons within the patient
Types of RF Coils Transmit Receive Volume Coil Coil Surface Coil Receive Only Coil Gradient Coil Transmit Only Coil Multiply Tuned Coil
MR SIGNAL Collected by a coil Encoded through a series of complex techniques and calculations (magic?) Stored as data Mapped onto an image matrix
TR - REPETITION TIMETime from the application of one RF pulse to another RF pulse TE - ECHO TIMETime from the application of the RF pulse to the peak of the signal induced in the coil
T1 WEIGHTING•A short TR and short TE will result in a T1weighted image•Excellent for demonstrating anatomy T2 WEIGHTING•A long TR and long TE will result in a T2weighted image•Excellent for demonstrating pathology MANY OTHER DIFFERENT TYPES OF IMAGES THAT COMBINE ABOVE AND INCLUDE OTHER PARAMETERS
If TR and TE are less (100-500/20ms), image contrast is due to differences in T1 relaxation time and we get T1 weighted image. Used to view anatomic details because of increased contrast. T1 images are also called FAT IMAGES because fat has shortest T1 relaxation time. Therefore high signal and bright image.
If TR and TE are more 2000 ms/80 ms, we get T2 weighted image. It is used for inflammatory changes, tumors, joint effusions perforations. T2 are WATER IMAGES because water has longest T2 relaxation time. Therefore produce high signal and bright image.
MR using Contrast Medium Lesions with increased blood permeability demonstrate higher signal intensity than that of normal tissue on T1-weighted images.
CONTRAST AGENTS Most commonly used- Gadolinium. Administered intravenously to improve tissue contrast. Shortens the T1 relaxation times of enhancing tissues, making them appear brighter. Could be a cause of nephrogenic systemic fibrosis in some patients with renal dysfunction.
MR Angiography For detection of vessel systems esp. carotid arteries and their branches. No contrast agents are injected. Vessels with flow appear as bright homogenous linear structures on MRA. Static tissues appear blurred. 2 widely available MRA methods are- time of flight (TOF) and phase-contrast (PC) approaches.
MR Sialography Requires no cannulation, contrast or ionizing radiation! Suitable for patients even with acute inflammation. This technique is more useful for salivary gland ducts. Virtual endoscopy using MR sialographic imaging data enables depiction of inner surfaces of parotid and submandibular ducts.
MR Cisternography No lumbar puncture, contrast medium or ionizing radiation! Cisterns of the brain are demonstrated as high signal intensity structures that coincide with the shape of the cistern. Invaluable for ruling out brain tumours in patients with trigeminal neuralgia.
Functional MRI New tool for evaluating specific hypothesis concerning the anatomical regions of the human brain involved in processing sensory and motor information. Small signal changes appear hyperintense on fMRI; these are related to alterations in blood oxygenation levels and therefore changes in the magnetization of protons within the blood. Enable identification of motor and sensory areas of brain related to oral functions eg. Occlusion.
UNITS OF MAGNETIC FIELD TESLA & GAUSS 1t = 10,000 g Earth‟s magnetic Field = 0.05 mt / 0.5 g Emf used in MRI = 0.15 – 1.5t 1 t = 10,000 x earth‟s magnetic Field.
ADVANTAGES1. Non-ionizing2. Non- invasive3. Excellent soft tissue imaging with high Contrast sensitivity.4. Transverse, saggital, coronal, oblique Images obtained without repositioning the patient.5. No bone or air artifacts.6. Equipment contains no moving objects7. Exposure of humans to static magnetic Field < 2.5 t has no adverse effects.
DISADVANTAGES Expensive ( 6500 – 9000/= per scan ) Available only in large set ups Needs trained staff Claustrophobic Long scanning time Movement artifacts Hard tissue details- difficult Absolutely contraindicated in patients with cardiac pacemakers, cerebral aneurism clips. 9. Joint & ear prosthesis, insulin pumps, distort the image 10. Not used in pregnancy
BIOEFFECTS of MRIReversible abnormalities may include: ↑ amplitude of T wave on an ECG due to magnetic hydrodynamic effect Heating of patients Fatigue Headaches Hypotension irritability
Time varying bioeffects may include: Light flashes in the eyes Alterations in the biochemistry of cells and fracture union Mild cutaneous sensations Involuntary muscle contractions Cardiac arrythmias
Projectiles The projectile effect of a metal object exposed to the field cans seriously compromise the safety of anyone sited between the object and the magnet system.
INDICATIONS To depict location, morphology & function of articular disc thus allowing diagnosis of internal derangement to be made. 2. In cases of bone marrow edema, joint effusion, fibrous adhesions & certain tumors. 3. Some osseous changes can be evaluated.
TECHNIQUE Patient is asked to remove all the metallic objects like hair pins, watches, ornaments, Credit cards. Patient is placed in supine position on removable table which is inserted into the magnet. Syringes of various sizes or commercially available bite blocks are used to stabilize the jaws in open mouth views. M.R.I is performed using body coil as transmitter and 2 surface coils as receiver. Surface coil improves spatial resolution and diagnostic details. Surface coil is placed adjacent to the structure being imaged. Patient and surface coil must be secured and motionless during image acquisition. Surface coils of diameter 6-12 cms provides optimal signal to noise ratio. Use of dual surface coil technique for imaging left and right TMJ at the same time is of great value because time on scanner can be reduced for bilateral TMJ imaging. Bilateral abnormalities are seen in up to 60% of patients with pain and dysfunction who initially present with unilateral symptoms. - Slice thickness of 3 mm or less is taken
STANDARD PLANES Oblique saggital Oblique coronal Images are taken perpendicular & parallel to long axis of condyle. Saggital Images should be obtained in both open and closed mouth positions to determine the function and position of disk in respect to condyle. Normal disk position is when posterior band is superior to condylar head in closed mouth position. In open mouth view- disk can be seen to be interposed between condyle and articular eminence (normal or reducing) or is anterior to the condyle (non-reducing). Coronal Images are taken only in closed mouth position. It gives better view of medial and lateral disk displacement. Osseous anatomy of condyle is better appreciated in coronal plane. Proton density images provide better view than T1 images for outlining the morphology.
MRI findings in normal TMJ Soft tissues of TMJ can be appreciated nicely by MRI images. Low intensity signal of normal meniscus makes it easily distinguishable from adjacent soft tissues of increased signal intensity. Saggital view: Open & Closed mouth - Normal disc is biconcave with posterior band lying superior to condylar head. - Because fibrous connective tissue of disc has low signal. Disc is usually distinguished from surrounding structures of high signal intensity. - Cortices of condylar & temporal components of joint appear dark because of low signal. - Posterior attachment has relatively more signal intensity compared to posterior part of disc because of more fatty tissue in posterior attachment. - M.R.I is the only modality that allows disc to be distinguished from post attachment. Coronal view: Normal disc is crescent shaped. In this view, disruption of disk .i.e. morphology or position of disc relative to condyle and articular eminence can be clearly visualized.
Significance of MRI in TMJdisorders Internal Derangement Disc Displacement Disc Deformation Pseudodisc Joint Effusion Fibrous Adhesions Muscle Atrophy Tumors Post Surgical Imaging
CT SCAN MRIComputed (Axial) Tomography Magnetic Resonance ImagingSited for hard tissue evaluation Suited for Soft tissue evaluationUses X-rays for imaging Uses large external field, RF pulse and 3 different gradient fieldsUsually completed within 5 minutes. Actual Scanning typically run for about 30 minutes.scan time usually less than 30 seconds.Therefore, CT is less sensitive to patientmovement than MRI.Despite being small, CT can pose the risk of No biological hazards have been reported withirradiation. Painless, noninvasive. the use of the MRI.Patients with any Metal implants can get CT Patients with Cardiac Pacemakers are notscan. allowed to get MRI scan, tattoos and metal implants may be contraindicated due to possible injury to patient or image distortion.Intravenous contrast agents- iodine based. Very rare allergic reaction. Risk of nephrogenicAllergic reaction is rare but more common than systemic fibrosis with free Gadolinium in theMRI contrast. blood and severe renal failure.Comparatively cheaper. More expensive.
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