Basics of CT & MRI

 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


 Synonyms;
 Computerized Axial Tomography
 Computerized Reconstruction
 Computerized Tomographic Scanning
 Axial Tomography
 Computerized transaxial Tomography

 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 patient's
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 body's
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 usually
10 mm through the body and brain

 5 mm through the head and neck, unless
three dimensional reconstruction is
anticipated.

 In such cases, the slice thickness is 1.0 to
1.5 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.

 4th
generation
configuration

 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 beam
2. 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.

DISTORTION
A signal can contain errors or distortions that are
repeatable (deterministic). For instance, if a patient
moves during the acquisition step, parts of the
anatomy may be blurred or in different positions
from their true location. If the image reconstruction
process requires linear and consistent data, but the
measurements for some reason are not consistent,
artifacts can arise that may result in lost information
or 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 ARTIFACTS
1. Partial volume artifact
2. 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 CT
Less radiation dose More
More time Less
Images are of lower Higher
contrast
Slice thickness 0.1 1-2 mm
mm
Less expensive More

SIGNIFICANCE OF CT IN
MAXILLOFACIAL REGION
 Trauma
 Neoplasms
 Inflammatory processes
 TMJ disorders

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 and
tumours involving the masticator spaces and
found that CT is helpful in differentiating
inflammation from frank abscesses.

Alan A Schwimmer et al (1988) emphasized the
role of CT in the diagnosis and management of
temporal 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 patients
with salivary gland neoplasm and concluded that
when CT is combined with the clinical
information and laboratory findings, the overall
specificity in identifying the tumour becomes
90%.

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 evaluated
mandibular keratocysts with CT and established
that CT provides better methods of accurately
displaying the margins of the keratocysts, the areas
of bony perforation and any extension into soft
tissues.

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 of
several mandibular tumours and established their
osseous and soft tissue extensions. CT was found
valuable in excluding the involvement of mandible
by primary osseous and soft tissue lesions of
adjacent 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 the
pretreatment evaluation of patients with carcinoma of the
oral cavity or oropharynx, the presence or absence of bony
invasion. CT was more specific than conventional X-ray
films in detecting bone invasion.

Mark A Cohen and Yancu Hertzanu (1988) in their study on
CGCG using CT, conventional tomogram and conventional
radiographs, proved CT to be superior in clearly
demonstrating the soft tissue mass of lesion, its extension
into 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-osseous
dysplasia with conventional radiography and CT and
observed thin, low density areas around high density masses
with expansion of buccal and lingual cortical plates. CT was
able to give additional information by identifying the density
of these masses which ranges from 772-1582 HU. These
values 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 early
diagnosis of the maxillary antrum carcinoma,
besides a conventional radiographic test, also of
more specific analysis, as the computed
tomography and radio therapy.

Colin P and Hodson N. Thirty-two patients with
histologically proved malignant disease involving
the paranasal sinuses were studied by CT.
Significantly greater tumor extent was
demonstrated 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 1
proton

THE HYDROGEN NUCLEUS HAS A NET
POSITIVE CHARGE

•Hydrogen nucleus is a spinning, positively charged
particle

LAW OF ELECTROMAGNETISM

•A charged particle in motion will create a
magnetic field
•The postitively charged, spinning hydrogen
nucleus generates a magnetic field

WHY HYDROGEN?

•Very abundant in the human body-H20
•Has a large magnetic moment

MAGNETIC MOMENT
The 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
BOTH
e.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 the
individual 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 MAGNETIZATION
VECTOR
 An excess of hydrogen nuclei will line up
parallel to B0 and create the NMV of the
patient

N
N

S S

size

direction The magnetic
vector

THE NUCLEI WILL ALSO
PRECESS…

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.

WE NEED THEM TO BE “IN-
PHASE” OR TO RESONATE…

RESONANCE
Occurs when an object is exposed to an oscillating
perturbation that has a frequency close to its own
natural frequency of oscillation

RADIOFREQUENCY
ENERGY
 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
CREATED

IF 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 body
part at a time

RELAXATION
When 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 TIME

Time from the application of one RF pulse to
another RF pulse

TE - ECHO TIME

Time 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 T1
weighted image
•Excellent for demonstrating anatomy
T2 WEIGHTING
•A long TR and long TE will result in a T2
weighted 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.

ADVANTAGES
1. Non-ionizing
2. Non- invasive
3. 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 objects
7. 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 MRI
Reversible 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.

Metallic implants and prostheses
 Intracranial aneurysm clips
 Cardiac pacemakers
 Prostheic heart valves
 Cochlear implants
 Intraocular ferrous foreign bodies
 Orthopaedic implants
 Abdominal surgical clips

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 TMJ
disorders
 Internal Derangement
 Disc Displacement
 Disc Deformation
 Pseudodisc
 Joint Effusion
 Fibrous Adhesions
 Muscle Atrophy
 Tumors
 Post Surgical Imaging

CT SCAN MRI
Computed (Axial) Tomography Magnetic Resonance Imaging
Sited for hard tissue evaluation Suited for Soft tissue evaluation
Uses X-rays for imaging Uses large external field, RF pulse and 3
different gradient fields
Usually 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 patient
movement than MRI.
Despite being small, CT can pose the risk of No biological hazards have been reported with
irradiation. Painless, noninvasive. the use of the MRI.
Patients with any Metal implants can get CT Patients with Cardiac Pacemakers are not
scan. 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 nephrogenic
Allergic reaction is rare but more common than systemic fibrosis with free Gadolinium in the
MRI contrast. blood and severe renal failure.
Comparatively cheaper. More expensive.

REFERENCES

 Oral Radiology- Principles and Interpretation-
White and Pharoah 5th Edition.

 Textbook of Dental and Maxillofacial
Radiology- Freny R Karjodkar 2nd Edition.

• Eric Whaites- Essential of Dental Radiology .

•Textbook of Oral Radiology- Ghom

• MRI at a Glance- Catherine Westbrook, 2nd Edition.

• MRI of the Musculoskeletal System
By Martin Vahlensieck, Harry K. Genant, Maximilian Reiser

• TMJ Disorders and Orofacial Pain: The Role of Dentistry in a Multidisciplinary Diagnostic
Approach
By Axel Bumann, Ulrich Lotzmann, James Mah
• Carl W,Hardin MD et al: Infection and tumours of the masticator space. Computed tomography
evaluation. Radiology 1985;157:413-17.

• Alan Schwimmer; S E Roth; S N Morrison: The use of computerized tomography in the
diagnosis and management of temporal and infratemporal space abscesses. Oral Surg Oral med
oral pathol 1988;66(1):17-20.

• Nick Bryan R et al: Computed tomography of major salivary glands. Am J Roentgenol
1982;139:547-54.

• Frame JW, Michael JC, Wake MB: CAT in the assessment of mandibular keratocysts. Br Dental
J 1982;153-93.

•Osborn et al: Normal and pathologic computed tomography anatomy of mandible. Am J Roentgenol
1982;139:555-59.

•Close LG, Merkel M et al: Computed tomography in the assessment of mandibular invasion by intraoral
carcinoma. Ann Otol RhinolLAryngol 1986;95(4-1):383-88.

•Cohen MA et al: Radiological features including those seen with computed tomography of central giant cell
granuloma of the jaws. Oral Surg Oral PAthol Oral Med 1988;65:255-61.

•Ariji Y et al: Florid cemento-osseous dysplasia radiographic study with special emphasis of computed tomography.
Oral Surg Oral Pathol Oral Med 1994;78(3):391-96.

•Brenna Betti N, Bruno E et al: Neoplasms of the maxillary sinus- the clinico-radiological considerations of dental
interest. Minerva Stomatol 1993;42(3):87-91.

•Colin P, Hodson N: Computed tomography of paranasal sinus tumours. Radiology 1979;132:641-45.

• Wang EY, Fleisher KA. MRI of temporomandibular joint disorders. Applied Radiol 2008;37(9):17-25.

•Belkin BA, Papageorge MB, Fakitsas J, Bankoff MS. A comparative study of magnetic resonance imaging versus
computed tomography for the evaluation of maxillary and mandibular tumors.J Oral Maxillofac Surg. 1988
Dec;46(12):1039-47.

•Harms SE, Wilk RM, Wolford LM, Chiles DG, Milam SB. The temporomandibular joint: magnetic resonance
imaging using surface coils.Radiology. 1985 Oct;157(1):133-6.

•Katzberg RW, Bessette RW et al. Normal and abnormal temporomandibular joint: MR imaging with surface coil.
Radiology. 1986 Jan;158(1):183-9.

Basics of CT & MRI

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