FIVE MARKS QUESTIONS & ANSWERS

Q1. Write a note on the History of Computed Tomography.

Answer:

The development of CT scanning is marked by several key milestones:

• 1961 — Oldendorff W.H. recognized the potential of reconstruction tomography.
• 1963 — Cornmark used a source and detector rotating around a non-symmetrical phantom and a computer 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 Nobel Prize in Physiology and Medicine.
• 1971 to 1975 — Within a span of just 4 years, four generations of CT scanners evolved, yielding shorter scan times and better control over the patient’s motion.
• In the fifth generation CT scanner, scanning time was reduced to 16 milliseconds.
• 1998 — CBCT (Cone Beam Computed Tomography) was invented.

CT was developed in the early to mid-1970s and is a radiographic technique for producing cross-sectional tomographic images. It is claimed to be 100 times more sensitive than conventional X-ray systems and demonstrated differences between various soft tissues never before seen with X-ray imaging techniques.

Q2. What are CT Numbers (Hounsfield Units)? Give the CT numbers for various body tissues.

Answer:

Each pixel in a CT image is assigned a CT number or Hounsfield Unit (HU) ranging from +1000 to -1000, depending upon the amount of X-ray absorption within that block of tissue.

Each pixel represents a calculation of the actual attenuation of the X-ray beam by materials within the body. It represents the absorption characteristics or linear attenuation coefficient of that particular volume of tissue in the patient.

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 constructs an image by assigning different degrees of greyness to each CT number. In some systems, the numerical values are translated into colours or brightness levels that can be displayed on a television screen or printed on paper.

Q3. Define Window Level and Window Width. Discuss bone and soft tissue windowing.

Answer:

Window Level and Window Width are two variables that enable the visual image to be altered by selecting the range and level of densities to be displayed.

Window Level: It is the CT number selected for the centre of the range, depending on whether the lesion under investigation is in the soft tissue or bone.

Window Width: It 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 images are usually viewed in two modes:

1. Bone Windowing: The contrast is set so that osseous structures are visible in maximal detail. This is used when examining bony structures and their pathologies.

2. Soft-tissue Windowing: With this setting, the bone looks uniformly white, but various types of soft tissues can be distinguished from each other. This is useful in evaluating soft tissue masses, muscles, glands, and vascular structures.

These two variables together give the radiologist the ability to examine the same CT scan data for both bony detail and soft tissue contrast, making CT a highly versatile diagnostic tool.

Q4. Write a note on Image Artifacts in CT scanning.

Answer:

Artifact is defined as any discrepancy between the CT numbers represented in the image and the expected CT number based on the linear attenuation coefficient.

Distortion: A signal can contain errors or distortions that are repeatable (deterministic). If a patient moves during the acquisition step, parts of the anatomy may be blurred or in different positions from their true location. If image reconstruction requires linear and consistent data, but measurements are not consistent, artifacts can arise resulting in lost information or spurious image features.

Types of Image Artifacts:

1. Partial Volume Artifact: Occurs when different tissues are included within one voxel, giving an averaged CT number that does not represent any single tissue accurately.

2. Beam Hardening Artifact: This occurs due to absorption of low energy photons from the beam. As the beam passes through dense structures, lower-energy photons are preferentially absorbed, making the beam “harder,” which leads to inaccurate CT number calculation.

3. Metal Artifact:

• Manifests as “Star Streaking” artifact
• Caused by the presence of metallic objects inside or outside the patient (e.g., dental fillings, implants, surgical clips)
• The metallic object absorbs the photons, causing an incomplete profile
• This results in characteristic radiating streak lines across the image

Metal artifacts are a significant disadvantage of CT in patients with metallic restorations, as metallic objects such as fillings produce marked streak artifacts across the CT image.

Q5. Write a note on Cone Beam Computed Tomography (CBCT) and compare it with conventional CT.

Answer:

CBCT — Cone Beam Radiology:

CBCT was invented in 1998. CBCT uses a round or rectangular cone-shaped x-ray beam centred 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.

Features of CBCT:

• Scan time ranges from 17 seconds to a little more than 1 minute
• Multiplanar reformatting allows both 3D and 2D images of any selected plane
• Resolving power is four times that of CT
• Less expensive than conventional CT
• Radiation dose is only 3–20% that of conventional CT

Comparison of CBCT vs Conventional CT:

Feature	CBCT	CT
Radiation dose	Less	More
Time required	More	Less
Image contrast	Lower	Higher
Slice thickness	0.1 mm	1–2 mm
Cost	Less expensive	More expensive

CBCT is particularly useful in dentistry and oral and maxillofacial imaging because of its lower dose, better spatial resolution, and lower cost compared to conventional CT.

Q6. Write the Advantages and Disadvantages of CT scanning.

Answer:

Advantages of CT:

1. Cross-sectional imaging — provides images in axial plane; coronal and sagittal reconstructions also possible
2. Superior contrast and resolution — can differentiate between soft tissues of similar density
3. Geometric accuracy — dimensions measured on CT closely reflect actual anatomical dimensions
4. Images can be manipulated — windowing, 3D reconstruction, multiplanar reformation
5. Axial tomographic sections are obtainable — eliminates superimposition of structures
6. Images can be enhanced by the use of i.v. contrast media, providing additional information about vascularity and pathology

Disadvantages of CT:

1. Expensive — high cost of equipment and maintenance
2. Facilities are not widely available — limited access in developing regions
3. Very thin contiguous or overlapping slices may result in a high dose of radiation
4. Geometric miss — lesions located between scan slices may be missed
5. Metallic objects such as fillings produce marked streak artifacts across the CT image, degrading image quality

Q7. Describe Precession in MRI. What is the Larmor frequency?

Answer:

Precession:

Due to the influence of the main magnetic field (B₀), the hydrogen nucleus “wobbles” or precesses — similar to a spinning top as it slows down and comes to rest.

The axis of the nucleus forms a path around B₀ known as the “precessional path.”

The speed at which hydrogen precesses depends on the strength of B₀ and is termed the “precessional frequency.”

The precessional paths of the individual hydrogen nuclei are random, or “out of phase” — they precess at the same speed but are not synchronised in the same position in their orbit.

Larmor (Resonance) Frequency:

The spinning protons wobble or “precess” about the axis of the external B₀ field at the precessional, Larmor, or resonance frequency.

The formula for resonance frequency is:

ω = γ B₀ where γ is the gyromagnetic ratio

This means the resonance frequency (ω) of a spin is proportional to the magnetic field (B₀).

The precessional frequency of hydrogen in a 1.5 Tesla magnetic field is 63.86 MHz.

Understanding precession is fundamental to MRI because resonance — applying radiofrequency energy at this exact precessional frequency — is the mechanism by which MRI signals are generated from hydrogen nuclei in the body.

Q8. Write the Indications of CT in Oral and Maxillofacial Region.

Answer:

Indications of CT in Oral and Maxillofacial Region:

CT has a wide range of indications in this region including:

1. Intracranial diseases and trauma
2. Malignancy of jaws — evaluation of extent, bone involvement, soft tissue spread
3. Infection — identifying abscesses, cellulitis, osteomyelitis
4. Post-irradiation — assessment of post-radiotherapy changes
5. Salivary gland pathologies — calculi, neoplasms
6. Temporomandibular joint (TMJ) disorders
7. Implants — pre-surgical planning and post-surgical assessment
8. Fracture — evaluation of complex facial fractures
9. Foreign body — localization of foreign bodies
10. Imaging of unerupted and displaced teeth, bone grafting

Significance of CT in the Maxillofacial Region:

• Trauma — CT accurately evaluates complex craniofacial fractures and intracranial injuries
• Neoplasms — determines extent of bone destruction and soft tissue involvement
• Inflammatory processes — differentiates cellulitis from abscess
• TMJ disorders — identifies bony changes including destruction of condylar head, wearing of articular elements, and traumatic lesions within and outside the capsule; advantageous over arthrography as it is a painless procedure with superior resolution

Q9. Describe the Basic Principle of CT scanning.

Answer:

CT scanners use X-rays to produce 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 is stored and can be manipulated by a computer. This numerical information is converted into a grey scale representing different tissue densities, thus allowing a visual image to be generated.

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.

CT permits 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:

• 10 mm through the body and brain
• 5 mm through the head and neck, unless three-dimensional reconstruction is anticipated
• 1.0 to 1.5 mm when 3D reconstruction is required, to provide adequate data

Image consists of a matrix of individual blocks called voxels (volume elements), which consist of an array of individual points or pixels.

Each pixel is assigned a CT number or Hounsfield Unit (HU) between +1000 to -1000, depending upon the amount of absorption within that block of tissue.

Q10. Write a note on CT Detectors.

Answer:

In CT scanning, 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.

Two types of detectors are used:

1. Gas-filled Ion Chamber Detectors:

• Made of high-pressure xenon gas
• Capture about 50% of photons in the beam

2. Solid State Detectors:

• Commonly used
• 80% efficient
• Usually made of cadmium tungstate

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).

Q11. Write a note on Conventional Tomography and its principle.

Answer:

Definition: Tomography is a generic term, formed from the Greek words tomo (slice) and graph (picture). It was adopted in 1962 by the International Commission on Radiographic Units and Measurements to describe all forms of body section radiography.

Definition of 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.

Principle of Conventional Tomography: 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 centre.

Tube Trajectories — 5 Basic Types:

1. Linear
2. Elliptical
3. Circular
4. Spiral
5. Hyocycloidal

Types of Tomography:

1. Conventional Tomography
2. Computed Tomography
3. Three-dimensional CT
4. Spiral Computed Tomography
5. Emission Computed Tomography

Q12. Write a note on Contrast Media used in CT.

Answer:

Purpose: 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.

Types of Contrast Media used in CT:

Iodine-based contrast media are most commonly used:

1. Iodine Monomers (Ionic):

• Iothalmate
• Diatrizoate
• Metrizoate

2. Non-ionic Monomer:

• Iopamidol
• Iotrioxol

Clinical Applications of Contrast Media in CT:

• Odontogenic infections — Contrast CT is used for detecting abscess (appears as irregular zone of low density with a peripheral rim of contrast enhancement) and cellulitis
• Salivary gland pathology — Non-contrast CT for detecting calculi; contrast CT for abscess/cellulitis; salivary calculi appear as high density, non-contrast enhancing mass along the course of the duct
• Malignancies — Demarcation from surrounding soft tissue is difficult without contrast aided imaging
• Odontogenic tumours — Show moderate enhancement in contrast-aided imaging (except cemental tumours)

Q13. Write a note on CT findings in Odontogenic Cysts and Tumours.

Answer:

Odontogenic Cysts on CT:

Odontogenic cysts appear as:

• Localized, expansile, degenerative areas having a fluid density throughout the lesion
• They 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 on CT:

Odontogenic tumours appear as:

• Expansile lesions having a soft tissue density which show moderate enhancement in contrast-aided imaging (except cemental tumours)
• Foci of cystic degeneration are commonly seen
• They show breach in cortical plates
• Foci of calcifications are noticed in maturing odontogenic tumours

Ameloblastomas specifically show:

• 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 may be noticeable in Pindborg tumour

Q14. Write a note on Malignancies of the jaw on CT.

Answer:

CT Appearance of Malignancies:

Malignancies of the jaw are seen as:

• Predominantly destructive lesions interspersed with focal high contrast enhancement areas
• Invasive lesions show no or 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

Key Studies:

• Close LG et al (1986) found that the critical factor in the pretreatment evaluation of patients with carcinoma of the oral cavity or oropharynx is 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.
• 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.

Q15. Write a note on Precession, Resonance, and Radiofrequency Energy in MRI.

Answer:

Precession: Due to the influence of B₀, the hydrogen nucleus “wobbles” or precesses (like a spinning top). The axis of the nucleus forms a path around B₀ known as the “precessional path.” The speed of precession depends on B₀ and is called the “precessional frequency.” Individual hydrogen nuclei precess randomly or “out of phase.”

Resonance: Resonance occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation. To generate an MRI signal, we need the nuclei to be “in-phase” or to resonate.

Radiofrequency Energy:

• Follows the Law of Electromagnetism — charged particles in motion generate a magnetic field
• The RF-generated magnetic field is 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”
2. The hydrogen atoms align with the RF’s magnetic field (B₁) and they flip

The resonance frequency is:

ω = γ B₀ (where γ = gyromagnetic ratio)

In a 1.5 Tesla magnetic field, hydrogen precesses at 63.86 MHz.

TEN MARKS QUESTIONS WITH ANSWERS

Q1. Describe in detail the Generations of CT Scanners from First to Fifth.

Answer:

CT scanner technology has evolved through five generations, each overcoming limitations of the previous design.

First Generation — Rotate / Translate, Pencil Beam

The original EMI unit was the first generation scanner. It was a 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 the brain; the head was enclosed in a water bath
• The linear motion was repeated 180 times, and after one linear movement, the gantry rotated 1 degree
• X-ray beam was on during linear motion and off during rotation
• The transmitted radiation was recorded 160 times during each linear movement
• Total number of transmissions = 160 × 180 = 28,800
• Scan time: 4.5 to 5 minutes
• Matrix: 80 × 80

Second Generation — Rotate / Translate, Narrow Fan Beam

Second generation scanners were also of translate-rotate type but 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.

• The source-detector assembly intercepted a fan-shaped beam with a narrow fan angle of 10°, rather than a pencil beam
• Instead of moving 1 degree after each linear scan, the gantry rotated through a greater arc of up to 30 degrees
• Linear movement had to be repeated six times to cover 180 degrees
• Scan time: 10 to 90 seconds

Third Generation — Rotate / Rotate, Wide Fan Beam

The translational motion of first and second generation was a major limitation — at the end of each translation, the translational inertia of the X-ray tube/detector system had to be stopped, the whole system rotated, and then the translation restarted. This design could never have led to fast scanning.

To overcome this limitation, third generation scanners evolved. They use:

• Increased number of detectors (up to about 750 detectors)
• Rotate-rotate system — both X-ray tube and detector array are rotated together
• The detector is aligned around an area of a circle whose centre is the focal spot
• X-ray beam is collimated into a fan beam with a fan angle of about 50°
• Scan time: 2 to 10 seconds

Fourth Generation CT Scanner — Rotate / Stationary

Fourth generation scanners were designed to overcome the problem of electronic drift between the many detectors used in the system, which caused ring artifacts in 3rd generation. This design eliminated ring artifact.

• Uses rotate only motion — the huge X-ray tube rotates 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
• X-ray beam was collimated to form a fan beam
• Was not faster in principle than third generation
• Provides easier detector calibration

Fifth Generation Systems

Developed by Dr. E Woods of Mayo Clinic. The system consists of multiple X-ray tubes and detectors. Such a unit is primarily used to image 3D sections of the heart and reduces artifacts caused due to cardiac rhythm.

• Scanning time is reduced to 16 milliseconds
• Since there are no moving mechanical parts in some designs, images may be acquired extremely rapidly, eliminating motion artifact

Summary of Generations:

Generation	Type	Detectors	Scan Time
1st	Rotate/Translate, Pencil beam	2	4.5–5 min
2nd	Rotate/Translate, Narrow fan	30	10–90 sec
3rd	Rotate/Rotate, Wide fan	~750	2–10 sec
4th	Rotate/Stationary	Ring (fixed)	Fast
5th	Multiple tubes & detectors	Multiple	16 msec

Q2. Describe CT Equipment, Technique, and Image Formation.

Answer:

CT Equipment

The CT equipment consists of:

1. Gantry — containing the X-ray source, detectors, and electronic measuring devices
2. Motorized table — used to position the patient within the gantry
3. X-ray power supplies and controls
4. Computer
5. Viewing devices

X-Ray Tubes

• Radiation source for CT ideally supplies a monochromatic X-ray beam by which image reconstruction is simple and more accurate
• Earlier models used oil-cooled, fixed anode, relatively large (2×16 mm) focal spot tubes at energies of about 120 kilovolt (constant potential) and 30 mA; the beam was heavily filtered to move low energy photons and increase the mean energy
• 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 0.6 mm, and generate X-rays in short bursts or pulses
• These tubes are air-cooled and operate at much higher currents, up to 600 mA
• 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 designed to withstand 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% of photons

2. Solid state detector:

• Commonly used, 80% efficient
• Usually made of cadmium tungstate

Technique

The process by which production of CT image occurs is called scanning.

1. The patient lies down with the part of the body to be examined within the circular gantry, which houses the X-ray tube head and detector
2. The level of plane and thickness of the section to be imaged are selected
3. The X-ray tube head rotates around the patient, scanning that section
4. As the tube head rotates, each set of detectors produces an attenuation or penetration profile of the region of the body being examined
5. These detectors produce electrical impulses proportional to the intensity of the X-ray beam emerging from the body
6. That intensity is determined by: (a) the energy of the X-ray source, (b) the distance between the source and the detector, and (c) the attenuation of the beam by the material in the object being scanned

Image Formation and Reconstruction

• The 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 elements), consisting of an array of individual points or pixels
• Pixel size is determined by: geometry of the scan, frequency and spacing of measurements, number of penetration profiles, and size of the X-ray source and detector
• Each pixel is assigned a CT number or Hounsfield Unit (HU) between +1000 to -1000

Image Reconstruction Methods:

• Images are typically 512 × 512 or 1024 × 1024 pixels
• Rapid image reconstruction is done by:
  • Two-dimensional Fourier Analysis
  • Filtered back projection

Image Display: Images can be displayed in two basic modes:

1. As a paper printout of CT numbers
2. As a grey scale image on a cathode ray tube or television monitor

Q3. Describe in Detail the Recent Advances in CT Technology.

Answer:

1. Three-Dimensional CT (3D CT)

In this technique, data obtained from CT scan is reformatted into 3D images.

3D CT requires that each voxel, shaped as a 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 voxels
• 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

2. Ultrafast CT

I-Matron (San Francisco) has developed a CT scanner capable of acquiring data up to 10 times faster than conventional CT.

• About 50 msec is able to freeze cardiac and pulmonary motion, enhancing quality without motion artifact
• Primarily used for cardiac imaging

3. Spiral CT Scanners

Discovered in 1989. In this technique, while the gantry containing the X-ray tube and detectors revolves 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.

Advantages of Spiral CT:

• Improved multiplanar image reconstruction
• Reduced examination time (12 sec vs 5 min for conventional)
• Reduced radiation dose (<75%)

Pitch in helical CT refers to the amount of patient movement compared with the width of image acquired:

Pitch = Table travel per X-ray tube rotation / Image thickness

Helical CT is now standard.

4. Multidetector Helical CT (MDCT / Multislice CT / 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: 0.35 sec
• The quality of axial, reformatted, and three-dimensional images is also improved with this as compared to single-slice machines

5. 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

6. Emission CT

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. This is the basis of PET (Positron Emission Tomography) and SPECT scanning.

Q4. Describe the Basic Principles of MRI including Hydrogen Atom, Magnetic Moment, Net Magnetization Vector, Precession, Resonance, and Radiofrequency Energy.

Answer:

Introduction

MRI is an imaging modality in which radiant energy is in the form of radiofrequency waves rather than X-rays. The father of MRI is Felix Bloch.

Types of Atomic Motion

There are three 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 (most important for MRI)

MRI Uses the Hydrogen Atom

MRI makes use of the hydrogen atom because:

• It has 1 electron orbiting the nucleus
• The nucleus contains no neutrons but contains 1 proton
• Therefore, the hydrogen nucleus has a net positive charge
• Hydrogen nucleus is a spinning, positively charged particle

Why Hydrogen?

• Very abundant in the human body (present in H₂O)
• Has a large magnetic moment

Law of Electromagnetism

• A charged particle in motion will create a magnetic field
• The positively charged, spinning hydrogen nucleus generates a magnetic field
• This gives the hydrogen nucleus its own magnetic moment — defined as the tendency of an MR active nuclei to align its axis of rotation to an applied magnetic field

MR Active Nuclei have an odd number of protons OR an odd number of neutrons OR both. Examples include: Hydrogen¹, Carbon¹³, Nitrogen¹⁵, Oxygen¹⁷, Fluorine¹⁹, Sodium²³, Phosphorus³¹.

Alignment in the Magnetic Field

When a body is placed into the bore of the scanner, the strong magnetic field (B₀) will cause the individual hydrogen nuclei to either:

• A) Align Anti-parallel to B₀ — high energy state
• B) Align Parallel to B₀ — low energy state

An excess of hydrogen nuclei will line up parallel to B₀, creating the Net Magnetization Vector (NMV) of the patient.

Net Magnetization Vector (NMV)

An excess of hydrogen nuclei will line up parallel to B₀ and create the NMV of the patient. This is the net sum of all individual magnetic moments of hydrogen nuclei aligned in the external field.

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”
• The speed of precession depends on B₀ and is the precessional frequency
• Precessional frequency of hydrogen in a 1.5 Tesla field = 63.86 MHz
• Individual nuclei precess randomly or “out of phase”

Resonance

Resonance occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.

To generate MRI signals, the nuclei need to be “in-phase” or to resonate.

The resonance frequency is:

ω = γ B₀ (where γ = gyromagnetic ratio)

Radiofrequency Energy

• Follows the Law of Electromagnetism
• The RF-generated magnetic field is known 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”
2. The hydrogen atoms align with the RF’s magnetic field (B₁) and they flip — the NMV is tipped into the transverse plane
3. This flip generates a detectable MR signal

Q5. Discuss in Detail the Significance of CT in the Maxillofacial Region with various conditions.

Answer:

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 software already allow 3D visualization of anatomy and pathology, but further improvement in clinical performance is expected.

Significance of CT in Maxillofacial Region — Main Areas:

1. Trauma
2. Neoplasms
3. Inflammatory processes
4. TMJ disorders

1. 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) used CT in evaluation of infections and tumours involving the masticator spaces and found CT 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.

2. Sialadenitis (Salivary Gland Pathology)

• 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 clinical information and laboratory findings, the overall specificity in identifying the tumour becomes 90%.

3. Odontogenic Cysts and Tumours

Cysts: Appear as localized, expansile, degenerative areas with fluid density; no contrast enhancement (except Aneurysmal Bone Cyst).

Odontogenic Tumours: Show expansile lesions with soft tissue density and moderate contrast enhancement (except cemental tumours); foci of cystic degeneration and calcifications; breach in cortical plates.

Ameloblastomas: Bicortical expansion, thinning and breach of bony walls, extension into soft tissue spaces. Plexiform ameloblastomas have high contrast enhancement; cystic forms show fluid density; malignant forms have grossly destructive patterns.

John W Frame and Michael JC Wake established that CT provides better methods of accurately displaying margins of keratocysts, areas of bony perforation, and extension into soft tissues.

4. Malignancies

• Seen as predominantly destructive lesions with focal high contrast enhancement
• Invasive lesions show no or minimal expansion
• CT was more specific than conventional X-ray films in detecting bone invasion (Close LG et al, 1986)
• Demarcation from surrounding soft tissue is difficult without contrast-aided imaging

5. Fibro-osseous Lesions

• CT pattern depends on the maturative stage of the lesion
• Cemental lesions are distinguished based on the continuity of the lesion with the roots of the tooth and the periodontal ligament space

Ariji Y et al (1994) studied cases of florid cemento-osseous dysplasia with CT and observed thin, low density areas around high density masses. CT gave additional information by identifying the density of these masses ranging from 772–1582 HU, which were suggestive of cementum or cortical bone.

6. Maxillary Lesions

Maxillary lesions share similar pictures in contrast to mandibular lesions, which makes them difficult to distinguish. CT helps define extent and involvement.

Brenna Betti N, Bruno E et al (1993) emphasized CT for early diagnosis of maxillary antrum carcinoma.

7. Temporomandibular Joint (TMJ)

CT helps identify bony changes in the TMJ like:

• Destruction of the condylar head
• Wearing of articular elements
• Traumatic lesions within and outside the capsule

It is advantageous over arthrography as it is a painless procedure with superior resolution.

8. 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.

Q6. Write in Detail the CT Equipment and Describe the Fifth Generation Advances — Spiral CT, MDCT, and Electron Beam CT.

Answer:

CT Equipment

CT equipment consists of:

• Gantry (X-ray source, detectors, electronic measuring devices)
• Motorized table
• X-ray power supplies and controls
• Computer
• Viewing devices

X-ray Tubes (detailed):

Earlier models used oil-cooled, fixed anode, relatively large (2×16 mm) focal spot tubes at energies of about 120 kilovolt (constant potential) and 30 mA. The beam was heavily filtered to remove low energy photons and increase mean energy.

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 0.6 mm, and generate X-rays in short bursts or pulses. These tubes are air-cooled and operate at much higher currents, up to 600 mA. They are (cathode-anode) perpendicular to the fan beam to avoid asymmetry due to the heel effect.

Recently, special types of X-ray tubes have been developed for CT to withstand the very high heat loads generated when multiple slices are acquired in rapid sequence.

Spiral CT (Discovered 1989):

The gantry containing the X-ray tube and detectors revolves around the patient while the patient table continuously advances through the gantry, resulting in a continuous spiral of data.

Pitch = Table travel per X-ray tube rotation / Image thickness

Advantages:

• Improved multiplanar image reconstruction
• Reduced examination time (12 sec vs 5 min)
• Reduced radiation dose (<75%)
• Helical CT is now standard

Multidetector Helical CT (MDCT) — Introduced 1998:

• Widely used
• Four to 64 adjacent detector arrays used in conjunction with helical CT
• Time for full cycle rotation: 0.35 sec
• Improved quality of axial, reformatted, and 3D images compared to single-slice machines

Electron Beam CT — Recent Development:

• Electron gun generates an electron beam focused electrostatically on a fixed tungsten target circling halfway around the patient
• X-rays expose the detector array on the other half
• Because there are no moving parts, an image may be acquired in less than 100 microseconds
• Primarily used for cardiac imaging to stop heart motion

Emission CT:

• 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