Computed Tomography Scan

Guided by: Dr. Deepanshu Garg Department of Oral Medicine and Radiology College of Medical sciences, Bharatpur

Submitted by: Shekhar Kumar Mandal BDS IV, Roll no:26 Batch 2nd

INTRODUCTION

Computed tomography is also known as "CAT scanning" (Computed Axial Tomography).

Tomography is from the Greek word "tomos" meaning "slice" or "section" and "graphia" meaning "describing".

Computed tomography is an examination that uses X-ray and computer to obtain a cross-sectional image of the human body

HistoryInvented in 1972 by British engineer Godfrey

Hounsfield of EMI Laboratories, England and by South Africa-born physicist Allan Cormack of Tufts University, Massachusetts. Hounsfield and Cormack were later awarded the Nobel Peace Prize for their contributions to medicine and science.

The first clinical CT scanners were installed between 1974 and 1976. The original systems were dedicated to head imaging only, but "whole body" systems with larger patient openings became available in 1976.

CT became widely available by about 1980.

Common uses of CT Scan• One of the best and fastest tools for examining the chest,

abdomen and pelvis• Severe injuries from incidents such as a motor vehicle

accident.• Acute symptoms such as abdominal pain or difficulty breathing• Detecting many different cancers and its size, precise location

and the extent of the tumor's involvement with other nearby tissue.

• Detection, diagnosis and treatment of vascular diseases• To assess for pulmonary embolism/ abdominal aortic

aneurysms (AAA). • Valuable in diagnosing and treating spinal and cranial

problems

Working Principle

Beams from one or several small X-ray sources are passed through the body and intercepted by one or more radiation detectors. These radiation detectors produce electrical impulses that are proportional to the intensity of the X-ray beam emerging from the body. The intensity of the X-ray beam exiting the body is determined by:a. The energy of the X-ray sourceb. The distance between the source of X-rays and thedetectorc. The attenuation of the beam by materials in the objectbeing scanned.

The efficiency of the scanning system can be increased multiple folds by using multiple X-ray beams and equivalent number of detectors. Each scan produces a penetration or absorption profile. However, construction of the image requires profiles obtained at different angles through the patient under study. The X-ray source and detector assembly are rotated around the patient (360⁰) to produce multiple profiles of the particular site of interest.

Components of CT Scan Gantry Data Acquisition System/computerOperating Console

Gantry

• The gantry assembly is the largest of these systems.

• Can be tilted upto 300 degree.• It is made up of all the equipment

related to the patient, including -the patient Support, -the Positioning couch, -the Scanner housing. -heart of the CAT scanner, the X-ray Tube, as well as -Detectors that generate and detect x rays

The CT X-ray Tube Tubes operates with three phase or high frequency

generators and rotating anode. Anode heat capacity: –3.5MHU upto 8MHU(MILLION HEAT

UNIT)

Operates at 120 kVp and 200-800 mA.High X-Ray output reduces the exposure time and

improves image quality as well as decrease bone absorption.

Focal spot size ranges from 0.5 to 2mmLow slice thickness result in higher spatial resolution

and contrast, less partial volume artifact and higher patient dose.

Collimator Thin fan shaped Collimator is used to reduce exposure. Postpatient collimator are also used to remove the scattered

photons as well as control slice thickness that ranges from 1 to 3 mm.

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 Made up of multiple discrete cells or detectors .Capture energy that has not been attenuated by the patient Two types of detectors are usedCrystal Scintillation DetectorsGas Filled Detectors

Scintillation DetectorsCoupled optically to photodiode Materials Used

I. Sodium IodideII. Bismuth Germanium Oxide III. Cesium IodideIV. Cadmium Tungstate

Gas Filled Detectors

Materials Used Xenon Krypton Xenon + Krypton

Have high atomic number, are inert, and display minimal afterglow. The gas is contained under high pressure in the detector array.

Since 90% of 50 is 45, the output is same. The overall efficiency of both the detectors is same

Gas Filled Detectors

Scintillation Detectors

Sensitive face: 100%Detection Efficiency: 45%

Sensitive face: 50%Detection Efficiency: 90%

Data Acquisition SystemThe DAS consists of the following partsX-ray photons come on the dectector The detector detects the intensity in form of current.The current is converted into voltage.The analog integrator removes spikes.The analog signal is converted into digital form.This signal can now be processed and reconstructed in

the computer. Detector

Current to Voltage Convertor

Pre Amplifi

er

Analog Integra

tor

Analog to Digital

Convertor

Computer

Image Reconstruction• After enough transmission measurements

(detector)• Sent to the computer for processing• A software called Fourier Slice Transform is used. • More than 250,000 reconstruction algorithms are

used (example: algebraic reconstruction technique) to compute the image.

Operating Console It is master control center of the CT scanner. It is used to input all of the factors related to taking a scan. Typically, this console is made up of a computer, a

keyboard, and multiple monitors. Often there are two different control consoles, one used

by the CAT scanner operator, and the other used by the physician.

GENERATIONS OF CT

• FIRST-GENERATION EMI CT SCANNER

• SECOND GENERATION OF CT : Reducing Scan time

• THIRD GENERATION OF CT : Rotate/rotate, wide fan beam

• FOURTH-GENERATION CT SCANNERS

• FIFTH GENERATION ELECTRON-BEAM CT (EBCT)

• SIXTH GENERATION OF CT: Slip ring Scanners/Helical/Spiral CT

• SEVENTH GENERATIONOF CT: Multiple detector array

FIRST-GENERATION EMI CT SCANNER

• It was a dedicated head scanner in which the patient's head was recessed via a rubber membrane into a water-filled box.

• The device was designed such that the water-filled box rotated (in 1° increments) along with the single-narrow-beam, single-detector assembly, resulting in a fixed path length through patient plus water for all rays and transmission measurements.

• Only 2 x-ray detectors used • About 4.5 minutes/scan with 1.5 minutes to reconstruct slice• Pencil beam geometry allowed very efficient scatter reduction, best

of all scanner generations

SECOND GENERATION OF CT

The first waterless full-body CT scanner was developed and installed by Ledley et al. at Georgetown University in February 1974.

Incorporated linear array of 30 detectorsFan shaped X-ray beam, translate rotate modeScan time: 10-90 sec

THIRD GENERATION OF CT• More than 800 detectors were used• It is characterized by linked tube–detector arrays undergoing

only rotational motion, is known as third-generation geometry. • Scan time range from 17 sec to little more than 1 minRING ARTIFACT If one of the detectors is out of calibration on a third-generation

(rotating x-ray tube and detector assembly) scanner, the detector will give a consistently erroneous reading at each angular position, resulting in a circular artifact called as RING ARTIFACT.

Xenon Detector Array: Ring artifacts were minimized by using an inherently stable

xenon array. Each small chamber acted as an ionization chamber.

Xenon detector array

FOURTH-GENERATION SCANNERS • Developed principally to suppress Ring artifacts.• Several thousands of ring detectors are used.• Mechanical motion is rotation of X-ray source around a fixed

detector array.• As fan beam passes across each detector, and image

projection is acquired• Scan time is: ½ - 1 second• During tube rotation, the part of the ring between the tube

and the patient would tilt out of the way of the x-ray beam (the peculiar wobbling motion of the ring was called nutation).

• Another disadvantage of fourth-generation designs was scattering.

FIFTH GENERATION -ELECTRON-BEAM CT (EBCT)

• Fast imaging • X-ray source is not an X-ray tube rather a focused,

steered, and micro wave- acclerated electron beam incident on a tungsten target.

• Scan time : <50 ms• Principally applied to cardiac imaging

SIXTH GENERATION OF CT• Helical CT scanners acquire data while the table is

moving• Mostly used nowadays

SEVENTH GENERATION OF CTMultidetector /Multislice /Multirow CT

• Most recent advancement, introduced in 1998.

• This uses usually 64-128 adjacent multiple detector arrays in conjunction with a helical CT scanner , the collimator spacing is wider and more of the x-rays that are produced by the tube are used in producing image data.

• Scanning time: 0.25 second

Mathematical principle of CT scan• Mathematical principles of CT were first developed in 1917 by

Radon• ”Proved that an image of an unknown object could be produced if

one had an infinite number of projections through the object.”• The internal structure of an object can be reconstructed from

multiple projections of the object. • Plain film imaging reduces the 3D patient anatomy to a 2D

projection image• Density at a given point on an image represents the x-ray

attenuation properties within the patient along a line between the x-ray focal spot and the point on the detector corresponding to the point on the image

Tomographic acquisition

Single transmission measurement through the patient made by a single detector at a given moment in time is called a ray

A series of rays that pass through the patient at the same orientation is called a projection or view

Two projection geometries have been used in CT imaging: Parallel beam geometry with all rays in a projection parallel to one another Fan beam geometry, in which the rays at a given projection angle diverge

Purpose of CT scanner hardware is to acquire a large number of transmission measurements through the patient at different positions

Single CT image may involve approximately 800 rays taken at 1,000 different projection angles

Before the acquisition of the next slice, the table that the patient lies on is moved slightly in the cranial-caudal direction (the “z-axis” of the scanner

IMAGE RECONSTRUCTION

The photon recorded by the detectors represent a composite of absorption characteristics of all elements of the patient in the path of x-ray beam. Computer algorithms use these photon count to construct one or more digital cross-sectional image.

The X-ray beam attenuation is collected in a grid like pattern called matrix.

These cross sectional image of the body is divided into tiny blocks called as VOXEL (VOlume Element){3D}.

Each square of image matrix is called as Pixel{2D}.

Image are typically 512 x 512 or 1024 x 1024 pixels.

Composition and thickness of voxel along with quality of beam determine the degree of attenuation.

Attenuation is followed by Lambert's law of absorption. So for a single block of homogeneous tissue and monochromatic beam of x–ray N = N0e-µx

since e is natural log, N0 is initial photon, N is transmitted photon, x is thickness of slab

Similarly if N no. of block is there then the equation becomes N=N0e-(µ1+µ2+µ3…………………µn)x

To solve this problem we must have transmission reading taken from at least to different direction .

Since the more is projection and lines more is equation formed As for example orignal EMI scanner 28,800 reading Fan beam scanner can take

1 lak to2 lak. Readings.

CT THEORY

Mathematical methods of image reconstruction

1.Back projection Also called summation method. It Is the oldest means of image reconstruction. Its principle demonstrates when a ray from two projection is superimosed, or back projected They produce a crude reproduction of original object.Filtered back-projection is one of this method. modification of these methods, called the Feldkamp reconstruction, is used for MDCT and cone-beam reconstructions to account for the diverging x-ray beam.

2.Iterative methods It start with assumption that all point in matrix have same value and it was compared with measured value and make correction until Values come with in acceptable range.It is used instead of filtered back-projection to reduce noise from images. This technique allows the use of low-dose protocols yet still produces images with comparable or better image qualit 3.Analytical methodsMost commonly used now. It start with assumption that all point in matrix have same valueAnd it was compared with measured value and make correction until Values come with in acceptable range

Fig: Image reconstruction. • A:Assume four volumes with differing linear attenuation coefficients (µ). A beam entering the

object with N0 photons is reduced in intensity by object. The intensity of the remnant beam is measured by the detector array. The value of each cell in the object can be determined by solving four (or more) independent simultaneous equations. Such a brute-force approach is computationally intensive, and in practice much faster algorithms are used to reconstruct images.

• B:This task is conceptually similar to sudoku problems in that the exposure to the detector is known, and the filtered back-projection algorithms estimate the exposure intensity at each voxel.

CT Tomographic images• For image display, each pixel is

assigned a CT number representing tissue density.

• CT numbers, aka Hounsfield units (HU), in honor of the inventor Hounsfield, range from −1000 to +1000, each corresponding to a different level of beam attenuation.

• Most monitors may display only 256 gray-scales and the human eye can perceive only 64 shades of gray.

Multiplanar Reformatted Imaging • Data from a single CT imaging procedure, consisting of either multiple

contiguous or one helical scan, can be viewed as images in the axial, coronal, or sagittal planes or in any arbitrary plane depending on the diagnostic task; this is referred to as multiplanar reformatted imaging

• Have the capability of viewing normal anatomy or pathologic processes simultaneously in three orthogonal planes often facilitates radiographic interpretation

• These are two-dimensional and require a certain degree of mental integration by the viewer for interpretation.

• This limitation is overcomed by computer programs that reformat data acquired from axial CT scans into three-dimensional images.

• Three-dimensional reformatting requires that each original voxel, shaped as a 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.

Fig: Three-dimensional rendering. Three-dimensional images can be reconstructed from the cuberilles, thresholded for bone (left) or soft tissue (right), oriented in any arbitrary direction, and made to appear to have depth by highlighting

structures near the front and shadowing structures near the back. This patient has hemifacial microsomia and demonstrates incomplete development of the left frontal, sphenoid, temporal maxillary, zygomatic, and mandibular

bones. Note also the reduced size of the left orbit, depression of the tip of the nose, missing and incompletely erupted left maxillary teeth, deviation of the right mandible to the left, sunken left midface, and malformation of the left ear.

CT Scan Methodology• X-ray tube and detectors rotate around the patient, with the axis of

rotation running from the patient’s head to toe.• Detectors measure the average linear attenuation coefficient, µ, between

the tube and detectors. Attenuation coefficient reflects the degree to which the X-ray intensity is reduced by the material it passes through

• 2D measurement are taken in a helical manner all around the patient• Attenuation data is summed up from thousands of angles used in a

process called reconstruction• Contrast dye is sometimes used to make the internal organs more visible

in the image• Bone appears white; gases and liquids are black; tissues are gray• Measurements taken in Hounsfield units (Hu• The same study data can show bone structure or soft tissue detail, simply

by altering the window and leveling (ie, which Hu range will the 0-255 greyscale values will correspond to)

Scanning methods

1. Digital projection 1. AP, PA, Lat or Oblique projection 2. Surview, Scanogram

2. Conventional CT 1. Axial -Start/stop

3. Volumetric CT 1. Helical or spiral CT - Continuous acquisition

Digital Projection X-ray tube and detector remain stationary .Patient table moves continuously with X-rays “on” Produces an image covering a range of anatomy similar to a conventional X-ray image, e.g. flat plate of the abdomen .Image used to determine scan location.

Volume CT X-ray tube and detector rotate 360°. Patient table moves continuously with X-ray’s “on” Produces a helix of image information. This is reconstructed into 30 to 1000 images .

Principles of CT InterpretationCT interpretation is based on an organized and comprehensive

approach. CT images are viewed in sequential anatomic order, examining

each slice with reference to slices above and below. This image analysis is made dramatically easier by viewing CT

images on a PACS workstation. The interpreting physician can scroll up and down the stacked image

display. The radiologist must seek to develop a three-dimensional concept

of the anatomy and pathology displayed. This analysis is fostered by the availability of image reconstructions in coronal and sagittal as well as axial planes.

Must be interpreted with reference to the scan parameters, slice thickness and spacing, administration of contrast, timing of scanning relative to contrast enhancement, and presence of artifacts.

In these images, bone is white or light grey, soft tissue is medium gray, and air is dark grey to black.

Axial images are oriented so that the observer is looking at the patient from below.

The patient’s right side is oriented on the left side of the image.

PACS workstation viewing of digital images allows the interpreter to actively manipulate the image, magnify, change image brightness and contrast, measure attenuation, and create oblique and three-dimensional image reconstructions to optimize interpretation.

Artifacts

Artifacts refer to components of the image that do not faithfully reproduce actual anatomic structures because of distortion, addition, or deletion of information. Partial volume artifacts/volume averaging:when a voxel contains tissues of different densities the resulting CT number for that voxel is an intermediate value that does not represent either tissue. The resulting image may be a blurring of the junction of the tissue or a loss of part of a thin cortical layer of boneBeam hardening artifact Results from greater attenuation(absorption) of low-energy x-ray photons as they pass through tissue. Mean energy of the x-ray beam is increased (the beam is “hardened”), resulting in less attenuation at the end of the beam than at its beginning. Beam-hardening errors are seen as areas or streaks of low density as darkening in middle of an axial slice.

Motion artifact Results when structures move to different positions during image acquisition. Motion is demonstrated in the image as prominent streaks from high- to low-density interfaces or as blurred or duplicated imagesMetal Streak artifacts:Occurs due to near complete absorption of x-ray photons by metallic restorations. Appears as opaque streak in occlusal plane.

Ring artifactsRing artifacts are seen as high- or low-density circular rings in the image.

Quantum mottle artifacts Produce noise in the image seen as salt-and-pepper pattern of random dark and light specks throughout the image. The image noise results from insufficient x-ray transmission data caused by inappropriate radiation settings for the size of the patient.

Contrast agents CT contrast is used to make specific organs, blood vessels and/or tissue types "stand out" with more image contrast to better show the presence of disease or injury.Route of contrast agent used in CT1. Intravenous injection2. Orally 3. Rectally 4. Intra articular - Arthrography or inhaled

COMPOUNDS USED:Iodide compound contrast a) Diatrizole ( urograffin hypaque )

1500 moSml/kg b) Metrizoate ( Isopaque ) 1500c) Iothalamate ( Conray ) 1500

Ionic -- LOCM d) Loxaglate ( hexabrix ) 490

None Ionic – LOCM I. Iopamidole ( Niopam , Isovue ) 470 II. Iohexol ( Ominpaque ) “III. Iomeprol ( Iomeron ) “ IV. Ioversol ( Optiray ) “V. Iopromide ( Ultravist ) “

None Ionic – ISO 1. Iotrolan ( Isovist ) 300 2. Iodixanol ( visipaque )

Barium compound contrast 3. Baritop 100 ( 100% all part of GIT ) 4. EPI -C ( 150% large bowel ) 5. E-Z HD ( 250 % esophagus ,

stomach and duodenum ) 6. E-Z paque ( 100% small bowel7. Polibar ( 115% large bowel )8. Polibar rapid ( 100% large bowel ) 9. Water -soluble 20 ml {Urograffin 150 ,

gastromiro }10.Barium suspension – low density ( 2%

w/v

• Once the iodine/barium contrast has been injected into the blood stream, it circulates through the heart and passes into the arteries, through the body's capillaries and then into the veins and back to the heart.

• As CT images are being acquired, the CT's x-ray beam is attenuated (weakened) as they pass through the blood vessels and organs flush with the contrast. This causes the blood vessels and organs filled with the contrast to "enhance" and show up as white areas on the x-ray or CT images. The kidneys and liver eliminate the contrast from the blood.

• Toxicity may occur rarely that include flushing , nausea , metal taste in moth , peripheral burning , rigors urticaria , warm , pain , abdominal pain bronchospasm , none cardiac pulmonary edema, arrhythmias, hypotension, nephrotoxicity , CIN , hematological crisis in sickle cell patient , neurotoxicity , thyroid diseases.

BENEFIT VS RISK OF CT SCAN

Benefits Risks Painless, noninvasive and accurate. Ability to image bone, soft tissue and blood

vessels all at the same time. Provides very detailed images of many types of

tissue Fast and simple; in emergency cases, they can

reveal internal injuries and bleeding quickly enough to help save lives.

Cost-effective imaging tool for a wide range of clinical problems.

Less sensitive to patient movement than MRI and CT can be performed if you have an implanted medical device of any kind, unlike MRI.

Provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle

Diagnosis may eliminate the need for exploratory surgery and surgical biopsy.

No radiation remains in a patient's body after a CT examination.

Usually have no immediate side effects.

Slight increase in cancer from larger amounts of radiation, such as from radiation therapy.

Not recommended for pregnant women unless medically necessary because of potential risk to the baby.

Manufacturers of intravenous contrast indicate mothers should not breastfeed their babies for 24-48 hours after contrast medium is given.

Rare risk of serious allergic reaction to contrast materials exist, so radiology department should be well-equipped to deal with them.

Limitations of CT Scanning of the Body

• Soft-tissue details in areas such as the brain, internal pelvic organs, and joints (such as knees and shoulders) can often be better evaluated with magnetic resonance imaging (MRI).

• In pregnant women, while CT can’t be performed safely, other imaging exams not involving radiation, such as ultrasound or MRI, is preferred if they are likely to be as good as CT in diagnosing your condition.

• A person who is very large may not fit into the opening of a conventional CT scanner or may be over the weight limit—usually 450 pounds—for the moving table

SIGNIFICANCE OF CT IN MAXILLOFACIAL REGION

❍ Evaluation of the presence and extent of clinically suspected pathology in the head and neck region including cysts, tumors and infections. ❍ Detection of the extension of disease process into the paranasal sinuses, base of skull and orbit. ❍ Determination of the location, extent and displacement of maxillofacial skeletal fractures, including detection of subdural and epidural hematomas. ❍ Salivary gland imaging. ❍ Evaluation of potential implant sites using 3D image reconstruction. ❍ Evaluation of the components of the TMJ. ❍ CT-guided fine needle aspiration biopsies. ❍ Virtual surgeries.

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 tumors involving the masticator spaces and found that CT is helpful in differentiating inflammation from frank abscess. Alan A Schwimmer et al (1988) emphasized the role of CT in the diagnosis and management of temporal and infra temporal space abscesses.

Mandibular body affected by suppurative osteomyelitis.

Salivary gland disease

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 in formation and laboratory

findings, the overall specificity in identifying the

tumor becomes 90%.CT of the neck demonstratesa stone (blue arrow) in the submandibular region of a dilated Wharton's Duct (red arrow)

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)

Fig: Axial CT images of a keratocystic odontogenic tumor in the mandible

ODONTOGENIC TUMOURS• Ameloblastomas-bicortical expansion, thinning

and breach of bony walls, extension of tumor into adjacent soft tissue spaces. Focal cystic degenerations commonly seen in multi-locular 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 hyper dense areas suggesting calcifications maybe noticeable in Pindborg tumour.

MALIGNANCIESSeen as 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.

Fig: Osteosarcoma of jaw in CT

Fibro-osseous lesionsCT 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.

Fig: Fibrous dysplasia of the mandible

MAXILLARY LESIONSMaxillary lesions share similar pictures in contrast to mandibular lesions which make this difficult to distinguish them.Significantly greater tumor extent is demonstrated by CT than by conventional methods.

Fig: Swelling in the left maxillary sinus diagnosed with Brown tumor

TMJCT 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.

Figure 2: Coronal CT scan view showing obliteration of left interarticular space of TMJ.

CONCLUSIONCT scan is a complex but effective imaging system that has a variety of clinical indications directly related to the diagnosis and treatment of oral and maxillofacial abnormalities.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 and conclusion of anatomy and pathology, but further improvement in clinical performance is expected.

References• Oral radiology, 1st south asian edition, white and pharoah• Oral radiology, 6e, White and Pharoah• Fundamentals of Diagnostic Radiology, 4e, Brant, William and Helms• Handbook of biomedical Instrumentation, R S khandpur • Lippincott Radiology• Computed Tomography - Special Applications Edited by Dr. Luca Saba• ww.contempclindent.org/article.asp• serc.carleton.edu/research_education/geochemsheets/

techniques/CT.html• http://pocketdentistry.com/• http://arcadiacachamber.org/hospitals-ct-scanner-state-of-art/