COMPUTED TOMOGRAPHY SCAN (C.T. SCAN)

COMPUTED TOMOGRAPHY SCAN(C.T. SCAN)Presented by:SHOUNAK NANDIBME 3rd Year, 6th semesterRoll-130206014Reg.n0. 131230110645

TABLE OF CONTENTSIntroductionHistory and TypesComponentsWorking principle and circuitrySignificanceRisks and PrecautionsDoseSummaryConclusion

INTRODUCTIONIt is evident that X-Rays are used to examine the internal structures of the human body. X-ray absorption relative to their surroundings cause sufficient contrast differences on the film to be distinguished by the eye. Thus whilst the bone structures are distinct, the shape and composition of the soft tissues and organs are hard to be discerned.

INTRODUCTION(contd..)However, the limitations of X-Ray are-The superimposition of the three dimensional information onto a single plane are too difficult and confusing to be diagnosed.The radiographic plate has a limited dynamic range, hence the large contrast differences are distinguishable by the eye.

INTRODUCTION(contd..)Hence Computed Tomography scan, in short the C.T. Scan has been designed which uses the X-Ray radiation technique but with a different mechanism so that the limitations could be bridged.The term tomos is a Greek word meaning to write a slice or section.

Head firstFeet firstComputed tomography

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HISTORY & TYPESCT Scan was invented and developed by G.N.Hounsfield at the Central Research Laboratories of EMI Ltd, UK and introduced on a commercial scale in 1972.As X-Ray CT is the most common CT in medicine and various contexts, the other types include PET (Positron Emission Tomography), SPECT(Single Positron Emission Computed Tomography), CAT(Computed Axial Tomography) Scan.

HISTORY & TYPESThe early CT Scanners were specifically designed for neuro-radiological investigations. But with the new age CT Scan machines the image resolution has increased, moreover it has enabled the doctors to distinguish between different brain tissue, or even normal and coagulated blood. Moreover, the ventricles if the brain and the repositories of the CSF are clearly observable from CT Scan. Whole body scanners are now commercially available.

HISTORY & TYPESBased on the scanning system the CT Scan has the following generations:First Generation: Parallel beam geometrySecond Generation: Fan Beam, Multiple DetectorsThird Generation: Fan Beam, Rotating DetectorsFourth Generation: Scanning Electron BeamSLICE: It refers to the number of images the X-ray detector can detect in one 360 rotation of the rotating anode. For e.g. A 64 slice CT Scanner produces 64 images as it contains 64 detectors in 1 complete rotation.

WORKING PRINCIPLE & CIRCUITRYCT Scan reconstructs the image from a large number of absorption profiles taken at regular intervals around a slice, each profile being made up from a parallel set of absorption values through the object.The X-Rays from a finely collimated source are made to pass through a slice of the object or patient from a variety of directions. Less X-Rays are transmitted through a longer tissue path-length, while more for shorter path length thus the radiation intensity of the regions in the image varies accordingly.

WORKING PRINCIPLE & CIRCUITRYFor a monochromatic X-Ray beam, the tissue attenuation can be described by-It=Ioe-xIt= Transmitted intensityIo =Incident radiation intensity = Characteristic attenuation co-efficient of tissueX = thickness of tissue

WORKING PRINCIPLE & CIRCUITRY

WORKING PRINCIPLE & CIRCUITRYThe image formed by the CT Scan is considered as per slice. This slice is considered to be as the sum of the volume elements or voxels, each voxel having separate attenuation coefficients. If a slice of heterogeneous tissue is irradiated, the sum of the attenuation co-efficients for each X-Ray beam is calculated by the measured intensities for a given voxel width. Computed Tomography uses the knowledge of attenuation co-efficient sums, thus derived, from all the irradiation directions to calculate the attenuation co-efficients to form the CT-image.

WORKING PRINCIPLE & CIRCUITRYThe X-Ray source and detectors are mounted opposite to each other in a rigid gantry with patient lying in between, by moving or both of these across relevant sections which is how measurements are made.The X-Ray transmission readings are taken and stored by the computer. In one system, there are 18 traverses in 20s scanning cycle by 30 detectors with approximately 600 position pulses that constitute the image.

WORKING PRINCIPLE & CIRCUITRY

COMPONENTSThe CT system consists of 4 major subsystems:Scanning system- Takes suitable readings for a picture to be reconstructed, includes X-Ray source and detectors.Processing unit- Converts the readings into intelligible picture informationViewing part- Displays the image and includes manipulative aids to assist diagnosis.Storage unit- This enables data storage.

COMPONENTSScanning systemX-ray tube:Rotating anode tubes produce pulsed X-Ray radiations having exposure time in the range of 2-20seconds.Power requirement: 100-160kV; optimum at 120kV, 200-500mA.Energy spectrum ranges from 30-120keV.

COMPONENTSScanning systemX-Ray detectorsXenon gas ionization detectors : Contains xenon gas inside the gas chamber. The X-ray falls on the chamber through a 1mm width slit and interacts with xenon, thus producing positive ions and negative electrons. The +ve voltage accelerates the ions to the collector plate and produces an electric current in the amplifier. The resulting current through the electrode is a measure of the x-ray intensity.

COMPONENTSScanning systemX-Ray detectorsScintillation PM detectors: Scintillation photo multiplier detectors comprise scintillation crystals and photodiodes.SOMATOM SCINTILLARC: 520 CsI crystals, assembled with photodiodes arranged on a 42 degree arc.Solid state detectors

COMPONENTS(b) Processing systemData Acquisition *The dynamic range of the detection is 1:4,00,000. In other words it can detect the smallest and the largest signal whether the subject is obese or thin and thereby produce an optimal image.**A data acquisition systems consists of a precision pre-amplifier, current to voltage converter, analog integrators, multiplexers and ADC.***The patient slice is divided up into several voxels, the 2D image of the slice contains several pixels which corresponds to the attenuation coefficient of voxel in the object slice.

COMPONENTS(b) Processing systemData Acquisition

computer

COMPONENTS(c)Viewing system Nowadays the CT image is obtained from a software known as DICOM which processes the data to reconstruct both cross-sectional as well as 3D image.The images are interpreted by the radiologist from the console room in front of a computer.(d) Storing and DocumentationThe data are stored in DVD or hard disk memory of the computer.

GANTRY GEOMETRYThe gantry looks like a doughnut which contains the X-ray tube and the detector and other associated mechanism.60-72cm diameter holeCan be tilted 20degrees Provided with a light which indicate the CT slice positionIntercom available for communication between the patient and the physician from the console room.

GANTRYCONSOLE (For intercom)

SPIRAL/HELICAL CTThis is a scanning technique in which the X-Ray rotates continuously around the patient while the patient is continuously translated through the fan beam. The focal spot traces a helix around the patient. The projection data is used for the reconstruction of multiple contiguous images.Spiral CT is used for 3D image- images taken are in smooth continuous motion. The projection data should be acquired in a single breath hold, at rates of one slice per second.

SPIRAL/HELICAL CT

SPIRAL/HELICAL CTHowever, the reconstruction algorithms are a bit complicated because the patient movement along z-direction needed to be taken into account.ADVANTAGE: Allows images to be reconstructed at arbitrary positions at arbitrary spacings, thus a 3D image can be constructed. Small spacings can locate small lesions.DISADVANTAGE: However, the z-direction movement of the patient should be controlled because the degree of blurring of the image depends on the speed with which the patient is moved.

3D RECONSTRUCT IMAGE OF THORACIC CAVITY

SIGNIFICANCEDetection of tumors- benign and malignant at various parts of the body.(PET Positron Emission Tomography is a specialized technique to detect cancer by allowing an isotope to the body which highlights the cancerous cells.)AngioplastyDetection of aneurysmBrain : vessel coagulation findings, strokeAbdominal injury findings

SIGNIFICANCE

RISKSIodine is the usual contrast dye. Some patients are allergic to iodine and may experience a reaction that may include nausea, breathing difficulty or other symptoms.The amount of radiation used during a CT procedure is considered minimal; therefore, the risk for radiation exposure is very low.Radiation exposure during pregnancy may lead to birth defects. Radiation is harmful for the foetus.

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Before the Procedure Explain the procedure to the patient. If the procedure involves the use of contrast dye, consent should be signed form the patient. Ask the patient if he has ever had a reaction to any contrast media. Generally, there is no fasting requirement prior to a CT scan, unless a contrast media is to be used. (abdomen and pelvis need fasting and cleaning the colon iv contrast need fasting 4 hours before the examination).

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DOSEDOSE depends on BMI of the subject. However, the optimum dose level ranges between 1.30.1 rad(J/kg) to 2.5 rad.

SUMMARYWhen X-rays are irradiated on the human body, some of the rays are absorbed and some pass through the body to produce an image. In plain X-ray imaging, the film directly absorbs penetrated X-rays. In CAT scanning, an electronic device called a "detector array" absorbs the penetrated X-rays, measures the X-ray amount, and transmits the data to a computer system. A sophisticated computer system, in turn, calculates and analyzes data from each detector in each level, and finally reconstructs multiple, two-dimensional, cross-sectional images. Presently, 32, 64, 128 slice CT Scanners are available.

REFERENCESR.S.Khandpur, HANDBOOK OF BIO-MEDICAL INSTRUMENTATION, 2nd edition, Tata McGraw Hill

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