FUNDAMENTALS OF CT AND ITS APPLICATIONS

NISHA KARNA

MSCMIT 1^ST YR

ROLL NO 25

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Introduction

• One of the major disadvantages associated with conventiona l radiography is its inability to produce sectional information.
• Computed tomography is a digital-imaging process which produces separate axial sectional images (transverse slices) having no intersection interference.
• The method was first developed in a commercial X-ray machine by Godfrey Hounsfield (UK) in 1973.

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COMPARISON OF CT WITH CONVENTIONAL RADIOGRAPHY

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A conventional x-ray image is basically a shadow.

Shadows give you an incomplete picture of an object’s shape.

COMPARISON OF CT WITH CONVENTIONAL RADIOGRAPHY

CT

• Highly tissue sensitive
• Provides more anatomical detail and differentiation
• Scan data can be manipulated into different views without additional imaging(i.e.: Axial, Sagittal, Coronal and 3D reconstruction)
• Increased exposure dose
• Increased examination time
• Increased examination cost

X-Ray

• Less tissue sensitive
• Provides less anatomical detail and differentiation
• 2 dimensional, superimposing anatomical information
• Required separate exposures for each unique view(i.e. AP, Lateral, and oblique views)
• Decreased exposure dose
• Decreased examination time
• Decreased examination cost

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COMPARISON OF CT WITH CONVENTIONAL RADIOGRAPHY

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COMPARISON OF CT WITH CONVENTIONAL RADIOGRAPHY

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HISTORY

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HISTORY

• 1979- Hounsfield and Cormack receive noble prize in medicine for their contribution in the development of CT
• 1974- first whole body scanner by Dr. Robert Ledly
• 1975- dynamic spatial reconstruction (DSR)

- Image dynamics of organ system with high spatial resolution

• 1983- EBCT scanner introduced, first cardiac imager
• 1989- first practical spiral Ct scanner introduced at RSNA.
• 1998- MSCT- four or more slices per revolution
• 2005- DSCT by Siemens

- Advanced cardiac imaging by utilizing 2 x-ray tubes

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HISTORY

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HISTORY

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BASIC PRINCIPLE

• Internal structure of an object can be reconstructed from multiple projections of the object.
• Basically a narrow beam of x-ray scans across a patient in synchrony with an array of radiation detector on the opposite side of the pt.
• A sufficient no. of transmission measurements are taken at different orientation of x-ray source and detectors, the distribution of attenuation coefficients within the layer are determined.
• By assigning different gray levels to different attenuation coefficients, an image can be reconstructed with aid of computer that represents various structures with different attenuation properties.

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BASIC PRINCIPLE

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Tomographic reconstruction

• Each ray that is acquired in CT is a transmission measurement through the pt. along a line, where the detector measures an x-ray intensity, N.
• The un-attenuated intensity of the x-ray beam is also measured during the scan by the reference detector, N0
• The relationship between N0 and N is given by

Where t is the thickness of the pt along the ray and µ s the average linear attenuation coefficient along the ray.

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For single tissue element

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For multiple tissue elements

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Total linear attenuation

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Where

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Fig : showing a small section of the final matrix showing

individual attenuation values combined as a ray-sum.

What are we measuring

• The average linear attenuation coefficient between tube and detectors
• Attenuation coefficient reflects to the degree to which the intensity of the x-ray is reduced by the material.

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Ct number- Hounsfield unit

• The Ct scanner calculates from the collected data, the linear attenuation coefficient (µ) of each pixel.
• after a computer calculates a value for the linear attenuation coefficient of each pixel, the value is converted to anew number called a CT number.
• The calculation allows the computer to present the information as a picture with a large gray scale.
• Ct number is calculated as

Ct number= k(µtissue- µwater)

µwater

Where µtissue is the linear attenuation coefficient of measured tissue, µwater is the attenuation coefficient of water and k is the constant factor.

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Ct number- Hounsfield unit

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Ct number- Hounsfield unit

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Ct number- Hounsfield unit

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Advances of CT through various generations

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First generation CT

• The first CT scanner used clinically was the EMI Mark 1, in 1972
• It employed a pencil like x-ray beam and a single detector.
• The x-ray tube and detector movements were both linear and rotary( translate- rotate)
• The x-ray tube and detector assembly on the opposite side makes a linear motion across the pt. making 160 measurements in one linear motion using pencil beam.
• After one linear motion the tube detector assembly is rotated 1 degree and makes similar measurement which is repeated over 180 times making a total of 28800 measurements.

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GENERATION 1

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Second generation

• Narrow fan beam geometry(10)
• Faster than 1^st generation, 5-30 detectors
• The movement of the tube detector are translate and rotate
• Second gen scanners produced image in between 10 and 90 sec, depending on the manufacture.

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GENERATION II

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3^rd generation

• Rotate – rotate mechanism
• More than 800 detectors
• Ring artifact
• .700,000 measurements/slice
• ,0.5 sec/ slice

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3^rd generation

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4^th generation

• Rotate –stationary tube detector design
• 600-800 ring of stationary detector
• No ring artifact
• 1,20,000 measurements
• 2-10 sec

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4^th generation

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5^th generation-EBCT

• Stationary stationary system developed specifically for cardiac tomography imaging .
• No conventional x-ray tube, no moving parts
• Large arc of tungsten (210) encircling patient and lying directly opposite to the detector ring.
• The whole heart can be acquired in 0.2 sec. These scanners were useful in cardiac imaging, pediatric and trauma pts.

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5^th generation-EBCT

• Used an electron gun that deflects and focuses a fast moving electron beam along tungsten target ring in the gantry.
• The detector is also in the form of ring , permitting simultaneous acquisition of multiple image sections.
• Images are obtained in 50ms and can produce fast frame rate CT of the beating heart rate with minimum motion artifacts.

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GENERATION 5

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Sixth generation

• Third generation + slip ring technology + helical /spiral motion= sixth generation
• Also k/a helical / spiral CT
• A truly revolutionary advancement (1990) in CT scanning that finally allowed true 3D image acquisition within a single breath hold
• Three technological developments were required; slip ring gantry designs, very high power x-ray tubes, interpolation algorithms to handle the non coplanar projection data.

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SIXTH GENERATION-Helical scanning

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Slip ring technology

• Early CT design limited the scan method to the step and shoot mode considerably limiting the gantry rotation time.
• It is a circular contact with sliding brushes which supplies electrical power to the CT system and allows the gantry to rotate continuously.

Slip ring design

• Disc type
• Cylinder type
• There are usually three slip rings on a gantry
• One provides high voltage power to the x-ray tube or low voltage power to the high tension generator
• Second provides low voltage power to control systems on the rotating gantry
• The third transfers digital data from the rotating detector array.

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Slip ring technology

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Slip ring technology

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Interpolation algorithms

• Reconstruction of an image at any z-axis position is possible because of a mathematical process called interpolation .
• Interpolation algorithm estimates a value between known values and extrapolation estimates the value beyond known values.
• During spiral CT , image data are received continuously.
• When an image is reconstructed, the plane of the image does not contained enough data for reconstruction.
• Data in that plane must be estimated by interpolation.

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Interpolation

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Interpolation algorithms

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Interpolation algorithms

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Seventh generation

• Third generation+ slip ring technology+ helical / spiral motion+ multiple detector array along z-axis= seventh generation
• Also k/a MDCT/ MSCT

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Multiple detector array

• Multiple detector arrays are a set of linear detector arrays, tightly abutted.
• Assembly of multiple solid state detectors.
• With multiple detector arrays, slice thickness is determined by the detectors, not by collimators.
• Types of detector array in MDCT
• Uniform( GE)
• Adaptive ( Siemens, Phillips)
• Hybrid( Toshiba)

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Pitch

• For SDCT, it is defined as the ratio of table increment per 360 degree gantry rotation over collimator width.
• Pitch=table travel per 3600 / collimation
• It is generally expressed as 0.5, 1 .0, 1.5, 2
• Pitch= 1- coils of the helix are in contact
• Pitch <1 – coils o the helix overlap, increased pt dose
• Pitch> 1 - coils of the helix are separated, reduced pt dose.

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Pitch

• Definition of pitch differs in case of MDCT
• Beam pitch and slice/detector pitch

Beam pitch can be defined as table movement per rotation divided by beam width

• Beam pitch = table travel per 360 ° / beam width
• Detector pitch is defined as table movement per rotation time divided by the selected slice thickness of the detector
• Detector pitch= table travel per 360 ° / detector thickness
• In general, in both SDCT and MDCT systems a pitch between 0.75 and 1.5 is most common.
• Collimator pitch= detector pitch/ N

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Pitch

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Data acquisition

• The scanning process begins with data acquisition.
• Data acquisition refers to a method by which the patient is systematically scanned by the x-ray tube and detectors to collect enough information for image reconstruction.
• Ray: - A single transmission measurement through the patient made by single detector at a given movement in a time is called ‘Ray’
• Projection: - A series of Rays that passes through the patient at same orientation called a Projection or View.
• Basically in CT image acquisition, Parallel Beam Geometry & Fan Beam Geometry are used.

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Sinogram

• The set of projections of a single slice is called sinogram.
• The data acquired for one CT slice can be displayed before reconstruction.
• This display is called sinogram.
• Sinograms are not used for clinical purposes, but the concepts helps in understanding tomographic principles.
• The horizontal axis of the sinogram corresponds to the different rays in each projection and the vertical axis in the sinogram represents each projection angle.

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Ct data acquisition components

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Ct data acquisition components

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X-ray tube

• Rotating anode x-ray tube
• More heat loading and dissipation capabilities
• Small focal spot size to improve spatial resolution.

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Filters

• Compensation filter is used(bow tie filter).
• PTFE is commonly selected as a filter material.
• Copper is used as an additional filter material(harden the beam)
• Filters are used:
• To absorb low energy x-rays
• To reduce pt. dose
• To provide a more uniform beam

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Filters

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Collimators

• To decrease scatter radiation
• To reduce pt. dose
• To improve image quality
• Collimator width determines the slice thikness in SSCT
• Pre pt. collimation
• Determines dose profile and pt dose
• Pre detector collimation
• Restrict the x-ray beam viewed by detector array.
• This reduces scatter on detector array.

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Detectors

• The detectors gather information by measuring the xrayy transmission through the pt.
• Two types:
• Xenon gas detectors
• Use high pressure(about 25 atm) nonradioactive xenon gas in long thin cells between two metal plates.
• Solid state detector
• Composed of a scintillator coupled directly to a photodetector.
• CdWO4, Lu2O:Ce, Gd2O:Ce, YAlO3:Ce

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Image reconstruction

• Image reconstruction can be done by:
• Simple backprojection
• Filtered backprojection
• Iterative method

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Windowing

• Windowing is the process in which the gray level can be manipulated using the CT numbers to provide the optimum demonstration of different structures seen in the image.
• Window width is the range of CT numbers for the gray scale and WW control alters the image contrast.
• Window level is the center of the gray scale image and WL control alters the CT number of the tissue to be displayed.

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Clinical applications of CT

• Isotropic viewing: MPR, MIP, MinIP, VRT, SSD, 3D Recon
• Musculoskeletal applications
• CT myelography
• Long coverage and multiphase studies
• Abdominal applications
• Ct angiography/ vascular application
• Ct perfusion
• Imaging of large patients
• Virtual endoscopy / colonoscopy/ bronchoscopy

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Clinical applications of CT

• Ct fluoroscopy
• Chest application
• Cardiac CT
• Evaluation of acute chest pain or dyspnea(TRO CT)
• Application in radiation therapy: CT simulation
• Portable CT imaging
• Screening and quantitative CT
• DECT

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Post processing

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Post processing

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Post processing

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Post processing

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Comparative routine and minIP images of HRCT lungs in axial plane showing

hypoattenuating areas(arrow) in apt with airway dx. The hypo attenuating area is more

conspicious in minIP image.

Post processing

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Post processing

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Brain CT scan a) axial image and b) 3D reconstruction.

Axial image shows focal area of hemorrhagic contusion with surrounding low density due to edema in the rt frontal lobe, and depressed fracture in frontal bone; b) 3D recon shows the extension and the morphology of the depressed skull fracture.

CT Angiography

• CT angiography defines as CT imaging of blood vessels opacified by contrast media
• Images can be captured when vessels are fully opacified to demonstrate either arterial or venous phase enhancement through the acquisition of both data sets.

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CT Angiography

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Occluded MCA on CT angiography

CT Angiography

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MCA ischemic stroke

CT Angiography

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CT PA shows multiple pulmonary emboli . 2D axial images shows presence of thrombus

In the main PA with extension to the segmental and sub segmental artery on both sides resulting in filling defects(A-C). Coronal reformatted image demonstrates multiple emboli on PA(D). Arrow indicate the thrombi in PA.

CT Angiography

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Cardiac application

• The advent of systems with subsecond rotation times and ECG synchronized scanning brought ct into the domain of cardiac imaging.
• Retrospectively ECG gated multi-slice spiral scanning , has the potential to completely cover the heart volume in single breath hold.

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Cardiac application

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Cardiac application

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Volume rendered(a-c) and curved MPR

images used in coronary artery evaluation

on most dedicated workstations.

Cardiac CTA in estimating left ventricular volumes and ejection fraction-Functional CT study

• Left ventricular ejection fraction(LVEF) is one of the most commonly reported measures on left ventricular systolic function.
• It is the ratio of blood ejected during systole( stroke volume) to blood in the ventricle at the end o the diastole.
• LVEF= stroke volume (EDV-ESV) / EDV
• LVEF is an important predictor of cardiac outcome and helps in making important diagnostic and therapeutic decisions such as the treatment of different types of congestive heart failure.

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Cardiac CTA in estimating left ventricular volumes and ejection fraction-Functional CT study

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MPR of left ventricular cavity in

4 chamber views and 2 chamber

and semi automated calculation of

left ventricular volume in both end

systolic and end diastolic in biplane

and 4 and 2 chamber views. Pt with

cardiomyopathy having low ejection fraction

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TRO-CT

• Triple rule out CT angiography can provide a cost effective evaluation of the coronary arteries, aorta , PA, and adjacent intrathoracic structures for the pt. with acute chest pain
• Recent MDCT scanners can scan high quality TRO CT studies by tailoring the injection of iodinated CM to provide simultaneous high levels of arterial enhancement in Coronary arteries, Aorta and PA

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TRO-CT

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CT Perfusion

• Perfusion CT allows functional evaluation of tissue vascularity.
• It measures the temporal changes in tissue density after the intravenous injection of a contrast medium bolus using a series of dynamically acquired CT images.
• The greatest impact of perfusion CT has been on the assessment of patients who have had strokes, wherein the rapid scanning and faster image processing have cemented its role as the modality of choice for evaluation of structural and functional status of cerebral vasculature.

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CT-Perfusion

• Blood flow(CBF) = flow rate of blood through cerebral vasculature(ml/100gm/min)
• Blood volume= volume of flowing blood (ml/100gm)
• Mean transit time= time required or blood to pass through tissue(sec).

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CT-Perfusion

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Ncct (A) and CTP parameters, CBF(B), CBV©, AND MTT(D), demonstrate normal symmetric

Brain perfusion. By convention, all color maps are coded red for higher values and blue

for lower values.

CT-Perfusion

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Multiphasic CT

• Hepatic CT
• CT Urography
• Donor and Recipient CT

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Hepatic CT

• Triple phase CT done to characterize the liver masses.
• Arterial, Portal venous and equilibrium(delayed) phase done to differentiate hyper and hypo vascular lesions.

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Ct enterography

• Non invasive technique for the diagnosis of small bowel disorders.

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Obstructing lesion in the ileum with shouldering leading to small bowel obstruction

CT myelography

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CT myelogram help in confirming intra-dural

extramedullary arachnoid cyst that is partially

opacfied by intrathecal CM.

CT Fluoroscopy

• Real time guidance
• Great image quality
• Low risk
• Faster procedures

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CT Fluoroscopy

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DECT

• In dual energy CT, two different x-ray spectra are used to acquire two image datasets of the same anatomic region, allowing analysis of energy dependent changes in the attenuation of different materials.
• Every material shows a relatively specific change in attenuation between images obtained with a high and a low energy spectrum and this attenuation difference allows better characterization of the tissues.

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Applications of DECT

• Material differentiation
• Bone removal in angiography
• Urinary stone characterization
• Metal artefact reduction
• Clinic application including and not limited to:
• Cardiac, renal
• Vascular, oncology, pulmonary
• Hepatic

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Applications of DECT

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Applications of DECT

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Coronal reformatted image(a) VRT image (b)

Showing fracture, DECT images(c,d) showing

bone marrow edema

Applications of DECT

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Another example of urinary calculi (arrows), two calculi are seen in the kidney

in the CT image(A). DECT the calculus red (B) in colour

and falling below the line, thus confirming uric acid calculi.

CT Simulation for radiotherapy

• CT simulation includes a CT scan area of the body to be treated with radiation.
• The images acquired during the scan will be reconstructed and used to design the best and most precise treatment plan for the pt.
• The simulation portion of patients adiation treatment regiment ensures that the treatment s will target the area of concern
• cT imaging data provde a complete 3 D view of the pt’s anatomy, allowing for more accurate delineation of the tumor and the surrounding normal tissues.

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Fig :CT simulation for radiaotherapy with avoidance to exposure to the gastric antrum.

CT Thermometry

• Much recent research effort has been devoted to introduction of non-invasive techniques for monitoring of temperature during hyperthermal treatment.
• This technique is based on the dependency of the attenuation coefficient , and consequently the CT number on temperature.
• CT thermometry refers to the acquisition of detailed thermal information throughout a slice of the subject obtained from a CT scan, providing indirect measurement of temperature from CT values.
• Research efforts have been devoted to the investigation of the above mentioned non-invasive techniques due to the growing clinical use of several minimally invasive procedures for the local treatment of solid malignancies i.e., RFA, LITT, MWA,HIFU and cryotherapy.

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Virtual bronchoscopy

• Virtual bronchoscopy (VB) is a computer-generated 3D CT post-processing technique that produces high-resolution images of the tracheobronchial tree and endobronchial views that simulate the findings at conventional bronchoscopy
• VB is being increasingly used to detect and grade benign and malignant airway stenosis.
• VB can be easily used for identification of anatomic variants such as tracheal and bronchial diverticula�������

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Fig : Example of effect of rendering algorithm on luminal size.

Volume-rendered 3D image of airway using translucent preset

shows trachea measures 1.76 cm��

Virtual bronchoscopy

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Fig : 45-year-old woman with cough.

Coronal multiplanar reformation shows level

of D. Arrow indicates

subsegmental branch of right

lower lobe bronchus.�

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Conclusion

• With the development of multidetector system, CT has reached the extreme of new era.
• CT has become as first line of imaging technique in diagnosis and treatment of some dx due to its superior speed.

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