UNIT I Basics of COMPUTED TOMOGRAPHY(CT)

UNIT I Basics of

COMPUTED TOMOGRAPHY(CT)

www.Vidyarthiplus.com


COMPUTED TOMOGRAPHY(CT)

A Multi slice CT Scanner: Philips ‘Brilliance’ 64 channel thin slice

Godfrey Newbold Hounsfield



http://en.wikipedia.org/wiki/Image:64_slice_scanner.JPG



INTRO • X-ray computed tomography, also computed

tomography (CT scan) or computed axial tomography (CAT scan), is a medical imaging procedure that utilizes computer-processed X-rays to produce tomographic images or 'slices' of specific areas of the body.

• These cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines.

• Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.



• CT produces a volume of data that can be manipulated, through a process known as "windowing", in order to demonstrate various bodily structures based on their ability to block the X-ray beam.

• Historically the images generated were in the axial or transverse plane, perpendicular to the long axis of the body.

• modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.

• Although most common in medicine, CT is also used in other fields, such as nondestructive materials testing.

• Another example is archaeological uses.



Early Attempts at CT

• Gabriel Frank: 1940 Patent: described CT principles using optical back projection reconstruction (but no filter)

• Takahashi (Japan, ‘40s, published 1956): describes equipment to image slices by backprojection

• Tetel’baum et al (Russia, 1957): Accurate formulation of inverse Radon Transform; TV-based reconstruct

• Kuhl & Edwards: (1963): cross-sectional images by back-projecting transmission data on oscilloscope

• Alan Cormack: built simple CT to measure densities for radiotherapy. Shared Nobel Prize.



HISTORY OF CT SCAN

• Originally known as "EMI SCAN" as the first CAT scanner, a device which revolutionised medical imaging developed in early 1970s, at EMI Central Research Laboratories in UK by Godfrey Newbold Hounsfield and Allan McCormack in 1972 (They shared the 1979 Nobel Prize)

• CT stands for Computerized Tomography • Sometimes referred to as CAT(Computed Axial

Tomography), or CT Scan or Body Section Roentgenography.



HISTORY OF CT SCAN (CONTD)..

• The word "tomography" is derived from greek tomos (section or slice) and graphein (to write).

• Utilizes X-Rays and computers for cross sectional slice of the body

• Each picture is like a slice from a loaf of bread, showing both the outline and internal structure

• The scan provides highly detailed images of all parts of the body (soft tissue, bone, organs, etc)



• CT was originally proposed and used as an extension of the basic X-ray: doctors wanted to see inside the head, but standard X-ray techniques could not penetrate the dense skull while distinguishing soft tissues

• The first scanners could only do one slice at a time and each slice took 4 minutes to complete

• By 1976, whole body scanners were developed • Today’s machines can acquire a slice in less

than half a second (.1 second for the GE top-end multislice LightSpeed Pro16 with Xtream)

• Advancements still occurring through new technology



Circa 1975, in the early days of the CT scan.

A present-day scan, showing a six-fold increase in detail

(images courtesy Siemens Medical Systems and Imaginis.com)




http://siemensmedical.com/

http://imaginis.com/


Original "Siretom" dedicated head CT scanner, circa 1974



Specifications First CT (circa 1970) Modern CT Scanner (2001)

Time to acquire one CT image

4-5 minutes 0.5 seconds

Pixel size 3 mm x 3 mm 0.5 mm x 0.5 mm

Number of pixels in an image

64,000 256,000

Table Data: http://www.physicscentral.com/action/action-02-3.html

HISTORY OF CT SCAN (CONTD).. www.Vidyarthiplus.com


TRADITIONAL X-RAYS

• X-rays pass through the body and project a shadow of the contents of the body onto the detectors, which record this projection in various shades of gray

• Only reveals bones clearly, while soft tissue just appears black

• Three-dimensional body parts are projected onto two-dimensional film, losing valuable information



PROJECTION IMAGES: The creation of a two-dimensional image “shadow” of the three dimensional body. X-ray are transmitted through a patient, creating a radiograph.



Limitations of Radiography

• Inefficient x-ray absorption: typically ~25% for par speed cassette (prior to rare earth technology)

• High Scatter-to-Primary Ratios: may have >50% scatter at receptor with large beams even with high ratio grid

• Receptor Contrast vs latitude: required film dynamic range limits film contrast

• Superposition/Conspicuity: overlapping structures with 3D anatomy rendered on 2D image



• Provides 3D images and cross-sectional views instead of basic 2D images • CT Scans can show soft tissue as well as bone, allowing physicians to detect problems such as cancerous tumors • Extremely helpful in determining organ anatomy, especially following

trauma

IMPROVEMENTS OVER X-RAYS www.Vidyarthiplus.com


CT SCANNING OF HEAD • CT scanning of the head is typically used to detect

infarction, tumours, calcifications, haemorrhage and bone trauma.

• Of the above, hypodense (dark) structures can indicate infarction and edema,

• hyperdense (bright) structures indicate calcifications and haemorrhage and

• bone trauma can be seen as disjunction in bone windows. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema.

• Ambulances equipped with small bore multi-sliced CT scanners respond to cases involving stroke or head trauma.



LUNGS • CT can be used for detecting both acute and chronic

changes in the lung parenchyma, that is, the internals of the lungs.

• It is particularly relevant here because normal two-dimensional X-rays do not show such defects.

• A variety of techniques are used, depending on the suspected abnormality.

• For evaluation of chronic interstitial processes, thin sections with high spatial frequency reconstructions are used;

• often scans are performed both in inspiration and expiration.

• This special technique is called high resolution CT.



ADVANTAGES OF CT • There are several advantages that CT has over traditional

2D medical radiography. • First, CT completely eliminates the superimposition of

images of structures outside the area of interest. • Second, because of the inherent high-contrast resolution of

CT, differences between tissues that differ in physical density by less than 1% can be distinguished.

• Finally, 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, depending on the diagnostic task.

• This is referred to as multiplanar reformatted imaging.



• CT is regarded as a moderate- to high-radiation diagnostic technique.

• The improved resolution of CT has permitted the development of new investigations, which may have advantages;

• compared to conventional radiography, for example, CT angiography avoids the invasive insertion of a catheter.

• CT Colonography (also known as Virtual Colonoscopy or VC for short) may be as useful for detection of tumors, but may use a lower radiation dose.



• The radiation dose for a particular study depends on multiple factors:

• volume scanned, • patient build, • number and type of scan sequences, and • desired resolution and image quality.



Conventional vs Axial Tomography

Conventional Cut

CT Axial Cut



CT IMAGES

Normal Abdominal CT. Level of Liver and Spleen Level of Kidneys



Principle of CT

• A narrow x-ray beam is scanned across a patient in synchrony with a radiation detector on the opposite side of the patient. If the beam is monoenergetic, the transmission of x-rays through the patient is

I=Ioe-µx

• If the x-ray beam is intercepted by trwo regions with attentuation coefficients µ1 and µ2 and thickness x1 and x2, the x-ray transmission is

I=Ioe-µ1x1+µ

2x2

• If many(n) regions with different linear attentuation coefficients occur along the path of x-rays, the trransmission is

I=Ioe-∑i=1to n

µixi



CT - BLOCK DIAGRAM

Diagram showing relationship of x-ray tube, patient, detector, and image reconstruction computer and display monitor



•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

CT SCAN METHODOLOGY www.Vidyarthiplus.com


Radiation detection system is composed of detection elements, such as scintillating crystals and photodiodes

•Data acquisition system measures the radiation data transmitted through the object and digitizes it so the computer system can read it

•Computer reconstructs the image from raw scan data then a picture is created by a cathode ray tube

•Computer allows the technologist to shade, rotate, correlate and measure the organs in the image

•Bone appears white; gases and liquids are black; tissues are gray

•Measurements taken in Hounsfield units (Hu), calibrated universally with air at -1000 Hu and water at 0 Hu (other typical values include fat ~-50 Hu, muscle ~40 Hu, and bone ~1000 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)

CT SCAN METHODOLOGY (CONTD).. www.Vidyarthiplus.com


The three standard orientations of slice (or tomographic) images

Axial, Transaxial, Transverse

Coronal Frontal Sagittal

Oblique Slice: an orientation not corresponding to one of the Standard slice orientation.



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UNIT I Basics of COMPUTED TOMOGRAPHY(CT)