Computed Tomography

Introduction to computed tomography imaging technique

1. Medical Imaging Computed Tomography Imaging technique By Dr Ramy Abdlaty

2. Contents What is Computed Tomography? How CAT scan works? Scanning procedure Risks of CT/ CAT

3. What is Computed tomography (CT/ CAT)? Computed Tomography (CT) imaging is also known as " CAT scanning " (Computed Axial Tomography). Tomography is from the Greek word " tomos " which means " slice " or " section " and “ graphy ” means " describing ". Therefore its is accepted for evaluation of the entire body. CT was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories , England, and independently by South African born physicist Allan Cormack of Tufts University, Massachusetts. The first clinical CT scanners were installed between 1974 and 1976 . CT uses X-rays and RF electromagnetic radiation. The CT images are obtained in the axial plane with the help of computer. CT can capture images of the body in the sagittal and coronal planes. The first CT scanner took several hours to acquire the raw data for a single scan or "slice" and took days to reconstruct a single image from this raw data. The modern CT collect up to 4 slices in 350 ms and reconstruct a 512 x 512-matrix image in less than a second , and the entire chest (forty 8 mm slices) can be scanned in 5:10 seconds .

4. CT scan for brain

5. CT Vs. conventional X-ray A conventional X-ray image is a shadow: You shine a "light" on one side of the body, and a piece of film on the other side registers the silhouette of the bones. If a larger bone lies between the X-ray machine and a smaller bone, the larger bone may cover the smaller bone on the film. To see the smaller bone, you need to turn your body or move the X-ray machine. Therefore conventional radiography suffers from the collapsing of 3D structures onto a 2D image. In a CAT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. The computer takes all this information and puts together a 3-D image of the body. Doctors can even examine the body one narrow slice at a time to pinpoint specific areas.

6. Comparison between X-ray And CT Aspect of Comparison X-ray shadowgraphs Computed Tomography (CT) Availability Common and available Less common Resolution Moderate High Image dimensions Only 2-dimensional images 2-dimensional & 3-dimensional images Cost Cheap Expensive Time of imaging Short Intermediate Reason of imaging Used primarily to check broken bones fractures and early diagnosis of pneumonia (  an infection that inflames the air sacs in lungs. The air sacs may fill with fluid or causing cough with phlegm, fever, chills, and difficulty breathing ) Used primarily to diagnose problems in soft organ and tissues. It is also used to check the cartilage between backbone parts. X-ray dose Low High

7. Comparison between X-ray, CT, MRI, MRA, PET X-Ray: shows bone/skull only. Does not show the brain. Best used to detect if there are bone fractures. CT: a quick test. Shows brain but detail not great. Shows if any larger bleed, stroke, lesions, or masses. MRI: a long test. Shows brain and detail is great. Shows smaller bleeds, stroke, lesions, or masses. MRA : (Magnetic resonance angiogram) shows the flow of blood in the vasculature system of the brain. If there is vessel narrowing or blockage this test would show it. PET scan: shows how active different parts of the brain is. An active brain uses sugar as energy and pet scan detects how much sugar is being used by lighting up and turning different colors.

8. conventional X-ray disadvantages CT Advantages: 1- CT overcomes the superimposition of the human body structures 2- CT has a high contrast in the produced images of the human body compared to conventional X-ray 3- CT, due to high image contrast, differentiates between various tissues in the human body

9. CT principle of Operation 1- the internal structure of the object (organ/ tissue) can be reconstructed from multiple projections of the object. 2- the mathematical concept of CT was developed by Radon in 1917. 3- This concept provided an evidence that image of unknown object could be produced if multiple number of projections have been taken throughout the object.

10. CAT Scanning Procedure The CT machine is like a donut. The patient lies down on a platform, slowly moving through the machine, where the X-ray tube is mounted on a movable ring. The ring also supports an array of X-ray detectors opposite to the X-ray tube. A motor turns the ring so that the X-ray tube and the detectors revolve around the body. Each full revolution scans a narrow, horizontal "slice" of the body. The control system moves the platform farther into the hole so the tube and detectors can scan the next slice. The CT computer uses sophisticated mathematical techniques to construct a 2-D image slice of the patient. The thickness of the tissue represented in each image slice can vary depending on the CT machine used, but usually ranges from 1-10 millimeters. When a full slice is completed, the image is stored and the motorized bed is moved forward incrementally into the gantry. This process continues until the desired number of slices is collected.

11. CAT Scanning Procedure CT scanning can be divided into three actions Scanning and data acquisition X-ray generator (50:80 KW, 5:50 KHz, 80:120KVp, 80:500 mA) X-ray tube (separate cooling, focal spot = 0.6 mm, Anode heating 1:7 MHU) X-ray Filtration system (compensation filter is used to allow only monochromatic x-ray to pass) Pre patient collimators: reduces the patient dose Post patient collimators: reduces the scattered radiation detectors X-ray detectors Image reconstruction Initial back projection computation Iterative methods of computation Analytical methodology Image display 2-dimensional view 3-dimensional view

12. CT Scanning Procedure Pre patient collimators : reduces the patient dose Post patient collimators : reduces the scattered radiation detectors The functions of the collimators are: Decreasing the scattered X-ray radiation produced by the tube Reducing the x-ray exposure dose for the patient Improving the image quality due to using monochromatic x-ray radiation Determining the slice width

13. CT Scanning Procedure CT detectors : are responsible for gathering information via measuring the x-ray transmission through the patient. There are two types of detectors: Scintillation crystal detector Xenon gas ionization chamber

14. CT Scanning Procedure CT Display : The reconstructed image is displayed on the monitor which is a digital display. The digital display consists of a matrix of pixels that shows a 2-D representation of 3-D object in the human body. The pixel real size of the tissue is computed by dividing the FOV by the display matrix size (for example 512*512) Pixel size = FOV (mm) / Matrix size

15. Generation of Ct scanning machine 1 st Generation of CT : It has a narrow pencil beam of x-ray It has a single detector The detector is made of NaI The CT raw data is obtained from the translate-rotate movements of the tube-detector combination The average scan time for this generation is 5 minutes This generation is mainly designed for scanning the brain

16. Generation of Ct scanning machine 2 nd Generation of CT : It has a narrow fan beam of x-ray It has array of detectors (5 to 30) The CT raw data is obtained from the translate-rotate movements of the tube-detector combination The average scan time is 30 seconds Fewer linear movements are needed as there are more detectors to gather the data

17. Generation of Ct scanning machine 3 rd Generation of CT : It has a pulsed wide fan beam of x-ray It has arc of detectors (600 to 900), both scintillation and Xenon detectors can be used The CT raw data is obtained from the rotate-rotate movements of the tube-detector motion The average scan time is < 5 seconds The disadvantage of this generation is the presence of ring artifacts due to electronic drift between many detectors

18. Generation of Ct scanning machine 4 th Generation of CT : It has a wide fan beam of x-ray covers the entire patient. It has a complete circular array of about 1200 to 4800 stationary detectors. The CT raw data is obtained from the rotational movements of the X-ray tube motion. The average scan time is 0.5 second or << 2 seconds It is designed to overcome the ring artifact disadvantage of the 3 rd generation by keeping detector assembly stationary. The disadvantage of this generation is the high cost.

19. Generation of Ct scanning machine 5 th Generation of CT : It has no conventional x-ray tube, however, it has large arc of tungsten encircles the patient and lies opposite to the detector ring. Electron beam is steered around the patient to strike the annular tungsten target The CT raw data is obtained while there is no movement either from the tube or the detectors. The average scan time is 50 msec. It can produce fast frame rate CT movies of the beating heart It is designed specifically for cardiac tomographic imaging.

20. Comparison between generations of CT scanners?

21. Various parameters of CT scan Slice Matrix Pixel Voxel CT Number Windowing Window Width Window Level Pitch

22. Computed Tomography 1- Slice / CUT The cross section portion of the body which is scanned for production of CT image is called slice. The slice has width and therefore volume. The width is determined by width of the x-rays beam.

23. Computed Tomography 2- Slice Cross section Think like looking into the loaf of bread by cutting into thin slices and then viewing each slice individually. Each slice shows the constituents of tissues that it has inside. The various slices show the extension of each organ/ tissue within the body section of interest.

24. Computed Tomography 3- Matrix Pixel / voxel CT image is represented as a matrix of numbers. A 2-dimensional array of numbers arranged in rows and columns is called matrix. Each square in a matrix is called a pixel. Each element or number in the image matrix represents a 3-dimensional volume element called voxel

25. Computed Tomography 4- CT number The numbers in the CT image matrix is called CT numbers. Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object. Air: -1000 Water: 0 Bone: 1000 Blood: 40 to 70 CSF: 15 Fat: -50 to -100

26. Computed Tomography 5- Hounsfield units (HU) Related to different composition and nature of tissue. CT number is also known as Hounsfield Units (HU). CT number represents the density of the imaged tissue. Different tissues have unequal CT number ranges in HU.

27. Computed Tomography 6- Windowing / window width / window level Windowing is a system where the CT number range of interest spreads to cover the full grey scale available on the display system. Window width means total range of CT no. values selected for gray scale interpretation. It corresponds to contrast of the image. Window level represents the CT no. selected for the center of the range of the no. displayed on the image. It corresponds to brightness of the image.

28. Computed Tomography 7- Pitch CT detector pitch is defined as table distance traveled in one 360° gantry rotation divided by beam collimation. For example, if the table traveled 5 mm in one rotation and the beam collimation was 5 mm then pitch equals 5 mm / 5 mm = 1.0. If patient moves 10 mm during the time it takes for the x-ray tube to rotate through 360 , the pitch is 2. Increasing pitch reduces the scan time and patient dose.

29. Cat Scan: Contrast agents Intravenous contrast: It is a liquid used to enhance tissues and organs and to outline blood vessels, which would not be visualized without contrast. Intravenous contrast is administered through an IV, usually placed in the arm. The contrast is injected by a pump, which controls the rate and amount of the injection. The contrast travels throughout the body via the blood vessels. Images are taken at different times during the circulation of the contrast. This example illustrates how much we can miss when forced to perform a non-contrast CT in someone with renal failure or a contrast allergy . The image on the left, from a triphasic CT liver, has been performed without contrast; the liver looks a little bit nodular but otherwise fine. On the right is an image from the arterial phase of the CT, taken at the same position – it shows multifocal hepatocellular carcinoma (arrows)

30. Cat Scan: Contrast agents Oral contrast: It is used to outline the stomach and upper intestine. There are two types of oral contrast, barium and gastrograffin. Barium is somewhat chalky in texture, and is commonly used. Orange, strawberry, vanilla and chocolate flavors are used to make the barium mixture palatable. Gastrografin is a clear liquid sometimes used in place of barium, and has a bitter taste when mixed with water. Regardless of the type of oral contrast used, it takes time for the contrast to travel through the body. The patient is required to drink approximately 500 - 800 mls of oral contrast. Roughly 45 - 60 minutes after drinking the patient will be ready for scanning.

31. Cat Scan: Contrast agents Rectal contrast: When visualizing the lower bowel, rectal contrast is used. Barium and Gastrografin are used to fill the rectum and lower bowel. A thin tube is inserted in the rectum and the contrast agent is administered through the tube and into the rectum. It is uncomfortable and you may feel full. Fortunately, scanning is fast, and the discomfort will last only a few minutes. After scanning the contrast is drained and you will be escorted to a washroom.

32. Risks of CT/ CAT scanning The risk of a CT scan causing a problem is small. There is a slight risk of developing an allergic reaction to the iodine contrast material. The reaction can be mild (itching, rash) or severe (difficulty breathing or sudden shock). Death resulting from an allergic reaction is rare. Most reactions can be controlled using medication. The contrast material used during CT scanning can cause water loss or damage to the kidneys that may lead to kidney failure. This is a concern if you have poor kidney function. If you have a history of kidney problems. There is always a slight risk of damage from being exposed to any radiation, including the low levels of X-rays used for a CT scan. However, the risk of damage from the X-rays is usually very low compared with the potential benefits of the test.