1. Medical Imaging Lecture 2 1

2. Computed Tomography (CT Scan) Computer tomography (CT), originally known as computed axial tomography (CAT or CT scan) and body section rentenography. it is a medical imaging method employing tomography where digital geometry processing is used to generate a three- dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write). CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X- ray beam. 2

3. How is CT different from a regular x-ray? During a regular x-ray procedure, a stationary machine sends x-rays through the body to make a single “shadow” picture. In CT, the x-ray machine rotates around the body, taking multiple pictures at different angles that allow a computer to make a detailed image of the patient’s anatomy. This 3-D imaging system provides much more information than a regular x-ray. 3

4. Anatomy of CT Scan 4

5. Process 5

6. Process (Cont..) 6

7. The projection of the fan-shaped x-ray beam from one specific x-ray tube focal spot position produces one view. Many views projected from around the patient's body are required to acquire the necessary data to reconstruct an image. Process (Cont..) 7

8. Process Description X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. Many data scans are progressively taken as the object is gradually passed through the gantry. They are combined together by the mathematical procedure known as tomographic reconstruction. contrast materials such as intravenous iodinated contrast are used. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. 8

9. 9 Process Description (cont..)

10. Windowing Windowing is the process of using the calculated Hounsfield units to make an image. The various radio density amplitudes are mapped to 256 shades of gray. These shades of gray can be distributed over a wide range of HU values to get an overview of structures. Alternatively, these shades of gray can be distributed over a narrow range of HU values (called a "narrow window") centered over the average HU value of a particular structure to be evaluated. In this way, variations in the internal makeup of the structure can be discerned. This is a commonly used image processing technique known as contrast compression. 10

11. Windowing (Cont..) For example, to evaluate the abdomen in order to find Small masses in the liver, one might use liver windows. Choosing 70 HU as an average HU value for liver, the shades of gray can be distributed over a narrow window or range. One could use 170 HU as the narrow window, with 85 HU above and 85 HU below it, with 70 HU average value; Therefore the liver window would extend from -15 HU to +155 HU. All the shades of gray for the image would be distributed in this range of Hounsfield values. Any HU value below -15 would be pure black, and any HU value above 155 HU would be pure white in this example. Using this same logic, bone windows would use a "wide window" (to evaluate everything from fat-containing medullary bone that contains the marrow, to the dense cortical bone). 11

12. Artifacts Although CT is a relatively accurate test, it is liable to produce artifacts, such as the following: Aliasing Artifact or Streaks These appear as dark lines which radiate away from sharp corners. It occurs because it is impossible for the scanner to 'sample' or take enough projections of the object, which is usually metallic. It can also occur when an insufficient penetration of the x-ray occurs. Ring Artifact Probably the most common mechanical artifact, the image of one or many 'rings' appears within an image. This is due to a detector fault. Noise Artifact This appears as gaining on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the kV or mA is too low. 12

13. Artifacts (Cont..) Motion Artifact This is seen as blurring which is caused by patient movement. This is not so much a problem these days with faster scanning times in the use of MDCT(Multidetector computed tomography). Beam Hardening This can give a 'cupped appearance'. It occurs when there is more attenuation in the center of the object than around the edge. This is easily corrected by filtration. 13

14. Three dimensional (3D) Image Reconstruction Because contemporary CT scanners offer isotropic, or near isotropic resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner. 14

15. 3D rendering techniques Surface rendering : A threshold value of radio density is chosen by the operator (e.g. a level that corresponds to bone). A threshold level is set, using edge detection image processing algorithms. From this, a 3- dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various different thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. Volume rendering: Surface rendering is limited in that it will only display surfaces which meet a threshold density, and will only display the surface that is closest to the imaginary viewer. In volume rendering, transparency and colors are used to allow a better representation of the volume to be shown in a single image - e.g. the bones of the pelvis could be displayed as semi-transparent, so that even at an oblique angle, one part of the image does not conceal another. 15

16. Image segmentation Where different structures have similar radiodensity, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image. 16

17. 17 Advantages Better detail compared with ultrasonography. Relatively quick compared with MRI scanning. Most systems can be scanned - eg, brain to leg.

18. 18 Disadvantages Requires breath holding which some patients can not manage. Artifact is common - eg, metal clips. CT scans of the brain can be affected by bone nearby. High doses of radiation are involved in CT scanning - chest CT scan is equivalent to 350 chest X-rays; CT abdomen to 400 chest X-rays and CT pulmonary angiography 750 chest X-rays. There is also a risk of childhood cancer and leukaemia in mothers who have imaging during pregnancy.

19. 19 MRI Scan MRI scanning uses magnetism, radio waves, and a computer to produce images of body structures. MRI scanning is painless and does not involve x-ray radiation. Patients with heart pacemakers, metal implants, or metal chips or clips in or around the eyes cannot be scanned with MRI because of the effect of the magnet. Claustrophobic sensation can occur with MRI scanning.

20. 20 How does an MRI scan work? An MRI scanner contains two powerful magnets, which represent the most critical part of the equipment. The human body is largely made of water molecules, which each consists of smaller hydrogen and oxygen atoms. At the center of each atom lies an even smaller particle called a proton, which serves as a magnet and is sensitive to any magnetic field. Normally the water molecules in our bodies are randomly arranged, but upon entering an MRI scanner, the first magnet causes the body's water molecules to align in one direction, either north or south.

21. 21 How does an MRI scan work? (cont..)

22. 22 The second magnetic field is then turned on and off in a series of quick pulses, causing each hydrogen atom to alter their alignment and then quickly switch back to their original relaxed state when switched off. This creates a knocking sound inside the scanner and is a result of the gradient coils being switched on and off. When electricity is passed through the coil, a magnetic field is created and the coil vibrates, which accounts for the noise we hear. Although the patient cannot feel these changes, the scanner can detect them, and in conjunction with a computer, can create a detailed cross-sectional image for the radiologist. How does an MRI scan work? (cont..)

23. 23 Visualization  In CT scan you would use injectable contrast, or dyes to alter the X-ray intensity from different regions of the body.  The contrast used in MRI is fundamentally different.  MRI contrast works by altering the local magnetic field in the tissue being examined. Normal and abnormal tissue will respond differently to this slight alteration, giving us differing signals. These varied signals are transferred to the images, allowing us to visualize many different types of tissue abnormalities and disease processes better than we could without the contrast. This MRI scan shows the upper torso in side view so that the bones of the spine are evident

24. 24 What are MRI scans used for? Abnormalities of the brain and spinal cord Tumors, cysts, and other abnormalities in various parts of the body Injuries or abnormalities of the joints, such as back pain Certain types of heart problems Diseases of the liver and other abdominal organs Causes of pelvic pain in women (e.g. fibroids, endometriosis) Suspected uterine abnormalities in women undergoing evaluation for infertility.

25. 25 Advantages MRI is particularly useful for the scanning and detection of abnormalities in soft tissue structures in the body like the cartilage tissues and soft organs like the brain or the heart. There is no involvement of any kind of radiations in the MRI, so it is safe for the people who can be vulnerable to the effects of radiations such as pregnant women or babies. MRI scan can provide information about the blood circulation throughout the body and blood vessels and also enabling the detection of problems related to the blood circulation.

26. 26 Disadvantages MRI scan is done in an enclosed space, so the people who are claustrophobic, i.e. fearful of being in a closely enclosed surface, are facing problems with MRI to be done. MRI scans involve really loud noises while processing because they involve a really high amount of electric current supply. MRI scanners are usually expensive. Bone and calcium do not show up on an MRI scan. This means that tissue calcification, a feature of a number of diseases such as osteoporosis, cannot be detected using MRI scanning.

27. 27 fMRI Vs. MRI The MRI and fMRI differ from each other in a way that the MRI views the anatomical structure while the fMRI views the metabolic function. An MRI studies the water molecule’s hydrogen nuclei whereas an fMRI calculates the levels of oxygen. An MRI’s structural imaging views at a high resolution the differences between tissue types with respect to space. On the other hand, an fMRI’s functional imaging views the tissue differences with respect to time.

28. 28 The MRI has a high, spatial resolution while an fMRI has a long- distance, superior, temporal resolution. When talking about its technological advancements, the fMRI is still starting to build up its name unlike the MRI wherein it is already at its peak as one of the widely used equipment technologies in the medical world. The fMRI is yet to be introduced for diagnostic purposes and is only used in experiments unlike the revolutionary MRI. The fMRI is considered to be more expensive than the MRI because of the additional software and hardware required for it. fMRI Vs. MRI (Cont..)

29. Corresponding References CT scan Related Link: https://www.lf2.cuni.cz/files/page/files/2014/basic_principles_of_ct.pdf http://www.slideshare.net/likanpatra/computer-tomography-ct-scan?related=2 http://www.slideshare.net/drpramodkrishnan/ct-basics?related=1 https://www.ucdmc.ucdavis.edu/radiology/UCDHS_CT_FAQ_v1.pdf MRI Related Link: http://www.livescience.com/32282-how-does-an-mri-work.html http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1121941/ http://www.medicalnewstoday.com/articles/146309.php http://www.nps.org.au/medical-tests/medical-imaging/for-individuals/types-of- imaging/mri-magnetic-resonance-imaging/for-individuals/how-it-is-donehttp://www.nps.org.au/medical-tests/medical-imaging/for-individuals/types-of- imaging/mri-magnetic-resonance-imaging/for-individuals/how-it-is-done 29