BASICS AND PRINCIPLES OF CT SCAN and windowing.pptx

basics of ct scan, windowing, hounsfield and contrast

1. BASICS AND PRINCIPLES OF CT SCAN DR. GANESH SAI TEJA JR2

2. RADIOGRAPHIC DENSITIES The whiteness or blackness of an xray image depends on the density of the tissue between the emitter and the film More the density , whiter the image Same principles apply for ct , denser the tissue ,whiter the image.

3. HOUNSFIELD UNITS Each part of the body is divided into many voxels, the density of each voxel is calculated and assigned a number called a Hounsfield unit By convention, pure water has HU of 0 and air has HU of negative 1000 Rest of the tissues have a HU in between negative to positive 1000

4. FAT STRANDING Fat stranding  is a sign that is seen on CT. It describes the change in attenuation of fat around an inflamed structure and is a very helpful signpost for intra-abdominal pathology A lmost synonymous with inflammation on a CT exam

5. CONVENTIONS axial CT scan, the convention for  left and right  is established by imagining that you are looking at the scan while standing at the feet of the patient  your left is the patient's right side In coronal view, we imagine the patient facing us, so our left is the scan’s right

6. WINDOWING Many of the tissues in our body have similar densities, so our eyes couldn’t differentiate minor variations in the change in shades of grey Windowing helps distribute all the shades into the preset window Window width refers to the number of different HU units that will be represented by different shades of gray. The window level (WL), often also referred to as window centre , is the midpoint of the range of the CT numbers displayed. When the window level is decreased the CT image will be brighter and vice versa. The  Window Level (WL)  refers to the window centre or midpoint HU value that is represented on the window setting. .

7. IV CONTRAST

8. Arterial phase is used to evaluate arterial problems such as aortic diussection or pulmonary thromboembolism Portal venous phase is used for most of the abdominal organs Delayed phase is used for evaluation of the collecting system

9. GANTRY A Slip ring functions to allow the transfer of electrical information and power between a rotating device and external components. A slip-ring  enables continuous rotation of the CT scanner gantry without progressive twisting of cables as the scanner rotates. It brushes on the rotating gantry , through contact with the stationary ring, allows power to be supplied to the gantry and the signal to be passed to the computer. The gantry can be tilted  up to 30 degrees in the vertical axis ( cranio -caudal)

10. CT SCAN PRINCIPLE The basic principle of CT is that internal structure of an object can be reconstructed from multiple projections of the object. The CT scan machine uses x-rays to generate cross sectional, 2-D images of the body. These images are acquired by rotation of the x-ray tube around the patient . A series of x-ray images are taken from different angles around the body.

11. The transmitted radiation is measured by detectors. The detectors measure this transmitted radiation and produce electrical impulses according to the intensity of the x-ray.

12. BASIC CONCEPT A thin cross section of particular body part is examined from multiple angles with a pencil like x-ray beam. The transmitted radiation is counted by a detector. It is fed into a computer for analysis by a mathematical algorithm and reconstructed as a tomographic image.

13. Goal of CT is to calculate attenuation Attenuation is a result of absorption and scattering.

14. MATRIX, PIXEL AND VOXEL MATRIX: CT image is represented as the matrix of the number. A 2-D array of numbers arranged in the rows and columns is called matrix. Each number represents the value of the image at that location. 

15. 2 . PIXEL It is a 2-D picture element. Matrix consists of pixels. Each square is called as pixel. Each pixel represents a CT number. The pixel represents the x-ray attenuation of the related voxel.

16. 3.VOXEL It is known as the volume element. The image matrix represents a 3-D volume element called a voxel. The voxel represent a 2-D element called a pixel. The size of voxel depends on the matrix size, the selected field of view (FOV) and slice thickness

17. GENERATIONS OF CT As the generation of ct progresses, the number of detectors and emittors increase. 4 th generation is the most commonly used ct machine 5 th generation is used for cardiac imaging Increased generation gives rapid imaging and gives better resolution

18. MULTI SLICE SCANNING Rather than just have one row of detectors, we now have  multiple parallel rows  of detectors. Different combinations of the detectors rows  can then be selected to gives different possibilities of slice thickness (width).

19. Advantages of CT • Quick and painless Costs less than MRI • Better availability compared to MRI • Shows up acute bleed • Good visualization of bony structures and calcified lesions • Can detect or exclude the presence of more serious problems

20. Disadvantages of CT • Exposure to ionising radiation • Resolution • Injection of a contrast medium (dye) can cause kidney problems or result in allergic or injection-site reactions in some people Some procedures require anaesthesia

21. Thank you