CT

Question 1

definition of CT

Answer 1

cross sectional imaging modality that combines series of x ray images that are acquired in a rotational format around the body forming slices of blood vessels, bones and soft tissues

Question 2

key words / key features

Answer 2

• hounsfield unit
• ionising radiation
• multi slice
• CT
• axial
• coronal
• sagittal
• bastion protocol

Question 3

what do we use CT for

Answer 3

• diagnose muscle and bone disorders, such as tumours and fractures
• pinpoint the location of a tumour, infection or blood clot
• guid procedures such as surgery, biopsy and radiation therapy
• detect and monitor diseases
• visualise blood and blood vessels
• forensic investigation

Question 4

how does CT work

Answer 4

• X ray tube and detector are opposite sides of the gantry to one another, these will rotate around the patient in a synchronous manner
• x ray tube emits photons with a typical energy level between 20-150 keV
• the beam will image a slice of the body in each of its acquisitions, whilst rotating
• these photons will interact with matter ( our patient )
• the photons that have [assed through the patient will reach the detector
• detector will measure the number of photons that have been attenuated ( photoelectric absorption / Compton scatter ) by the patient, at different angles
• this acquisition from multiple angles allows the imaging in the third dimension
• these attenuation measurements are used to build a 3D representation of the area scanned using complex mathematical algorithms
• the amount of attenuation within the patients body is ultimatley determined by the density of their tissues
  
  • high density tissue absorbs radiation to a greater degree, therefore a reduced quantity is detected by the scanner on the exiting side of the body
  • low density tissue absorbs radiation to a lesser degree, therefore a greater signal is detected by the scanner
  • each voxel is assigned a hounsfield unit ( or CT number )

Question 5

hounsfield units

Answer 5

• quantify the attenuation coefficients of tissues within the bidy
• high attenuation co efficient = high absorption = white on image = high hounsfield unit
• low attenuation co efficient = lower absorption = dark on image = low hounsfield unit
• hounsfield units are measured and reported in a veriety of clinical applications
  
  • evaluating the fat content of the liver
  • assessing bone mineral density
  • predicting the presence of anaemia
  • managment of kidney stones

Question 6

windowing

Answer 6

• process in which CT greyscale images can be manipulated via the hounsfield units each voxel possesses
• windowing can allow certain structures to appear highlighted, brighter and more distinguishable from other structures
• the brightness of the image is adjusted via the window level
• the contrast is adjusted via the window width

Question 7

window width

Answer 7

measure of range of HU that an image contains
WW can be wide and is best used in areas where there is an acute difference is attenuation values i.e. cortical tissue where air and blood vessels sit side by side
WW can also be narrow and are useful when examining areas of similar attenuation i.e. soft tissues

Question 8

window length

Answer 8

midpoint of the range of the HU in an image
when the WL is decreased the CT Images will be brighter and vice versa

Question 9

clincical windowing figures - head and neck

Answer 9

• brain W:80, L:40
• subdural W:130-300, L:50-100
• stroke W:8, L:32 or W: 40, L:40
• temporal bones W:2800, L:600 or W:4000, L:700
• soft tissues W:350-400, L: 20-60

Question 10

clinical windowing figures - chest

Answer 10

• lungs W:1500, L:-600
• mediastinum W: 350, L: 50

Question 11

clinical windowing figures - abdomen

Answer 11

• soft tissues W: 400, L:50
• liver W:150, L:30

Question 12

clinical windowing figures - spine

Answer 12

• soft tissues W:250, L:50
• bone W: 1800, L:400

Question 13

image reconstruction

Answer 13

• MPR ( multiplanar reformatting )
  
  • converting data from a given plane, usually axial, and reconstructed into another plane i.e. coronal, sagittal, etc
• MIP ( maximum intensity projection )
  
  • projects the voxel with the highest attenuation value on every view throughout 3D volume onto a 2D image
  • this is useful for displaying bone, contrast filled structures and pulmonary/carotid vessels
• 3D
  
  • involves using software to create a 3D model of our patient from all the 2D slices we acquired

Question 14

MPR ( multiplanar reformating )

Answer 14

• MPR ( multiplanar reformatting )
  
  • converting data from a given plane, usually axial, and reconstructed into another plane i.e. coronal, sagittal, etc

Question 15

MIP ( maxiumum intensity projection )

Answer 15

• MIP ( maximum intensity projection )
  
  • projects the voxel with the highest attenuation value on every view throughout 3D volume onto a 2D image
  • this is useful for displaying bone, contrast filled structures and pulmonary/carotid vessels

Question 16

3D

Answer 16

• 3D
  
  • involves using software to create a 3D model of our patient from all the 2D slices we acquired

Question 17

contrast enhanced imaging

Answer 17

• CT can be performed with contrast media - a prescribed drug used to better visualise structures or aid identification of pathology within the body
• usually iodine based but can be barium based
• delivered intravenously, orally or via an enema
• safety

Question 18

advantages and disadvantges of contrast enhanced imaging

Answer 18

• advantages = more clinical info provided, could mean better diagnosis
• disadvantages = anaphylaxis, extravasation, renal dysfunction