CT physics 4

Question 1

what is the modern scanner design

Answer 1

a continuously rotating fan beam - third gen scanner design

Question 2

what are 3rd gen CT scanners

Answer 2

fixed X-ray source and curved array of detectors which reduces the mechanical complexity and improves image quality
- improves intrinsic collimation so stops x-rays being detected by incorrect detector channel

Question 3

typical scan time for 3rd gen scanner

Answer 3

scan time (rotation time) reduced to less than one second per 360 degree rotation

Question 4

what does the hounsfield unit represent

Answer 4

the density of different tissues in the body
- also used to assess contrast in CT scan

Question 5

what happens when an image in reconstructed in axial slice

Answer 5

made up in matrix of pixels each assigned a HU value which is determined by attenuation of various tissues relative to the attenuation of waterw

Question 6

what is the HU values given to tissues in the body relative to?

Answer 6

the attenuation of water

Question 7

what HU value is water given

Answer 7

0 HU

Question 8

what HU value is air given

Answer 8

-1000 HU

Question 9

average attenuation value of a bone

Answer 9

average value of 1000 HU
- tissues attenuate more than water

Question 10

filtered back projection

Answer 10

• images are ‘filtered’ to enhance certain features and remove noise (improve quality)
• back projection - images are ‘back projected’ onto single image involving taking the filtered images and projecting them back into a common space - stacked together to make a 3D image

Question 11

what is iterative reconstruction

Answer 11

• used to create clearer and more detailed images
• works more slowly
• starts with initial estimate of image then iteratively refines that estimate to match acquired data
• helps reduce noise, errors, and improve image quality of final picture

Question 11

advantages of IR

Answer 11

• noise reduction without degraded resolution
• improved spatial resolution + artefact reduction
• dose reduction + improved resolution
• lower mA or kV compared to when using filtered back projection

Question 11

disadvantages of IR

Answer 11

• high computation demands - slower reconstruction technique
• clinicians reported waxy nature to images - normally used to seeing certain amount of noise in CT images

Question 12

what happens in a helical CT scan

Answer 12

x-ray source and detectors continuously rotate around patient while patient is moved around the scanner creating a path of data acquisition
- no defined slice

Question 13

what kind of attenuation data does 360 degree interpolation use

Answer 13

uses a combination of attenuation data from multiple angles around the object being scanned
- raw projection data
- sinogram data
- filtered data
- interpolated data

Question 14

how many rotations is data required from

Answer 14

2 FULL rotations
- makes image width broader causing z-axis blurring

Question 15

180 degree linear interpolation simplified

Answer 15

• two data points and having to estimate a value at a point between them (point C)
• linear interpolation means finding value of point C

Question 16

what can interpolation be used for

Answer 16

• smooth transitions between pixels can help reduce jagged edges + improve visual quality of images
• can be used to estimate pixel values at new locations

Question 17

what does slice profiles in CT refer to

Answer 17

• used to create a cross-sectional image of patients anatomy

Question 18

when would you use overlapping reconstructed axial image slices in helical CT

Answer 18

• reduces stair step artifact and improves MPRs and 3D image reconstructions
• depending on acquiring of volume of data you can choose

Question 19

what does z-axis collimation determine

Answer 19

acquired slice thickness during each x-ray tube / detector rotation

Question 20

what does an increase in pitch lead to

Answer 20

reduction in z-axis resolution

Question 21

advantages of helical CT

Answer 21

• faster scan times due to acquiring volume of data instead of single slice
• better spatial + contrast resolution
• motion artefacts are reduced

Question 22

disadvantages of helical CT

Answer 22

• increased radiation dose
• motion artefacts
• complexity and cost

Question 23

single slice interpolation

Answer 23

2 nearest values used

Question 24

multislice scanners

Answer 24

• more data points available
• use all measurements within variable filter width for interpolation