1
Q
what are the sequences of events in CT
A
- tube and detectors rotation around the patient
- attenuation (loss of x-ray energy as it gets absorbed) of radiation as it passes through the patient and remnant beam measured by detectors
- beam converted into electrical signals followed by amplification and digitisation
- data sent to computers -> data processed
- data displayed, manipulated and recorded
2
Q
how does the computer construct image with mathematics formulae?
A
the computer calculates the CT number of each voxel using mathematics formulae
3
Q
what is basic assumption of CT image reconstruction?
A
that the imaged object is completely stationary during the course of scan
4
Q
what is the evolution of CT technology?
A
- 1st generation scanners
- 2nd generation scanners
- 3rd generation scanners
- 4th generation scanners
- 5th generation scanners (EBCT)
- Multi-slice technology
5
Q
which generation is commonly used? what are its advantages and limitations?
A
third generation
advantage - lower dose for patient, quicker
disadvantage - can miss something
6
Q
the multi-slice CT is based on which generation and what are the new modifications?
A
third generation and the only difference is it has multiple detectors at the bottom and covers a lot of area for patient due to diverging x rays
7
Q
what are the components of a multi-slice CT?
A
- collimator beam width
- isocenter
- detector width
8
Q
what are three principal system components of CT?
A
- Gantry
- Computer
- operating console
9
Q
what does the gantry consists of?
A
consists of X-ray tube, filter, collimator, detector array
aperture (controlled by filter and collimator)
10
Q
what is the aperture of gantry and why is it designed like that?
A
- Aperture - max 90cm diameter
1. to allow max gantry angulation
2. reduce patient anxiety due to claustrophobia - can be angled +/- 30 degrees on average (to avoid development of cataracts)
11
Q
what is the maximum patient size the table can hold?
A
around 22 stone, patients above 22 stones can be placed on table but it will not move in right manner
12
Q
why is gantry angled with +/-30 degrees?
A
to avoid development of cataracts
13
Q
what are the characteristics of X-ray tube in CT?
A
x-ray production similar to general radiography however
- rapid sequential exposures required - resulting in excess heat production
- large diameter of anode discs (20cm) - layered with alloy (rhenium) and thick graphite backing to aid heat sink
- modern units with pulsed beams of voltage (80-140kV) - to reduce heat loadings
- scanners have either liquid (oil) or air-cooled tube housing designs
14
Q
what is the life expectancy of CT X-ray tube?
A
1 year or so
15
Q
what are advantages of slips rings?
A
- allows faster conventional axial scanning
2. allows spiral scanning - continuous table feed
16
Q
what is acquisition with respect to spiral/helical scanning?
A
the process through which a single continuous set of spiral scan data is acquired with an intervening phase
17
Q
what is interpolation with respect to spiral/helical scanning?
A
modification of scan data before image reconstruction to produce clinically acceptable cross section anatomy
18
Q
why is interpolation/reconstruction of image crucial?
A
- to improve pixelate images
2. to produce clinically acceptable cross section anatomical images
19
Q
what is collimator and collimation?
A
Collimator - placed between the filter and patient
Collimation - it is adjusting the path length between x ray exposure and detector panel to obtain good quality of images
20
Q
what type of collimation is used in CT and why?
A
double collimation of radiation beam
- control patient dose - at tube and pre patient
- control slice thickness - post patient; restrict scattering from outside of the slice
21
Q
what is the range of collimation and how is it controlled ?
A
variable from 1.25mm to 80cm and it is controlled by a program software
22
Q
what is the detector array made up of?
A
- made from ultra fast ceramic material (scintillators) of solid state type
- they respond to x-ray exposures by emitting pulses of visible light in proportion to the absorbed energy
23
Q
why is the efficiency of detecter array crucial?
A
to determine tube loading and control patient dose
24
Q
why do detector arrays have high sensitivity?
A
they have high sensitivity for best low (optimum) contrast resolution
25
what are the feature of detector array?
1. high detection efficiency for x-rays in CT (reduce heat loading of X-ray tube)
2. High dynamic range - multiple detectors
3. Narrow gaps between active elements (good geometrical efficiency) - not tightly packed
4. Fast response
5. Low cost
6. Small physical size
26
why the detectors are not tightly packed?
there is interspace between them to reduce cross talks. (unwanted transfer of signals b/w detectors) and improve image quality therefore, resulting in reduced detection efficiency and increases patient dose (doubtful)
27
why multiple detector arrays are used?
to allow the collection fo two or more data image sets simultaneously
28
what are the two types of detector arrays?
1. fixed - 2.5mm width throughout
| 2. adaptive - lesser in the centre (1.25mm) and thicker towards the periphery (5mm)
29
a. what is the incident intensity with respect to beam attenuation in CT
b. what is it directly proportional to and its impact on tissue density?
- the intensity of X-ray beam striking the patient is called incident intensity
- it is directly promotional to tube output (mAs)
- the beam attenuated by the patient is dependent on the different tissue densities
30
how does the tissue density vary within a patient?
the thickness changes when you go from front to back and side to side
31
how is attenuated radiation measured?
attenuated radiation is measured by detectors situated opposite the X-ray tube
32
what are the characteristics of an ideal CT detector?
1. high detection efficiency - absorb all the emergent x-ray beam's energy to which they are exposed
2. high conversion efficiency - enables all the detected energy to contribute to image formation
3. wide dynamic range - to enable all x-ray intensities from weakest to strongest are converted into proportional output signals
33
what happens to light emitted by the detector?
the light is converted into electrical signals by photodiodes coupled to scintillators
34
what are the factors for measuring attenuation?
1. transmitted intensity
| 2. attenuation coefficient
35
what is transmitted intensity?
- the intensity of x-ray beam after passing through the patient is called transmitted intensity
- it is inversely proportional to the attenuation of x-ray beam
36
what is attenuation coefficient (µ)?
- it is the numerical expression of the decrease in intensity of x-rays after penetrating into matter
- i.e. the % of radiation remaining after the x-rays pass through the object
37
what is the change in intensity of attenuation used for?
- the change in intensity can be computed to produce an image section ('slice') of the scanned object
- The change in intensity is referred to x-rays before penetrating the body, post the absorbance and transmission. therefore, it is basically the remaining the x-rays that have been transmitted through the body and hit the detector panel to produce the image
38
what are voxels?
voxels are small volumes of tissues in a criss cross matrix of the body collected from hundreds of readings from the CT detectors
39
how do you calculate the resultant data for the attenuation?
resultant data (transmitted intensity) = incident beam intensity signal - attenuated signal from detector =
40
what are the three main steps in image construction?
1. pre-processing
2. convolution
3. back-projection
41
what is pre-processing? outline the steps involved
- raw data is produced by processing scan data
- the steps involved are
1. to undertake a series of dose measurement
2. dose measurement
3. raw data
42
what is raw data?
the information created by measuring the position of tube and detectors array and measure the voxels the beam has passed though
43
what is the fourier transformation?
- primary mathematical model to create computerised medical images
- it is the conversion of data to more useful recognisable form using mathematical algorithms
44
what is convolution?
Algorithm (mathematical formula) applied to raw data to provide a sharp or smooth image to enhance the appearance of the tissue of interest (determined by protocol selection prior to scanning the patient)
45
what is back projection?
- A complicated process in which the convolved scan data is projected back into the image matrix
- the process used to reconstruct image
46
what is hounsfield scale and what are its features
- named after sir Godfrey hounsfield
- each pixel is assigned a numerical value (CT number)
- this number is the average of all attenuation clues with the corresponding value
- this number is then compared to the attenuation value of water (i.e. 0 HU)
- the numbers are displayed on a scale called housnfield units (HU)
47
what does each number represent on a hounsfield scale?
each number resents a shade of grey
+1000 = white
-1000 = black
48
what is HU for
1. dense bone
2. water
3. air
1. dense bone = +1000HU (white)
2. water = 0HU
3. air = -1000HU (white)
49
what is the range of CT numbers for HU?
The range of CT numbers is 2000HU wide
50
why only a limited a number of HU are displayed despite of having 2000HU wide scale?
because the human eye cannot distinguish between 200 different shades of grey
51
how is the clinical grey scale set and what are the parameters used to set it?
- depending on the type of tissue being scanned
| - window level (brightness) and window depth (contrast)
52
what is mutiplanar reformatting (MPR)?
the original cross sectional images are used to produce image in another plane (coronal/sagital)
53
what is maximum intensity projection (MIP) ?
selected voxels along a row/column (e.g. blood vessels) in a volume of interest with specific CT numbers are separated
54
in which of brain/head disease/disorder is CT used?
1. Trauma (Traumatic brain injury - TBI)
2. Intra-cerebral haemorrhage (ICH)
3. Cerebro vascular accident (CVA) - stroke
4. infection/inflammatory disease - meningitis (ALS?)
5. Tumours (malignant and benign)/metastases
(CTA can be used for all them)
55
what is the Glasgow coma score (GCS)?
- the point system for responding to range of stimuli that includes eye opening, motor and verbal response
- the sum total of these areas/section are calculated and if the value is below a certain threshold (e.g. 11-13) then they will have a CT scan
- score of 3-2 indicates they have passed away or are dead
56
What does the following GCS score indicate?
1. 15 (maximum score)
2. 13-14
3. 9-12
4. 8 or less
5. 3 (lowest score)
1. 15 (maximum score) - normal consciousness
2. 13-14 - mild TBI + 15% patients having symptoms unto 1 year after injury
3. 9-12 - moderate injury + mortality rate of 2-3%
4. 8 or less - severe head injury + mortality rate upto 36%
5. 3 (lowest score) - probability of death or persistent vegetative state
57
what is the shape of extra dural and subdural haemorrhage?
extradural (meaning outside the dura) is biconvex (lenticular) shaped resulting in excess pressure on the brain tissue
subdural (meaning between the dura) is crescent shaped resulting in the shift of structures inside
58
what are the characteristics of intra cranial haemorrhage?
1. the fresh blood appears white on the CT due to high density
2. after 2-3 weeks, haematoma appears on CT with a similar density of brain
3. beyond this time, it turns less dense until it appears the same as CSF
- area without sulci represents pathology (gyrus present)
59
what is haematoma?
it is the bleeding outside the blood vessels due to a disease or trauma (surgery/injury) and may include blood leaking from from capillaries
60
what does it mean if the blood appears the following on the CT
1. dark
2. bright
1. dark - not fresh/active blood
| 2. bright - fresh blood
61
why is pre- and post- contrast done in a CT for brain tumour?
it is to highlight the tumour
62
what are the types of brain tumour?
1. glioma
| 2. meningioma
63
in which cases is CTA used?
CTA - computed tomography angiography; used in
1. CT pulmonary angiography
2. CT cardiac - catheters inserted in heart vessels which has high risk so CT helps to determine whether the intervention is needed
3. CTA - circle of wills (artery and venous system joined together)
4. CTA - trauma
64
under what conditions should whole body CT be carried out and not be carried out?
1. it should be carried out for:
- people aged 16 or over with blunt major trauma and suspected multiple injuries
2. it should not be carried out for:
- to image whole body CT for people under 16.
- use clinical judgement to limit CT to body areas where assessment is needed
65
what are the guidelines to be followed while doing a whole body CT in people aged 16 or over?
1. patients should not be repositioned during whole body CT
| 2. use clinical findings and scanogram to direct CT of limbs in people aged 16 or over with limb trauma
66
what is ROI in whole body CT
consists of vertex to toes scanogram followed by CT from vertex to mid-thigh
67
what does solid state detector consist of?
1. scintillator layer - converts x-rays into visible light photons
2. photodiode - coverts photons into electrons and thus, electrical signal
68
what are advantages of multiple slice scanning?
1. Faster scanning due to wider total active detector width
2. Better dynamic imaging due to faster scanning times
3. Thinner slices
4. 3D imaging is enabled by thin slices
5. Simultaneous acquisition of multiple slices
69
what are the two types of scanning used in CT and explain them?
1. axial - scanning of each slices individually
| 2. spiral - continuous movement and scanning of all the slice together
70
why spiral scanning is used as opposed to axial scanning in CT?
it is the continuous movement of the table and the patient through the gantry.
1. scan performed in one breath so avoids respiratory misregistration. The entire volume of tissue to be scanned in one acquisition.
2. faster scanning - allows effective use of contrast agents since multiple phases can be scanned with one contrast injection
3. overlapping slices - allow better reconstruction of images and show smaller lesions
71
what is pitch?
- Pitch is the measure of overlap during scanning
| - it is the ratio of distance travelled by couch and width of slice
72
what does the following value of pitch indicate?
1. pitch number > 1
2. pitch number < 1
1. pitch number > 1 - the couch travels more than the width of beam (slice) = GAPS
2. pitch number < 1 - the couch travels less than the width of the beam = OVERLAPS
73
what is the effect of increasing pitch value?
```
higher pitch value means
Advantages
1. faster scanning
2. low radiation dose
disadvantages
1. more sparsely sample
```
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