Hardware & Software Flashcards
What is beam pitch and how is it calculated?
Beam pitch is the calculation of pitch taking into account MDCT and it calculated by beam pitch = movement/rotation/(numberxthickness)
What is the HU for air?
-1000HU
What is the HU for lung?
-500HU
What is the HU for fat?
-100 to +50
What is the HU for water?
0
What is the HU for CSF?
+15
What is the HU for blood?
+30 to +45
What is the HU for muscle?
+10 to +40
What is the HU for white matter?
+20 to +30
What is the HU for grey matter?
+37 to +45
What is the HU for liver?
+40 to +60
What is the HU for calcium
+200
What is the HU for metal
+1000
How does pixel size impact image quality?
Smaller pixels result in better spatial resolution
What is an algorithm/kernel?
A type of spatial filtration applied to a dataset
What is interpolation and what scanning mode is it associated with?
Interpolation is the averaging of data in between 180 degree rotations in spiral (helical) CT
What is window width and how does it affect an image?
Range of HU shown on a greyscale and it affects image contrast
Decreasing window width does what to an image?
Increases contrast
Increasing window width does what to an image?
Decreases contrast
What is window length and how does it affect an image?
Centre of the window width and it affects image brightness
Where should a window length be centred?
Approximately centred on the average attenuation of the anatomy of interest
How is pixel size calculated?
Pixel size = dFOV/matrix size
What are 2 commonly used matrix sizes and why are they used?
512x512 in conventional CT
1024x1024 Ultra high res
Define partial volume averaging
When a CT number in a voxel is representative of the average attenuation of 2 structures that lie within that voxel
Define reconstruction interval
Spacing between adjacent reconstruction slices
How and why do thin reconstruction slices affect an image?
Thinner reconstructions result in sharper images with more noise and increased spatial resolution as there are less photons contributing to the image reconstruction
What is MTF?
Modulation Transfer Function is the spatial frequency response of an imaging system converts contrast values into contrast intensities. It is the capacity of the detector to transfer the input signal at a given spatial frequency to its output
What does linearity refer to and what is the acceptable range?
Linearity is a CT QA term referring to the value of a CT HU/number being proportional to a known density. The CT number should be within 50 HU
What does uniformity refer to and what is the acceptable range?
Uniformity refers to the pixel values in a homogenous phantom over various regions. There should be no more than 8HU difference between peripheral and central regions
Define temporal resolution
The time taken for the tube to rotate 180 or 360 degrees
Define spatial resolution and what unit is it measured in?
The ability to distinguish small independent objects in close proximity to one another. It is measured in line pairs/mm
What parameters can be changed to improve spatial resolution?
Focal spot, sharper kernels, thinner reconstruction slices, smaller display field of view
Define contrast resolution
The ability to distinguish differences in intensities in an image
What has the biggest influence on contrast resolution
Noise
How can contrast resolution be increased on CT?
IV contrast
How is the beam filtered before passing through the patient?
Bowtie filter filters the peripheral edges of the beam which result in the beam being ‘hardened’ through the parts that will pass through the thinnest parts of the patient
What generation scanners are currently used?
Sixth
What are the advantages of MDCT over SSCT?
Minimise motion artifacts, decreased misregistration, reduced patient dose and improved spatial resolution
What factors does collimation influence?
Collimation can influence scan time, noise, contrast resolution and radiation dose efficiency
What is beam hardening?
Lower energy photons being absorbed more rapidly than high energy photons resulting in overall mean energy of the beam increasing and becoming ‘harder’
How can beam hardening be minimised?
Avoid scanning dense bony regions, position patient differently, tilt gantry (if possible), select appropriate scan range, use of bowtie filter, beam hardening correction, iMAR, DE
Describe cupping artefact and give an example
X-rays passing through cylindrical object are hardened and result in loss of detail of defining structure. May be present in CT brain and can be corrected with bone correction
Describe streak artefact and give an example
Dark bands and streaks occur when a portion of the beam passes through dense structures and is hardened in some positions. Examples are petrous temporal bones and contrast in the SVC
Describe partial volume artefact
Portions of objects are averaged together in a slice resulting in decreased spatial resolution and inaccurate signal intensity
Describe the appearance of motion artefact and list some examples
Motion artefact appears as double lines of cortical bone, lung, skin edge. Examples include: breathing, cardiac motion, swallowing, peristalsis, movement
What artefact does a calibration error result in?
Ring artefacts
How can metal artefacts be reduced?
iMAR, DE, repositioning of anatomy if possible
Zebra artefacts are caused by what?
Spaces between measured data during helical interpolation process
What is aliasing artefact the result of and why?
Undersampling due to large intervals between projections resulting in misregistration of information relating to sharp edges and small objects
How does shaded surface display result in a 3D image?
User selects a threshold range to select only a specific tissue type to be rendered, as rays are projected through the volume they stop when the first ‘on’ voxel is identified which is used to create the surface
What does a volume rendered image display?
The entire volume with control over opacity or translucency of selected tissue types
What is a MIP?
Maximum Intensity Projection. Projects the highest attenuation value in the z axis on a preselected range of data through a volume onto a 2D image
Which vendor uses a fixed detector array?
GE
Which vendors use adaptive detector array?
Siemens & Philips
Which vendors use mixed detector array?
Toshiba
What is a fixed detector array?
All elements are the same size
What is an adaptive detector array?
Elements symmetrically increase in size moving towards the periphery of the array
What is a mixed detector array?
Four small equal sized elements in the center of the array and larger equal sized elements making up the rest of the array
