PET/CT exam Questions and Answers
Graded A+
Explain the difference between a pixel and a voxel - Correct answer-A pixel is a
2D unit of measurement while voxel is 3D
What is a CT number/ HU - Correct answer-CT number/HU: a relative quantitative
measurement of radio density, interpreted by a greyscale image
What filter is used in CT and why? - Correct answer-Bow tie filter, higher fluency
and energy density of x-rays at edges of FOV, the filter causes more attenuation in
the periphery so that a uniform signal results. Reduces patient dose w no loss of
image quality.
What do Pre patient collimators do? - Correct answer-controls patient dose, shapes
beam
what is post patient collimation? - Correct answer-removes scattered radiation,
improve image quality
Over beaming - Correct answer-beam produced is wider than the detector
©COPYRIGHT 2025, ALL RIGHTS RESERVED 1
,over ranging - Correct answer-patient exposed beyond length of scan, increases
patient dose
What do CT detectors do? - Correct answer-captures x-rays and converts x-ray
photons to digital data
What is SFOV (scan field of view) - Correct answer-Determines area from which
raw data is acquired around isocentre middle of gantry. anything outside SFOV not
imaged
What is DFOV? - Correct answer-How much raw data is used to create image, can
choose target body part can be = or less SFOV
Relationship between DFOV, metric and pixel size - Correct answer-· Pixel size
(mm) = DFOV/
matrix
what are reconstruction kernels and how to they affect the image? - Correct
answer-soft tissue - smoothing kernel
Bone kernel - sharpening kernel
corrects image by reducing blurring/ sharpens
Window Width WW - Correct answer-The range of HU values that an image
contains
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
, Window Level WL - Correct answer-centre of range of greyscale midpoint
Helical scanning interpolation - Correct answer-Approximating data values to
make axial slices, helical data is interpolated to make a full dataset. use of
projection data from outside the reconstructed volume leads to reduction in axial
resolution.
What is Pitch - Correct answer-Table distance travelled in one 360 degree gantry
rotation
Pitch in Helical MSCT - Correct answer-Wider beams of MSCT accentuate cone
beam shape and lead to rays that do not even lie within same plane.
Pitch = T/D
Pitch in SSCT - Correct answer-Pitch = T/W, movement per rotation.
Pitch and dose relationship when increasing pitch - Correct answer-decreases dose
= decreases image quality
Pitch and dose relationship when decreasing pitch - Correct answer-increases dose,
increases image quality
5 physical factors affecting image quality - Correct answer-Contrast, Spatial,
Noise, Artefacts, Temporal resolution
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
Graded A+
Explain the difference between a pixel and a voxel - Correct answer-A pixel is a
2D unit of measurement while voxel is 3D
What is a CT number/ HU - Correct answer-CT number/HU: a relative quantitative
measurement of radio density, interpreted by a greyscale image
What filter is used in CT and why? - Correct answer-Bow tie filter, higher fluency
and energy density of x-rays at edges of FOV, the filter causes more attenuation in
the periphery so that a uniform signal results. Reduces patient dose w no loss of
image quality.
What do Pre patient collimators do? - Correct answer-controls patient dose, shapes
beam
what is post patient collimation? - Correct answer-removes scattered radiation,
improve image quality
Over beaming - Correct answer-beam produced is wider than the detector
©COPYRIGHT 2025, ALL RIGHTS RESERVED 1
,over ranging - Correct answer-patient exposed beyond length of scan, increases
patient dose
What do CT detectors do? - Correct answer-captures x-rays and converts x-ray
photons to digital data
What is SFOV (scan field of view) - Correct answer-Determines area from which
raw data is acquired around isocentre middle of gantry. anything outside SFOV not
imaged
What is DFOV? - Correct answer-How much raw data is used to create image, can
choose target body part can be = or less SFOV
Relationship between DFOV, metric and pixel size - Correct answer-· Pixel size
(mm) = DFOV/
matrix
what are reconstruction kernels and how to they affect the image? - Correct
answer-soft tissue - smoothing kernel
Bone kernel - sharpening kernel
corrects image by reducing blurring/ sharpens
Window Width WW - Correct answer-The range of HU values that an image
contains
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
, Window Level WL - Correct answer-centre of range of greyscale midpoint
Helical scanning interpolation - Correct answer-Approximating data values to
make axial slices, helical data is interpolated to make a full dataset. use of
projection data from outside the reconstructed volume leads to reduction in axial
resolution.
What is Pitch - Correct answer-Table distance travelled in one 360 degree gantry
rotation
Pitch in Helical MSCT - Correct answer-Wider beams of MSCT accentuate cone
beam shape and lead to rays that do not even lie within same plane.
Pitch = T/D
Pitch in SSCT - Correct answer-Pitch = T/W, movement per rotation.
Pitch and dose relationship when increasing pitch - Correct answer-decreases dose
= decreases image quality
Pitch and dose relationship when decreasing pitch - Correct answer-increases dose,
increases image quality
5 physical factors affecting image quality - Correct answer-Contrast, Spatial,
Noise, Artefacts, Temporal resolution
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
