Computed Tomography (Answered)
225 Questions and Correct Answers.
Updated 2025/2026. 100% Verified.
For a typical 64 to 128 detector array CT scanner, the fan angle is approximately ___ degrees, while
the full cone angle is about ___ degrees
60, 2.4
CT Fan beam projection diagram
CT Cone beam projection diagram
CT True cone beam projection diagram
Why is an arc-shaped detector beneficial in CT?
By mounting the detector modules on a support structure that is aligned along the radius of
curvature emanating from the x-ray source, there is very little difference in fluence to the detectors
due to the inverse square law. Very importantly, the primary x-rays strike the detector elements in a
nearly normal manner, and so there are effectively no lateral positioning errors due to x0ray beam
parallax
Features of true cone beam systems
Cone angle nearly as great as the fan angle. Planar flat detector with a 2D bank of detectors.
Correction methods needed for different distances from source to detector ,giving rise to inverse
square and hell effect-based differences in fluence to deach dexel. Some parallax.
Some uses of true cone beam imaging
Dental/maxillofacial CT cystems, breast CT systems, radiation therapy
,Typical true cone beam specifications
30x40cm flat panel detector, SSD 90 cm, 25 deg fan angle and 19 degree cone angle
General CT Geometry Diagram
Third Generation (Rotate-Rotate) CT Geometry
Magnification factor at the CT scanner's isocenter
M - B/A where B is the SID and A is the source to isocenter distance
Detector dimensions in CT are generally referred to their size as projected to the _______
isocenter
The maximum FOV of a CT is defined by ______
The physical extent of the curved detector arrays (the fan angle)
If the (minimum) CT slice thickness is quoted on a scanner, the actual detector width is _______
Larger by magnification factor M = B/A. That is, for a 0.5 mm min slice thickness, and B = 95 and A =
50, the actual detector width is 0.95 mm. That is why most dimensions in CT are related to the plane
of the isocenter
Do CT scanners use 2D antiscatter grids?
Sometimes
CT antiscatter grid diagram
CT Tube Orientation Diagram
, CT Beam Profile Diagram
Typical CT gantry rotation speed
Approaching 5 rotation per second (0.20 s rotation time)
Why is the plane of the anode disk parallel to the plane of gantry rotation?
To reduce gyroscopic effects that would add significant torque to the rotating anode if mounted
otherwise. Also, the anode-cathode axis (heel effect) runs parallel to the z-axis of the scatter,
eliminating heel effect-induced spectral changes along the typically wide fan angle. The fan beam is
much more limited in the anode-cathode angle
Approximate fan angle of most CT scanners
60 degrees
CT fan angle on anode diagram
CT cone angle on anode diagram
Most modern CT scanners use a (continuous/pulsed) x-ray source
continuous
The detector array sampling time in effect becomes the acquisition interval for each CT projection
that is acquired, and the sampling dwell times typically run between ______ ms, meaning that
between _______ projections are acquired per 360 degree rotation for a 0.5s gantry rotation period
0.2 and 0.5, 1000 and 3000
225 Questions and Correct Answers.
Updated 2025/2026. 100% Verified.
For a typical 64 to 128 detector array CT scanner, the fan angle is approximately ___ degrees, while
the full cone angle is about ___ degrees
60, 2.4
CT Fan beam projection diagram
CT Cone beam projection diagram
CT True cone beam projection diagram
Why is an arc-shaped detector beneficial in CT?
By mounting the detector modules on a support structure that is aligned along the radius of
curvature emanating from the x-ray source, there is very little difference in fluence to the detectors
due to the inverse square law. Very importantly, the primary x-rays strike the detector elements in a
nearly normal manner, and so there are effectively no lateral positioning errors due to x0ray beam
parallax
Features of true cone beam systems
Cone angle nearly as great as the fan angle. Planar flat detector with a 2D bank of detectors.
Correction methods needed for different distances from source to detector ,giving rise to inverse
square and hell effect-based differences in fluence to deach dexel. Some parallax.
Some uses of true cone beam imaging
Dental/maxillofacial CT cystems, breast CT systems, radiation therapy
,Typical true cone beam specifications
30x40cm flat panel detector, SSD 90 cm, 25 deg fan angle and 19 degree cone angle
General CT Geometry Diagram
Third Generation (Rotate-Rotate) CT Geometry
Magnification factor at the CT scanner's isocenter
M - B/A where B is the SID and A is the source to isocenter distance
Detector dimensions in CT are generally referred to their size as projected to the _______
isocenter
The maximum FOV of a CT is defined by ______
The physical extent of the curved detector arrays (the fan angle)
If the (minimum) CT slice thickness is quoted on a scanner, the actual detector width is _______
Larger by magnification factor M = B/A. That is, for a 0.5 mm min slice thickness, and B = 95 and A =
50, the actual detector width is 0.95 mm. That is why most dimensions in CT are related to the plane
of the isocenter
Do CT scanners use 2D antiscatter grids?
Sometimes
CT antiscatter grid diagram
CT Tube Orientation Diagram
, CT Beam Profile Diagram
Typical CT gantry rotation speed
Approaching 5 rotation per second (0.20 s rotation time)
Why is the plane of the anode disk parallel to the plane of gantry rotation?
To reduce gyroscopic effects that would add significant torque to the rotating anode if mounted
otherwise. Also, the anode-cathode axis (heel effect) runs parallel to the z-axis of the scatter,
eliminating heel effect-induced spectral changes along the typically wide fan angle. The fan beam is
much more limited in the anode-cathode angle
Approximate fan angle of most CT scanners
60 degrees
CT fan angle on anode diagram
CT cone angle on anode diagram
Most modern CT scanners use a (continuous/pulsed) x-ray source
continuous
The detector array sampling time in effect becomes the acquisition interval for each CT projection
that is acquired, and the sampling dwell times typically run between ______ ms, meaning that
between _______ projections are acquired per 360 degree rotation for a 0.5s gantry rotation period
0.2 and 0.5, 1000 and 3000
