l 3 ct physics, shahid younas, sr. medical physicist
TRANSCRIPT
Introduction to CT Physics
Shahid YounasLecture 3
MCQs
Q: Melting point of Tungsten is,
1: 3370◦C
2: 337◦C
3: 370◦C
4: 730◦C
MCQs
Q: Modern X ray tubes have,
1: one filament
2: two filaments
3: three filaments
4: four filaments
5: five filaments
MCQs
Q: Space Charge Cloud is produced around filament when
1: Anode is given negative Voltage
2: Cathode is given positive Voltage
3: No voltage applied between anode and cathode
4: Tube is over heated
5: impurities increase in the filament
MCQs
Q: the effect of space charge cloud is dominant up to
1: 10 kVp
2: 20 kVp
3: 30 kVp
4: 40kVp
5: 50 kVp
MCQs
Q : Fixed anode x-ray tubes are usually not embedded in
1: Fixed Dental x-ray units
2: Portable dental machines
3: Portable fluoroscopy
4: Portable CT Scanner
5: Portable x-ray machines
MCQs
Q: rotating anodes are used for most diagnostic x-ray applications mainly because of their
1: greater heat loading capabilities
2: higher x-ray output capabilities
3: longer anode life
4: All of above
MCQs
Q: The focal track area of the rotating anode is equal to the
1: product of the track length and the track width
2: sum of the track length and the track width
3: ratio of the track length with the track width
4: inverse of track width
5: inverse square of track width
MCQs
Q: Anode angles in diagnostic x-ray tubes range from
1: 7 to 20 degrees
2: 10 to 20 degrees
3: 12 to 20 degrees
4: 15 to 20 degrees
5: 18 to 20 degree
MCQs
Q: Which one is the best relation between focal spot length and effective focal spot length?
1: Effective focal spot length = Focal spot length x sin θ
2: Effective focal spot length = Focal spot length x cos θ
3: Effective focal spot length = Focal spot length x tan θ
4: Effective focal spot length = Focal spot length x 0.5
5: Effective focal spot length = Focal spot length x 5
MCQs
Q: What is the tool for measuring focal spot size?
1: Pinhole camera
2: Slit camera
3: Star bar pattern
4: Resolution bar pattern
5: All of above
MCQs
Q: A small anode angle (approximately 7 to 9 degrees) is desirable for
1: small field-of-view
2: cine-angiographic equipment
3: neuro-angiographic equipment
4: the decreased size of the usable x-ray field
5: All of above
ANODE-ROTATING
The anode angle is
defined as the angle of
the target surface with
respect to the central ray
in the x-ray field.
Introduction to CT Physics
Introduction to CT Physics
Large anode angles (~ 12 to 15o ) are
necessary for general radiographic
work to achieve large field area
coverage at short focal spot-to-image
distances.
Introduction to CT Physics
smaller effective focal spot for the
same actual focal area.
provides better spatial resolution
limits the size of the usable x-ray field
Introduction to CT Physics- Focal Spot Size Measurement
Heel Effect
Heel Effect
Introduction to CT Physics- Heel Effect
Heel Effect
The heel effect is a loss of
intensity on the anode side of the
x-ray field of view.
It is caused by attenuation of the
x-ray beam by the anode.
Heel Effect
Heel Effect
Heel Effect
X-rays are produced isotropically at depth in the anode
Photons directed toward the anode side of the field transit a
greater thickness of the so experience more attenuation than
those directed toward the cathode side of the field.
Heel Effect
The heel effect is less prominent with a longer source-to-image
distance (SID).
Because the image receptor subtends a smaller beam angle.
Heel Effect
The x-ray tube is best positioned with the cathode over thicker
parts of the patient and the anode over the thinner parts, to
better balance the transmitted x-ray photons incident on the
image receptor.
Off Focus
Beating about the bush
Off Focusing Radiation
When electrons strike the
anode outside the focal
spot area.
Off Focusing Radiation
A small fraction of electrons scatter from the target and are
accelerated back to the anode outside the focal spot.
These electrons create a low-intensity x-ray source over the face
of the anode.
Off Focusing Radiation
This increases patient exposure, geometric blurring, and
background fog.
A small lead collimator placed near the x-ray tube output port
can reduce off-focus radiation by intercepting x-rays that are
produced at a large distance from the focal spot.
Computed Tomography
With plain film imaging, 3D anatomy of the patient is reduced to 2D projection image.
Computed Tomography
The density at a given point on an image represents the x-ray
attenuation properties within the patient.
patient's anatomy, information with respect to the dimension
parallel to the x-ray beam is lost.
Computed Tomography
This limitation can be overcome, at least for obvious structures,
by acquiring both a PA projection and a lateral projection of the
patient.
Computed Tomography
Imagine that instead of just
two projections, a series of
360 radiographs were
acquired at 1-degree
angular intervals around
the patient's.
Computed Tomography
The tomographic image is a picture of a slab of the patient's
anatomy.
The 2D CT image corresponds to a 3D section of the patient,
with slice-thickness l to 10 mm.
Bremsstrahlung X Rays
Do you know what do you mean by pixel and voxel?
Picture element
Volume element
Computed Tomography
Each pixel on the CT image
displays the average x-ray
attenuation properties of the
tissue in the corresponding
voxel
MCQs
Q : Off-focus radiation increases
1: patient exposure
2: geometric blurring
3: background fog
4: All of above
MCQs
Q: The permitted leakage radiation exposure rate from x ray
tube housing is,
1: 100 mR/hour at 1 m at the maximum kVp
2: 100 mR/hour at surface at the maximum kVp
3: 100 mR/hour at 1 m at the minimum kVp
4: 100 mR/hour at surface at the minimum kVp
MCQs
Q: In general diagnostic radiology, the most common added
filter materials are,
1: Aluminum, copper and acrylic
2: Aluminum, copper and silver
3: Aluminum, copper and gold
4: Aluminum, copper beryllium
MCQs
Q: In mammography, the most common added filter
materials are,
1: molybdenum and rhodium
2: molybdenum and beryllium
3: molybdenum and acrylic
4: molybdenum and tungsten
MCQs
Q: In diagnostic energy range the exposure is,
1: directly proportional to kVp
2: directly proportional to kVp2
3: inversely proportional to kVp
4: inversely proportional to kVp2
MCQs
Q: HU provides a simple way of expressing the energy
deposition on and dissipation from the anode of an x-ray tube. HU
stands for,
1: Hounsfield unit
2: Heat Unit
3: Housing Unit
4: Heel Unit
Wake up please!
Thanks You.