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.