MDCB Board Prep
The line focus principle is used in diagnostic x-ray tube design in order to:
A. make the apparent anode smaller than the actual anode angle
B. make the apparent focal spot size smaller than the actual focal spot size
C. make the apparent focal spot size larger than the actual focal spot size
D. make the apparent anode angle smaller than the actual anode angle - answerB.
make the apparent focal spot size smaller than the actual focal spot size
The apparent focal spot size equals the actual focal spot size x the sin of the anode
angle. This allows a larger portion of the anode to be irradiated because the heat is
more evenly dispersed, while keeping the spatial resolution in the image as good as
possible.
Insufficient vacuum inside the x-ray tube can lead to___________.
A. reducing image contrast
B. increased patient dose
C. tube arcing
D. need to increase tube voltage - answerC. tube arcing (no useful beam is produced
when the tube is arcing.
The output of an x-ray tube is approximately proportional to:
A. square of tube current at a given kVp
B. square of filament current at a given kVp
C. thickness of xray target
D. square of tube voltage at a given tube current - answerD. square of tube voltage at a
given tube current
Output (kVp)^2 (mAs)
Standards defining the minimum necessary amount of inherent filtration to be used in x-
ray tubes are intended to ____________.
A. increase image contrast
B. reduce patient skin dose
C. increase x-ray tube life
D. allow reduced tube voltage - answerB. reduce patient skin dose
Filtration hardens the beam, increasing the portion of the beam that penetrates the
patient and contributes to the image. Increased filtration reduces the image contrast.
The surface dose rate of Sr-90 eye applicator was 4.5 cGy/ second on May 25. If a
patient is to be treated 1 year later, what is the treatment time if 500cGy is prescribed?
(half life of Sr-90 is 28 years). - answer114 sec
, When 10 mCi of I-125 (half life =60 days) and 25 mCi of Pd-103 (half life = 17 years)
have equal activities? - answer43.17 days
A 10 MeV photon undergoes a Compton interaction, and the scattered photon is emitted
perpendicular to the direction of the incident photon. What is the approximate energy of
the scattered photon?
A. 0.51 MeV
B. 1.02 MeV
C. 1.25 MeV
D. 2.04 MeV - answerB. 0.51 MeV
For megavoltage x-rays, the photon energy is much larger than the rest mass of
electron, which is 0.511 MeV. Compton photons scattered at 90 degrees approach 511
keV energy because of the conservation of energy and momentum. Similarly,
backscattered Compton photons approach 256 keV energy.
In Compton interactions, when is the maximum energy imparted to the electron? -
answerWhen the photon is scattered at 180 degrees.
A FFF beam of the same energy as a regular beam has a ______ energy and ______
dmax depth (penetration) because there is no filter in place to remove the lower energy
photons. - answerDecreased, decreased
What is the dmax and SSD of a Co-60 machine? - answer0.5cm dmax and 80 cm SSD
In a linac, to produce the same dose rate for a 15MV x-ray beam as a 15 MeV electron
beam, the electron current in the accelerating cavity must be:
A. Higher for electron treatment than for photon treatment
B. Lower for electron treatment than for photon treatment
C. About the same for both modalities - answerB. Lower for electron treatment than for
photon treatment
Bremsstrahlung production efficiency is always <1. (Not the most efficient process.)
Many x-rays are attenuated by the flattening filter, and many x-rays pass completely
though the patient without depositing any dose. Therefore, much higher electron current
is required when the linac is running in x-ray mode as compared to electron mode.
A patient is treated with a single posterior 6 MV beam, 15 x 15 cm at 100 cm SAD, 8 cm
depth,
by using an isocentric technique. The spinal cord is located 5 cm from the posterior
surface. If
40 Gy is delivered to the isocenter, the dose to the spinal cord is:
TISSUE-MAXIMUM RATIO for 6 MV PHOTONS
Depth (cm) 12 x 12 15 x 15 20 x 20
1.5 1.000 1.000 0.999
The line focus principle is used in diagnostic x-ray tube design in order to:
A. make the apparent anode smaller than the actual anode angle
B. make the apparent focal spot size smaller than the actual focal spot size
C. make the apparent focal spot size larger than the actual focal spot size
D. make the apparent anode angle smaller than the actual anode angle - answerB.
make the apparent focal spot size smaller than the actual focal spot size
The apparent focal spot size equals the actual focal spot size x the sin of the anode
angle. This allows a larger portion of the anode to be irradiated because the heat is
more evenly dispersed, while keeping the spatial resolution in the image as good as
possible.
Insufficient vacuum inside the x-ray tube can lead to___________.
A. reducing image contrast
B. increased patient dose
C. tube arcing
D. need to increase tube voltage - answerC. tube arcing (no useful beam is produced
when the tube is arcing.
The output of an x-ray tube is approximately proportional to:
A. square of tube current at a given kVp
B. square of filament current at a given kVp
C. thickness of xray target
D. square of tube voltage at a given tube current - answerD. square of tube voltage at a
given tube current
Output (kVp)^2 (mAs)
Standards defining the minimum necessary amount of inherent filtration to be used in x-
ray tubes are intended to ____________.
A. increase image contrast
B. reduce patient skin dose
C. increase x-ray tube life
D. allow reduced tube voltage - answerB. reduce patient skin dose
Filtration hardens the beam, increasing the portion of the beam that penetrates the
patient and contributes to the image. Increased filtration reduces the image contrast.
The surface dose rate of Sr-90 eye applicator was 4.5 cGy/ second on May 25. If a
patient is to be treated 1 year later, what is the treatment time if 500cGy is prescribed?
(half life of Sr-90 is 28 years). - answer114 sec
, When 10 mCi of I-125 (half life =60 days) and 25 mCi of Pd-103 (half life = 17 years)
have equal activities? - answer43.17 days
A 10 MeV photon undergoes a Compton interaction, and the scattered photon is emitted
perpendicular to the direction of the incident photon. What is the approximate energy of
the scattered photon?
A. 0.51 MeV
B. 1.02 MeV
C. 1.25 MeV
D. 2.04 MeV - answerB. 0.51 MeV
For megavoltage x-rays, the photon energy is much larger than the rest mass of
electron, which is 0.511 MeV. Compton photons scattered at 90 degrees approach 511
keV energy because of the conservation of energy and momentum. Similarly,
backscattered Compton photons approach 256 keV energy.
In Compton interactions, when is the maximum energy imparted to the electron? -
answerWhen the photon is scattered at 180 degrees.
A FFF beam of the same energy as a regular beam has a ______ energy and ______
dmax depth (penetration) because there is no filter in place to remove the lower energy
photons. - answerDecreased, decreased
What is the dmax and SSD of a Co-60 machine? - answer0.5cm dmax and 80 cm SSD
In a linac, to produce the same dose rate for a 15MV x-ray beam as a 15 MeV electron
beam, the electron current in the accelerating cavity must be:
A. Higher for electron treatment than for photon treatment
B. Lower for electron treatment than for photon treatment
C. About the same for both modalities - answerB. Lower for electron treatment than for
photon treatment
Bremsstrahlung production efficiency is always <1. (Not the most efficient process.)
Many x-rays are attenuated by the flattening filter, and many x-rays pass completely
though the patient without depositing any dose. Therefore, much higher electron current
is required when the linac is running in x-ray mode as compared to electron mode.
A patient is treated with a single posterior 6 MV beam, 15 x 15 cm at 100 cm SAD, 8 cm
depth,
by using an isocentric technique. The spinal cord is located 5 cm from the posterior
surface. If
40 Gy is delivered to the isocenter, the dose to the spinal cord is:
TISSUE-MAXIMUM RATIO for 6 MV PHOTONS
Depth (cm) 12 x 12 15 x 15 20 x 20
1.5 1.000 1.000 0.999
