Transcript Physics Practice Questions 2009

effective dose for
-chest radiograph
-skull radiograph
-abdomen radiograph
-Lower extremity CT
-head ct
-abdomen CT
-chest Ct
Chest radiograph- 0.05 mSv
Skull radiograph- 0.1 mSv
Abdominal radiograph- 0.5 mSv
Lower extremity CT- < 1 mSv
Head CT- 1- 2 mSv
Abdomen CT- 4 mSv (book says 5-10 for Abd/Pelvis)
Chest CT- 6 mSv (book says 5-10)
entrance skin exposure (ese) for-chest
-skull
-abdomen
-l spine
-Fluoro (1 min)
-body ct
-head ct
chest 0.2 mGy
skull 1.5 mGy
abdomen 3 mGy
L spine 10 mGy
Fluoro 20 mGy
body ct 30 mGy
head ct 60 mGy
receptor exposures
-Fluoro
-Cine
-photospot
-film screen (200 speed)
-detail film screen
-mammo
Fluoro- 0.01- 0.02 μGy
Cine- 0.15- 0.30 μGy
Photospot- 1 μGy
Film screen (200 speed)- 5 μGy
Detail screen film- 20 μGy
Mammo- 100-200 μGy
what is the typical equivalent dose for a
body ct midway through the body?
5-10 mSv
What is linear energy transfer?
(LET)
•
Energy absorbed by medium per unit length of travel
(kEv/um)
- Proportional to particle charge squared.
- Inversely proportional to particle kinetic energy
what is 10th value layer
(tvl)?
It is the thickness of material that attenuates the beam by 90%
what is the quality factor?
Linked to relative biologic effectiveness (RBE)
-orThe radiation weighing factor
-It compares the ability of different radiation to cause biologic
change.
WHAT IS THE BIOLOGICAL
HALF LIFE (Tb)?
It measures the clearance of the radionuclide from the organ,
tissue, or body.
7th hvl
1% transmitting
(1/2)^7
compton interactions are
proportional to _______?
The # of outer shell electrons (electron density)
-Inversely proportional to photon energy (1/E)
Lateral resolution on ultrasound is
dependent on _________?
The width of the beam and the line density.
what is vignetting?
- Image intensifier artifact
- Fall off in brightness at the peripheral of II field
- Decreases 25%
- Caused by curvature of II faceplate
What is pincushion
distortion?
- Image intensifier artifact
- Straight lines appear curved
- 3% for 23 cm II
- Caused by curvature of II faceplate
ct is advantageous
because...?
- Low contrast detectability
- Increased dynamic range
What is the typical percent
conversion of a photon into light?
10%
What is beta plus decay?
- Proton converted into a neutron
- Excess energy released as a positron and a neutrino
- C 11, O 15, F 18, and Ga 68
- Z decreases by 1, A stays the same
what is Beta minus decay?
- A neutron is converted to a proton.
- Excess energy released as a Beta particle and an antineutrino.
- P 32, H 3, C 14
- Z increases by 1, A stays the same
what is the tr and te for T1
weighted sequences?
-Short TR (less than 500 msec)
- Short TE (less than 8 msec)
what is the tr and te for T2
weighted sequences?
-Long TR (more than 1500 msec)
- Long TE (more than 60 msec)
what causes ghost signal in
mr?
- Caused by mismapped signal
- in the phase encoding direction (duration of RF pulse)
spin echo
Multiphase encoding steps
- 1st, 90 degrees
- then, multiple 180 degree
what percent of an x-ray is
characteristic?
approximately 10%
what exposure is needed to
blacken film?
-0.5 mRad (film), results in OD of 1.5
-5-20 mRad (mammo)
-0.5 mRad/5mGy produces satisfactory image
what is the ct attenuation
coefficient?
Attenuation coefficients are expressed in Hounsfield units.
Air -1000
Lung -300
Fat -90
Water 0
White matter +30
Gray +40
Muscle +50
Bone +1000
Metal +3000
What is electron capture?
-A proton is converted to a neutron by capturing an electron
from another shell.
-Vacancy is filled by an outer shell electron.
-Excess energy is emitted as characteristic x-ray or an Auger
electron.
-Z decreases by 1, A stays the same.
What is exposure and in what unit is
it measured?
The ability to ionize air measured in Roentgens (R).
what is dose and in what unit is it
measured?
Dose is the absorbed energy
-Given in rad or Gy
what is dose equivalent and what
unit is it in?
The measure of biologic damage likely to occur from dose.
-Given in rem or Sv
limiting fields close to
magnets
0.5 mT= pacemakers, cathode x-ray tubes
1 mT= credit cards, watches
2 mT= floppy disks
5 mT= power supplies
what is dqe?
Detected Quantum Efficiency
SNR^2(out)/ SNR^2 (in)
-SNR performance of the imaging system
QUANTITY e^Mx
REPRESENTS?
-Fraction of primary photons transmitted by x cm of
attenuation.
-Linear attenuation coefficient (μ =cm^-1)
-Formula N=No^(e^(μ t))
production of radiation slowing down
and changes direction?
Brehmstralung increases withaccelerating voltage (kV) &
Z (atomic #)
quench
MR wire heats up too much and loses superconductivity & the
magnetic field is converted to heat.
physical half-life (t1/2)
Time required for a radionuclide to decay to half it’s original
activity.
F18
2 x 511 KeV photons
produced in cyclotron
Beta + decay
t1/2 = 110 minutes
noise level of mr systems?
65- 120 dB
1 curie=
3.7 x 10^10 Bq
1 mCI=
37 kBq
proton density
tr/te
long TR, short TE
film temperature, speed & fog are
inversey related to ____?
Exposure, Dose & Contrast
what improves pin-cushion &
vignetting artifacts?
Smaller field size (less minification)
Technesium 99m
-140 KeV photons
-produced in generator
-decays by isomeric transition
-T1/2 = 6 hrs
imaging system with resolving power of 10
lp/mm represents system mtf response of?
10 lp/mm = 10% or 0.1
30 lp/mm = 30% or 0.3
lambda decay constant
0.693/ T(1/2)
dose limit for the general
public?
2 mRem/hour average
100 mRem/year
For radiologists- 5000 mRem/year
in nm,
as resolution decreases, distance
____
increases
in nm,
as sensitivity decreases, distance
____
increases
in nm,
as sensitivity increases, resolution
____
increases
focal spot size on ct?
0.7- 1 mm
hvl calculation
0.693/ μ
why is a1 filtration done?
To decrease entrance dose to skin.
what is the inverse square
law?
If you double the distance, you decrease the exposure by 25%.
1/distance^2
Double distance= 1/(2^2)= 1/4= 25%
what is zipper artifact with
mri?
Dotted line across an image caused by hardware or software problem such as
radiofrequency entering the scanning room when the door is open during
acquisition.
what is a slow screen?
Thinner, increased resolution, used in extremity imaging,
higher patient dose, absorbs less photons.
-10 lp/mm
what is a fast screen?
Thicker, decreased resolution (have more blur), used in
abdominal imaging, absorbs more photons (increased
absorption efficiency).
-5 lp/mm
what is the focal spot size
for angio?
0.3- 1.2 mm
fwhm for nm?
3- 5 mm
higher resolution gamma
camera fwhm?
7.5 mm
storage for single ct image?
0.5 MB
male sterility occurs at?
5-6 Gy
500-600 rad
dose that results in mammalian cell
death?
-deterministic
More than 0.5 Gy or 50 rad
dose limit for workers
extremities (hands)?
500 mSv/year
50,000 mrem/year
annihilation pair
production
-Proton energy threshold= 1.02 meV
-Makes 2- 511 keV photons at 180 degrees.
ct artifacts
1. streak
2. beam hardening
3. ring
4. partial volume average
1. Random motion produces it and it’s in the direction of
motion
2. Low energy photons absorbed- causes underestimation of
HU usually at higher contrast interfaces such as bone/brain.
3. 3rd generation CT scanners- detector failure
4. averaging linear attenuation coefficient in a voxel.
bandwidth
Horizontal resolution
TV systems
intensity is directly
proportional to ____?
mAs
ADC
Analog to digital converter
Changes varying voltages to closest binary equivalent
order of beam traveling in
fluoro?
Grid
input phosphor (converts xray to electron)
photocathode
focusing lenses (electrostatic lenses)
anode
output phosphor (converts to visible light)
camera
quantum mottle
-Noise
-Random variation of photon incident based on poisson stats.
types of us transducers?
1. Linear
2. Curvilinear (sector scanner)
3. Endocavitary
4. Phase array
5. Annular
federal law requires ____ in
an x-ray unit?
Filtration
256 shades of gray = how
many bits?
8 bits (1 byte)
n bits codes for 2^n gray levels
what is the largest contributor to
background radiation?
Radon
what modality does not use filtered
back projection?
MRI
dose response curve for radiation and
cancer risk is _____?
Linear
which of the following is not directly ionizinga. proton
b. electron
c. neutron
d. alpha particle
Neutron
power is measured in
_____?
Watts (W)
W= Amp x Volt
MC grid used
12:1
lead equivalent for shielding
apron?
0.5 mm
units of dose equivalent?
Sv
Rem
best detector for gamma
radiation?
Geiger- Mueller counter
speed or velocity of us is inversely
proportional to _____?
Compressibility
Velocity = 1/compressibility
example- bone is non-compressible so velocity increases.
maximum allowable nonuniformities in nm?
Less than 10%
why is csf bright on t2?
Long T2
most to least radiosensitive
a. skin
b. breast
c. lung
d. gi
GI- most radiosensitive
Skin
Lung
Breast- least radiosensitive
how can you increase mean photon energy
without affecting max photon energy?
Filtration
stir
T1 weighted
saturates fat
Atomic #?
Z= # of protons
Electron interacts with a positron to
produce _____?
2- 511 keV photons
difference between3rd generation ct &
4th generation ct
3rd Generation CT- Fan beam and Detectors rotate
4th Generation CT- Fan beam rotates, Detectors are fixed
US can cause what at increased
power levels?
Cavitation= creation and collapse of microbubbles
functional mri
uses changes in oxygenation
5 mm lead shield blocks ___% of
tc99
___% of x-rays
70% of Tc99
>90% of X-rays
most radiosensitive part of the
cell cycle?
Mitosis- metaphase
ratio of heat to x-ray
production
99-1
do you need a grid with 10cm air gap
to decrease scatter?
No
how do you decrease
scatter?
- Collimation
- Air gaps
- Grids
what is not associated with digital
radiology screens?
Silver halide grains
If hu changes by 3, it represents what
% of attenuation change?
0.3 %
units of exposure?
Roentgen (R)
Coulombs/ kg (C/ kg)
units of radioactivity?
Ci
Bq
units of absorbed dose?
Gy
rad
Threshold dose for cataract formation for
chronic radiation dose, &
acute radiation dose.
Chronic radiation dose threshold- 5 Gy or 500 rad
Acute radiation dose threshold- 2 Gy or 200 rad
Focal spot size for mag
mammo?
0.1 - 0.15 mm
greatest amount of scatter
occurs where?
At the skin entrance
Coulomb is a measure of
what?
Charge
are there different mutations seen in
hiroshima survivors?
No
result of major radiation exposure
prior to implantation?
Intrauterine death
radionucleus with excess neutrons
decays by _____?
Beta minus decay
increase scatter by _____?
Increasing field size, or,
Increasing patient thickness
what changes when us passes from
one medium to another?
Velocity and wavelength
(frequency doesn’t change)
frequency= velocity x wavelength
put the following in order of increasing
wavelengthuv, ir, visible, gamma
Gamma, Xray, UV, Visible, IR, Microwave, Radiowave.
what are the units for the sfactor?
H-157
S-factor= absorbed energy/ target organ mass
mGy/Bq S (SI)
rad/μGi hv (common)
increase noise in film by
_____?
Increasing conversion efficiency
-ability of intensifying screen phosphors to emit a large
amount of light per x-ray absorption.
phase contrast mr
Background is taken out and flow enhancement is noted