Terms in this set (239)
composition of ...
ALPHA ("the alpha male"
-has 2 protons and 2 neutrons
-electron emitted from the nucleus (not from an outer ring)
why is the x-ray tube in a glass enclosure
maintain a vacuum
-amount and speed of electrons can be controlled
quantity of electrons, or the current
what is a rotating anode?
anodes spins so heat is dissipated better
what is the focal spot of general XR vs. mammo
0.6mm and 1.2 mm
0.1 mm and 0.3 mm
how is anode angle related to focal spot
smaller anode angle denotes a smaller focal spot
what is heel effect? what parameters change it?
inhomogeneity of the x-ray produced
cathode side is always stronger
worsens with smaller angles
heel effect more pronounced when the tube to film distance is smaller
interactions of electrons with a tungsten atom. what three things can happen?
1. excitation - just makes heat
- electron wiggles but doesn't actually leave
2. Bremsstrahlung - makes an XR
- protons slow an electron, which gives off an XR
3. Ionization - makes an XR
- K shell electron removed, characteristic XR peak seen
peak energy (not average)
the average is typically 1/3 to 1/2 the peak
what elemental parameter affects Bremsstrahlung XR production
higher Z number gives more (protons cause the electron to slow and give off XRs)
by which process are the most XR produced for general diagnostic imaging
what is the K-shell binding energy of Tungsten
how can the X-ray beam be kept the same when changing mAs or kVp
increasing kVp by 15% and cutting mAs in half
decrease kVp by 15% and double mAs
what is half-value layer
what parameters increase/decrease it?
HVL is the amount of amaterial required to attenuate an x-ray to 1/2 strength
Amount of filtration directly correlates to HVL (because more filtration = stronger beam, harder to attenuate)
what is an auger electron
ionization occurs, energy is dissipated as an electron ejected from outer shell instead of giving off an x-ray
how are kVp and mAs related to focal spot
increased mA widens focal spot (more blooming)
increased kVp narrows focal spot
how does DEXA work?
shoot a low and high energy beam, subtract to difference to calculate the bone density/attenuation
low energy X-ray bounces off of an atom
absorbed by body
NO EFFECT ON IMAGE QUALITY
small dose increase
which is good and which is bad?
BAD = compton
GOOD = photoelectric effect
HIGH energy (>30 keV) X-ray hits outer shell electron and causes:
1. knocks out an outer shell electron
2. ionizes the atom
3. deflects the incoming x-ray scattered photon
-fogs the image
-radiation exposure risks are related to this
probability of compton depends on the density of the material (NOT THE Z NUMBER)
DOMINATES AT LOWER ENERGIES (<30 keV)
strikes inner shell electron and x-ray is completely absorbed
1. inner shell electron ejected
2. other electrons shift down
3. characteristic X-ray released (or an Auger electron)
HELPS PRODUCE GOOD CONTRAST (this is why low energies are used for mammo)
probability of photoelectric effect occuring is proportional to ____?
the Z number cubed
(higher Z is more like to undergo PE effect)
for example, lead is radioopaque because of a high Z number
what is the K-edge
photoelectric effects occur more frequently near the binding energy of the atom, so an appropirate kVp must be selected
ex. Iodine k-edge is 33keV, so a kVp of 70 is appropriate to maximize contrast of iodine
(remember about 1/3 to 1/2 of the kVp value is the average beam energy - kVp refers to the peak, not average)
what factors would maximize scatter in X-rays
high kVp (more compton)
large field of view
thick body parts or fat people
decreases noise, should filter out the unhelpful photons on the side from the x-ray tube
what does grid ratio mean?
average grid ratio of mammo vs. general x-ray?
blocks off-angle x-rays
will increase dose, but gets a better image (decreased noise)
grid ratio describes density of the grid (height of grid / distance between grid holes)
what is a bucky grid?
basically a grid that slides back and forth quickly ... if it fails you will see grid lines
increasing which factor improves SNR?
mAs or kVp?
mAs increase improves signal to noise
increased kVp will cause more compton scatter
magnification factor in x-ray
source to image (detector)
source to object
modulation transfer function
information recorded / information available
defines how accurate a transfer is (per system) for a set spatial resolution
Detective quantum efficiency
required exposure level to make an optimal image
a better DQE requires lower dose (and vice versa)
directly proportional to MTF
adjusting which parameter improves _______ on x-rays?
-mAs increase makes x-ray more "black"
-low kVp improves contrast
-collimate, reduce kVp, compression (mammo)
considerations when doing X-ray of babys
NO GRIDS in PEDS
lower the mAs
digital x-rays ...
pixel density vs. pixel pitch
high density of pixels means better resolution
pitch refers to spacing of pixels (so lower pitch is better for spatial resolution)
what does automatic exposure control
DOES NOT AFFECT kVp
changes mAs to get adequete x-rays to the detector
CR: cassette systems
this is susceptible to what artifact?
storage phosphor holds image
cassette returned to station where it is read by a laser, converts to light, converts to electronic signal
susceptible to ghosting artifact if the cassette is not properly wiped with white light after the last image
DR: flat panel detectors
-cesium iodide activated by photon
-converted by photodiode to electric charge
-TFT makes image
DIRECT (removes the light before electric signal step - prevent light from lateral dispersion)
-selenium hit by photon, releases electrons
-charges storage capacitors on TFT
-TFT makes image
Mammo tasks, when are these performed?
localization/accuracy for stereo?
optimal kVp for mammograms
16-23 keV ... which requires kVp of 25-30
remember average keV is about 1/3 to 1/2 of kVp
which anode/filter combos can be used for mammo
binding energies of Mo and Rho
Mo - 18 keV
Rho - 20 keV
focal spots for mammograms
0.1mm for mags
0.3mm for regular mammo
what other parameters must change in mammo when using a small focal spot (ex. mag views)
lower the mA (small focal spot can't take the heat)
longer exposure time
how should the cathode and anode be positioned relative to the breast? why?
cathode on chest wall side
anode on nipple side
heel effect issues, chest wall is denser, beam will be denser along cathode side
what type of tube window is used in mammo systems
glass is used for normal diagnostic XR (not mammo)
Why is the breast compressed for mammo?
less scatter, can decrease kVp --> improved contrast
less mAs needed because of reduced thickness
less motion artifact
when is a grid used in mammo
what is grid ratio? what number used for mammo?
Grid used in all normal mammograms
Grid REMOVED for mag views
ratio 5:1 for mammo (around 10:1 for normal diagnostic XR studies)
grid ratio is height/width between grids
how is a breast mag view done
increased source to detector distance (move it closer to source
air gap (reduce scatter)
what is an air gap
for mag views, an air gap allows errant xrays to scatter before hitting detector (kind of acting like a grid)
a grid IS NOT used for mag views because it would lead to increased dose
what are the MQSA line pair requirements for digital mammo
there are no digital requirements
you go by manufacturer specifications
what are the MQSA line pair requirements for analog mammo
12 lp/mm average
who oversees MQSA nationally?
how often is accredidation?
every 3 years accredidation is done
how does an image intensifier work (fluoro)?
CsI captures x-rays, transitions to light
photocathode --> lights to electrons
focusing electrons accelerates electrons
smaller output phosphor, minimizes image and increases brightness (minification gain)
what happens to older image intensifiers as they fail?
conversion gain (product of minification and flux gain) decreases and the efficiency is bad ... leads to much higher doses to get bright enough images
geometric vs. electronic magnification?
geographic, bring object closer to x-ray tube
electronic mag - shrink FOV, automatic exposure control increases dose to maintain brightness (2x dose per setting increase)
ideal positioning of the image intensifier and x-ray tube to the patient
-I.I. is very close
-X-ray tube is further
automatic exposure control limits by law
87 mGy/min or 10 R/min
IF patient is way too obese, you can activate "high level control" which requires audible alarms
limit is: 176 mGy/min or 20 R/min
what is Kerma-area product?
If electronic magnification is used in fluoro, what happens to AIR KERMA and KAP?
amount of x-ray photons per unit air in front of the patient
KAP is a product of dose and cross sectional area (so it is normalized per area)
-AK goes up (more dose)
-KAP is the same (more dose, less area - balances out)
image intensifier pincushion artifact
due to large field of view, inward bowing of beam on the periphery
image intensifier S distortion
interference of Earth's magneitc field on flow of electrons towards image intensifier
fix by using mu-metal
image intensifier flare artifact
bright white glare at periphery of field, due to thin tissue and too many x-rays
image intensifier lag artifact
I.I. is moved and ghosted image stays briefly
image intensifier saturation artifact
dose cranked up to get through metal, and stuff around the metal is too lucent
how does fluoro flat panel detector work
xrays to grid to carbon fiber shield to CsI needles to photodiode array to readout element
pros and cons of Binning with flat panel detectors in fluoro
signal to noise
-ghosting and susceptibility to motion artifact
-spatial resolution worsened because binning increases pixel size (by combining them)
difference between regular and pulsed fluoro in regards to mAs
low mA, higher seconds
higher mA, fewer seconds
at what pulse rate, does pulsed fluoro reduce the dose?
around 30 frames per second or lower will reduce dose
pulsed has more mA per pulse, but fewers "seconds", therefore less dose overall
how is spatial resolution checked on fluoro machines?
lead bar pattern
ideal kVp for fluoro studies ...
with iodine (all digital subtraction angios)
trying to stay around k-edge
what is the dose spreading concept for fluoro/IR
move the gantry around so the same skin point isn't repeatedly irradiated
where should the angiographer stand relative to the I.I. and x-ray tube
always at the detector end
regulatory dose limit for radiation workers per year
what about for the fetus?
50 mSv (normally is about 5 mGy)
fetus is 0.5 mSv
above which threshold for fluoro skin dose do symptoms occur? when must action be taken?
above 2 Gy
less than 2 Gy : do nothing
2-5 Gy: advise patient to watch for burns for 10 days
above 5 Gy: physicist review
skin symptoms of radiation by dose
2 Gy: transient erythema
3 Gy: temporary epilation
6 Gy: chronic erythema
7 Gy: permanent epilation
10 Gy: telangiectasia
13 Gy: dry desquamation
18 Gy: ulceration
24 Gy: secondary ulceration
what determines the minimal slice thickness on CT
detector element aperature
how does mA affect dose in CT. for example, mA is doubled, what happens to dose?
dose is doubled
mA linearly increases dose
how is pixel size on CT calculated
FOV / matrix size
what happens on CT if mA is too low
too much noise
how is CT pitch defined
what is pitch >1 ?
what is pitch < 1?
table movement / beam width
pitch > 1 will have gaps that are not imaged, so spatial reolution is worse, but dose is lower
(do this for peds)
pitch < 1 will have overlaps for better spatial reolution but increased dose
(do this for cardiacs)
how is an HU calculated
1000 x ((atten of material - atten of water)/atten of water)
what are the classic window and level settings on CT for ...?
BRAIN W 80 L 40
LUNG W 1500 L -400
ABDOMEN W 500 L 50
SUBDURAL/LIVER W 150 L 50
BONE W 1500 L 500
effect of increasing kVp on a CT?
increased dose (opposite of XR)
what CT factors will improve SNR?
longer rotation time
larger slice thickness
how do you calculate relative noise for a set mA on CT?
square root N divided by N
ex. mA of 4 and 16
root 4 / 4 = 0.5
root 16 / 16 = 0.25
hence, 4 times the photons are needed to result in a doubling of signal to noise ratio
what is CTDIvol
what are ACR reference values of CTDI vol for head, adult abd, peds abd?
CT dose index per unit volume
HEAD: 75 mGy
ADULT ABD: 25 mGy
PEDS ABD: 20 mGy
what is a CT's DLP?
dose length product
CTDI vol x length of scan (in cm)
if you multiply by a "body part constant" you will get the effective dose
risk of radiation induced cancer per dose ...
5% per Sv in adults
up to 15% per Sv in kids
ideal CT kVp for contrast enhanced study
kVp of 100
will be around k-edge of iodine (k-edge of 33) and will still give a nice not-noisy image
what is CT beam hardening
low energy beams are lost in an object, so the remaining beam is hardened (higher energy)
can cause cupping (dark in the middle of object) or streak artifact
how can you fix CT photon starvation
1. increase tube current (auto modulation)
2. adaptive filtration
how can CT metal artifact be fixed
1. increase kVp
2. remove the metal
3. use thinner slices
4. software options
CT ring artifact
how is it fixed?
calibration error or defective detector
re-calibrate or replace to fix it
stair step artifact (CT)
jagged edges on oblique angled structures or sag/coronal reformatted images
fixed by making thinner slices
ultrasound machine assumes what speed of sound in tissues?
when ultrasound waves hit different tissues does wavelength of frequency change
how many decibels correspond to 50% reduction in signal intensity (ultrasound)? how about 90% reduction?
dB is based on a log10 scale
so 50% is 3dB
90% is 10dB
what is ultrasound impedence
relates to the stiffness of a material which determines reflection
higher impedence will cause greater reflection of sound waves
what four interactions can ultrasound waves have with matter?
change in direction of wave at tissue interface when beam is angled relative to surface (not perpendicular) ... speed may change but frequency DOES NOT CHANGE
two types of ultrasound scatter
-angle dependent reflection along a smooth surface
-rough surface scatters multiple different ways (angle becomes irrelevant)
what types of frequency and wavelength (small or large) will increase scatter
high frequency and small wavelength
surfaces will appear more rough which increases non-specular scatter
which frequency (high or low) attenuates ultrasound more?
high frequency (that's why high frequency probes are only used for superficial stuff)
how does an ultrasound transducer work?
matching layer: minimizes acoustic impedence between tissue and crystal/transducer
crystal: PZT (lead-zinc-titanate). mecahnically compressed and makes an image.
dampening layer: absorbs backward directed US waves ... dampens vibration
differences between thin and thick dampening blocks on US transducers
-long spatial pulse length
-used for Doppler (preserve velocity info)
-short spatial pulse length
-high axial spatial resolution
what is the optimal matching layer thickness on US transducer?
1/4 of the wavelength
difference between (curvi)linear and phased array probes?
phased steer the beam and can be made smaller
linear activates all elements and repeats the cycle
phased activates and reactivates elements in phased pattern
shape of the normal ultrasound beam
hourglass shaped with a near zone (fresnel) and far zone (fraunhofer)
the middle part is the focal zone
why are stand off pads used in ultrasound
to shift the focal zone to something very superficial
what is spatial pulse length and what effect does it have on axial resolution of US?
number of cycles emitted per pulse multiplied by wavelength
if objects are spaced less than 1/2 SPL, they will not be differentiated on an ultrasound image
what factors improve axial resolution on US?
low spatial pulse length and lower transmit cycles
where is lateral resolution best on ultrasound
in the focal zone, independent waves are closer to each other so more accurate margins are drawn for two adjacent objects
lateral resolution degrades in the near and far zones
high frequency and high scan density (more density of waves) improve lateral resolution
do axial or lateral US resolution change at different depths
AXIAL - NO
-revolves completely around spatial pulse length, depth doesn't matter
LATERAL - YES
-relies on the focal zone location
what improves and what worsens lateral resolution on US
high frequency and high scan density (more density of waves) improve lateral resolution
increasing gain (or wider beam) will worsen resolution laterally
how is temporal US resolution improved
reduce the depth, which will increase the frame rate
US - side lobe artifact
LINEAR ARRAYS are susceptible
side waves can reflect back in, making it look like an external object is in the middle
classic example is pseudo sludge in the GB
US reverberation artifact
sound wave reflected between two parallel high reflective surfaces
recorded as multiple echoes
equally spaced linear reflections
US comet tail artifact
closely spaced reflective parallel surfaces
closer than 1/2 SPL, so looks like one object, rather than multiple lines
classic for adenomyomatosis
ring down artifact on US
fluid trapper between air bubbles
parallel band extending posterior to gas
mirror image artifact US
false assumption that echo returns after one reflection
echo stuck between reflective surfaces
classic is duplicated liver behind the diaphragm
US speed displacement artifact
speed of sound slows in fat (ex. hepatic steatosis) which will perceive an organ border as further away
US artifact with "two aortas"
most often refracted beam along the rectus abdominal muscles (US machine assumes all waves are straight), which makes aorta or sma look duplicated
shadowing vs. through transmission on US
-US hits material that attenuates sound more than surrounding tissue (ex. stones)
-US hits material that attenuates sound less than surrounding tissue (ex. fluid/cyst)
what are ultrasound A/B/M modes
A mode (amplitude) - outdated, processed info from receiver vs. time
B mode (brightness) - current grayscale technique ... brightness corresponds to echo signal amplitude
M mode (motion)- b mode info used to display motion in a specific organ from fixed position
US doppler assesses for what change in sound
changes in frequency (due to doppler effect of moving objects)
ideal doppler angle
doppler equation has cosine in the numerator (cosine of 0 deg = 1. cosine of 90 deg = 0).
so if you image from 90 degrees, you should get 0 signal
theoretically imaging at 0 degrees is perfect, but this is impractical and causes refraction and loss of signal
clinically imaging from 30-60 degrees is considered ideal
spatial resolution of color/spectral doppler vs. grayscale
spatial resolution much better on grayscale
what is power doppler
VERY sensitive for flow, registers total number of pixels of motion as frequency shifts
this is a single number, NOT DIRECTIONAL, therefore NO ALIASING
NOT DEPENDENT ON DOPPLER ANGLE
what is the nyquist limit
1/2 of the pulse repitition frequency
when doppler shift is greater than nyquist limit you get aliasing
what is the pulse repetition frequency
the number of pulses that occur in 1 second
what is US aliasing?
how do you reduce it?
very high velocities are displayed as low
from "Wrap around" the baseline
occurs when doppler shift > nyquist limit (1/2 of PRF)
-increase PRF and increase scale
-use lower frequency transducer or go close to 90 doppler angle
US tissue vibration
secondary to turbulent flow
classically AV fistula in kidney post bx
doppler mirror image US
duplicated vessel appearance near a reflective surface (ex. liver at diaphragm)
US twinkle artifact
noisy spectrum and rapid fluctuation of red/blue color due to strongly reflecting surface (ex. kidney stones)
greater sensitivity for small kidney stones than shadowing
psuedoblood artifact US
things that move but aren't blood (ex. ureteral jets)
flash artifact US doppler
moving object (patient of transducer) causes flash of color (ex. baby kicking)
color bleed US doppler
how is it fixed?
color outside of vessel wall
decrease the color gain
what effect does increasing transmit gain have on lateral US resolution
will decrease resolution because it widens the beam (narrower beam is preferred)
what is the time gain compensation (US)
manual manipulation of amplification by depth
compensates for loss of echo strength in deep tissues
what is US harmonics
harmonics involves transmitting at one frequency and receiving at a different one (due to distortion by body tissues)
needs to travel before producing these (NO HARMONICS IN NEAR FIELD)
IMPROVES LATERAL RESOLUTION
Lose some depth penetration
what is US compound imaging
uses steering of beams to sharpen edges of objects
will cause loss of posterior shadowing (could make a cyst look solid)
how is US intensity calculated
power / area
power is related to amplitude of waveform
relative intensity of B-mode, M-mode, and doppler mode on US?
B - mode ~ 10
M - mode ~ 40
Doppler ~ 500
this is why we don't doppler the fetus
acoustic output parameter indices of US
Thermal Index (TI)
-max temp rise in tissue due to energy absorption
Mechanical Index (MI)
-how likely cavitation (bubbles popping) will occur
-important for contrast enhanced US
upper limit of mechanical index on US
US recommendations to minimize risk to fetus
NO pulsed doppler
M-mode for heart rate
-keep thermal index under 1
BETA MINUS (for radionuclide therapy)
-neutron turns into proton and electron (electron ejected from nucleus)
-isobaric (atomic number increase, but no mass change)
BETA PLUS (used for PET)
-proton gives off positron to become a neutron
-has 1.02 MeV energy
-hits regular electron and gives off two 511 keV photons
ELECTRON CAPTURE (used for gamma camera imaging)
-electron leaves K-shell to come to nucleus
-isomeric transition gives off characteristic X-rays
how is effective half life calculated
1/effective = (1/physical) + (1/biologic)
if physical or biologic are very different, the half life will approach the shorter one
what is a curie?
what is a Becquerel?
curie = 3.7 x 10 to the tenth disintergrations per second
Becquerel is a newer unit
describes on disintegration per second
components of a gamma camera receptor?
relationship of sensitivity and resolution to nucs collimators?
sensitivity and resolution are inversely proportional
narrow collimator increases resolution but degrades sensitivity and vice versa
general type of collimator used for nucs - high vs. low energy
-narrow holes and thin septa
-wider holes and long thick septa
how does distance from the collimator in nucs affect sensitivity and resolution
NO EFFECT on sensitivity
WORSENED RESOLUTION with increased distance
effect on nucs crystal size on sensitivity and resolution
-better resolution, worse sensitivity
-opposite of above
nuclear medicine downscatter concept
alwasy image with lower energy things first
xenon (81 keV peak)
Tc99m (140 keV peak)
always do ventilation first, otherwise tech counts will pour over xenon if it is done after tech
star artifact on nucs
septal perforation of hexagonal collimator holes
GAMMA CAMERA QC
center of rotation
uniformity - "flood"
-daily (extrinsic) with collimator
-weekly (intrinsic) without collimator
-test energy peak windowing with set source
-cobalt57 sheet with lines
center of rotation (for SPECT)
-use small sources of Tc99m
where is the radiation film badge worn?
where is the ring badge worn?
-collar at chest/neck level
-index finger of dominant hand. label towards source
sodium-iodine well counter
good for "wipe test" or in-vitro blood/urine samples
easily overwhelmed if sample exceeds 5000 counts per second
has a single photomultiplier tube
thyroid probe for nucs
modified Na-I well counter
shielding with small opening pointed at patient at a precise distance
dose compared to calibrated capsule of same radionuclide
used to detect radioactive contamination
-"dead time" concept
-maximum dose it can handle is 100mR/hr
-if exceeded, it may stop working for a few minutes when removed from the area
similar to geiger counter, used when higher doses are expected
can detect from 0.1 to 100 R/hr (as opposed to max of 100mR/hr on geiger counter)
this is the most often used dose calibrator clinically
nuclear dose calibrator
like a well counter
intra-operative probes for nucs
dose calibrator QC
-should be within 5% accuracy of dose amount
accurate readout of different energy over a large range (comes as a kit with varying lead thickness with Tc99 to simulate different energies)
-at installation or annually
-at installation or relocation of device
what constitutes a MAJOR vs. MINOR spill?
"100 for the T's"
-100 mCi or more of Thallium or Tc99m
-10 mCi or more of indium, I-123, or gallium
-1mCi or more of I-131
anything less is a minor spill
NEXT STEP after major nucs spill?
cover with absorbent paper, but don't clean it
shield source if possible
NEXT STEP after minor nucs spill?
protect the patient
confine the spill
clean up the spill
survey cleaning items and people
a minor nucs spill was just cleaned
next step if ...
spilled on clothes?
spill touched skin?
there is a xenon leak?
-take off the clothes
-held by RSO until decayed to safe levels
-wash with soap and water
-don't scrub too hard, don't want to break skin
-leave room as quickly as possible and close the door
-a wipe test will NOT work with xenon
nuclear dose limits to general public
rate limit in "unrestricted area"
100 mrem to the public
UNRESTRICTED RATE LIMIT
2mrem per hour
NUCLEAR MED SIGNAGE
when do these signs need to be placed ...
high radiation area?
very high radiation area?
0.005 rem (0.05 mSv) per hour at 30cm
0.1 rem (1mSv) per hour at 30cm
500 rads (5 Gy) per hour at 1 meter
OCCUPATIONAL DOSE LIMITS
total body per year?
dose to lens per year?
dose to embryo/fetus over 9 months?
extremity dose per year?
BODY: 50mSv per year (5 rem)
LENS: 150 mSv per year (15 rem)
FETUS: 5mSv over 9 months (0.5 rem)
EXTREMITY 500 mSv per year (50 rem)
UNIT CONERSION (NUCS)
1 rad = ____ rem
1 rad = ____ Gy
1 mSv = ____ mrem = ____ rem
1 Curie = ____ Bq = ____ GBq
1 rad = 1 rem
1 rad = 0.01 Gy
1 mSv = 100 mrem = 0.1 rem
1 Curie = 3.7 x 10 to the tenth Bq = 37 GBq
per the NRC, how much can prescribed dose differ from administered dose?
difference between recordable event and reportable medical event
RECORDABLE (kept for 5 years in-house)
-wrong drug, wrong dose, wrong patient, or wrong body part
-whole body dose < 5 rem (50 mSv)
-single organ dose < 50 rem (500 mSv)
-any doses greater than above that were unintended
-CALL the NRC within 24 hours
-LETTER to the NRC within 15 days
-NOTIFY referring doctor and the patient
when must a new nuclear delivery be inspected
within 3 working hours
geiger counter at 1 meter must be less than 6600 dpm / 300 square cm
Shipping labels on nuclear deliveries
what is the transportation index?
WHITE 1: no special handling
surface rate <0.5 mRem/hr
T.I. is about 0 mRem/hr at 1 meter
YELLOW 2: special handling needed
surface rate < 50 mRem/hr
T.I. is <1 mRem/hr at 1 meter
YELLOW 3: special handling needed
surface rate <200 mRem/hr
T.I. is >1 mRem/hr at 1 meter
T.I. is the max dose at 1 meter (actual measured dose)
basic process of Tc99m generation from Moly generator
Mo adheres to aluminum column
wash off with saline, gives of NaTc99mO4 (must be reduced with stanous ions because of its positive charge)
confirm no Moly contaminant by checking photopeaks (moly is high around 700 keV)
confirm no aluminum contamination with pH paper
what is the limit for Moly contamination in Tc99 from the generator?
limit is ...
0.15 microCi of Mo per 1 milliCi of Tc99m (note units differences)
NEEDS TO BE KNOWN AT TIME OF ADMINISTRATION (NOT THE ELUTION)
where does free Tc go?
what is nuclear SPECT?
what advantages over planar imaging?
single photon system (as opposed to PET which is two photon system)
makes a scroll-able 3D look to the images
is depth dependent - unlike PET
COMPARED TO PLANAR:
PET vs. SPECT
PET is double photon (spect is single)
PET has ring of detector (vs. single cameras)
PET uses thicker crystal
PET counts are way more
concept of coincidence in PET
both photons must come to the detector on opposite sides (from which the machine can calculate an exact location)
theoretically the photons should travel 180 degrees from each other (TRUE COINCIDENCE)
if slightly scattered, this is SCATTERED COINCIDENCE
two different events could fool the camera as well (RANDOM)
what is PET noise equivalent counts?
the same concept as SNR
varieties of PET crystals
which one is best
BGO - worst
GSO - middle
LSO (lutetium oxyorthosilicate) is the best
LSO has best denstiy, fastest decay, highest light output
PET time of flight
quicker detection to maximize change for good localization
IMPROVES spatial resolution and contrast
which organs are typically hotter on non-atten corrected images?
SKIN and LUNGS
how is PET SUV calculated
SUV = (tissue activity at a given time x patient weight) / injected dose activity
what parameters affect SUV measurements?
size of the lesion?
fat people have lower SUV
timing must be consistent (SUV will get higher as time goes on)
high glucose = lower SUV (tumor takes other glucose)
smaller than 1 cm = lower SUV
dose extravastation = lower SUV
what is PET truncation artifact
fat people can have artifically high/low SUV at body surfaces due to limited CT attenuation at those sites
patient preparation for PET-CT
fast for four hours
minimize cardiac activity with low carb diet
hydration and voiding (minimize bladder dose)
don't use short-acting insulin
decrease brown fat (make room warm)
1. using a straight positron source (no phantom)
2. using a uniform cylinder source
-would reveal broken crystal if image isn't uniform
-take a point source a rotate detectors around it
what MRI factor affects precession frequency?
field strength (what Tesla the magnet is)
so if Tesla doubles, frequency doubles
T1 is defined as the time when longitudinal magnetization returns to ____ % of its final value?
T1 is defined as the time when transverse magnetization has decayed to ____ % of its original value?
63% for T1
37% for T2
what TR/TE parameters make T1, T2, and PD images
what factors determine MRI acquisition time?
what about a 3D scan
TR x number excitations x number phase encoding steps
each phase step requires 1 TR
SLICE THICKNESS DOES NOT EFFECT THE TIME OF SCAN in 2D
3D scan time is increased by increased number of slices
when is the frequency encoiding gradient applied
same time as readout
basic order of a spin echo sequence
how would you make thinner or thicker MRI slices?
-steep slice select gradient
-thin transmit bandwidth
-thick transmit bandwidth
effect of changing MRI TR and TE on SNR?
long TR improves SNR
short TE improves SNR
what MRI factors improve spatial resolution?
what MRI factors improve SNR?
more number of excitations
which MRI sequence has the best SNR?
concept of gradient sequences
use low flip angle
NO 180 degree pulse
so you deal with T2 star effects
what is echo-planar imaging
rapidly cycle PE and FE on and off, causes fast filling of k space
used for diffusion sequences
very vulnerable to susceptibility
how does DWI work?
TWO signals fired. if the molecules hit on 1st firing can't diffuse, they will be hit again and be bright. stuff that diffused will not be bright.
higher b-values mean greater diffusion weighting
a b0, basically is just a long TR and TE, so a "poor man's T2"
b-500 or b-1000 become more of a true diffusion image
is fat suppression more successful at 1.5 T or 3 T?
at 3T it is better because the fat and water peaks separate more
how does in/out of phase work?
does it matter which is done first?
GRE sequence where fat/water imaged when in and out of phase at precession
every 2.2 msec on 1.5T and every 1.1 msec on 3T
OUT OF PHASE ALWAYS FIRST
-if in phase is done first, IRON depositiliver signal loss as time goes on
chemical shift artifact
what direction does it occur in?
what are the two types?
what parameters would fix or worsen it?
ALWAYS FREQUENCY ENCODING (different from other artifacts)
Type 1 - light and dark rim on objects (IMAGE)
Type 2 - india ink on out of phase
-increases with field strength
-decreases with increased gradient
-decreases with wider readout bandwidth
what effect does Gadolinium have on T1 time?
what about T2?
shortens T1, makes it brighter
also shortens T2, makes stuff darker (would see this as a level in the bladder at high concentrations)
which Gadolinium agent has highest NSF risk?
-is it linear or macrocyclic? ionic or non-ionic?
-probably Magnevist (gadopentetate)
-linear and ionic
features of Eovist?
what is the formal name?
50% hepatocyte uptake
linear and ionic
features of gadavist?
macrocyclic and non-ionic
LOW risk of NSF
gadolinium agents are hydrophilic or hydrophobic?
occurs in phase encoding direction
image wraps around the sides
-flip PE/FE directions
interference in lines in k-space at abrupt high contrast tissue differences
-limited sampling of free induction delay
ex. fake syrinx in the cord
phase encoding direction
FIX by increasing MATRIX (will increase acquisition time)
spin echo - flow looks dark
GRE - flow looks bright
FIX by applying a saturation band
magic angle artifact
short TE sequences (T1, PD, GRE)
angle of 55 degrees
artifact will go away with T2 (long TE)
MRI zipper artifact
related to surrounding RF signals (radio, TV, etc.)
-could be caused by a pulse ox monitor
phase encoding direction
CLOSE THE DOOR
REPAIR BAD RF SHIELDING
MRI inhomogeneous fat suppression
how can it be improved?
how can inhomogeneity be improved?
use STIR instead (especially with metal)
adjust the shimming
what are active and passive MRI shimming
-done at installation
-positioning of shimming plates
-done by electromagnetic coil
-can adjust to patient size
which sequences is it worst on? why is it worse on in phase than out of phase?
how to make it better?
worse on GRE/T2* sequences
worse on in phase (than out of phase) because in phase is done later (more time for blooming - longer TE = more T2* effects)
MRI eddy current
stretch/smearing of the image
worst on DWI
optimize gradient pulses to fix it
MRI dielectric effects/standing wave
what makes it worse?
what makes it better?
biologic tissues may cause wavelength reduction and local eddy currents
WORST with stronger magnet (3T) and ascites
FIX by placing dielectric pads or parallel RF transition
MRI herringbone/criss cross
oblique stripes in image
due to improper processing of K-space
REPROCESS THE IMAGE TO FIX
chemical shift artifact
how could you fix it?
at interface of fat/water in breast
fix by increasing receiver bandwidth (but at the expense of worsened SNR)
how are phase/frequency encoding directions placed for breast MRI? why?
phase across the chest (to avoid cardiac motion)
MRI signal flair
body part is too close to coil element and fat will not sat out properly
leads to poor fat suppression
When should an MRI be quenched?
What happens? Risks?
ONLY QUENCH IN EMERGENCY
1. person pinned by metalic things in MRI room
2. fire in the MRI room endangering people
Quench will vent all liquid helium out of the room
IF THERE IS A BROKEN VENTILATION PIPE
-helium will leak into room
-will kill you
-pressure will pin door closed
CODE occurs while patient is in MRI scanner
start the immediate steps of code in the MRI room
DO NOT QUENCH MAGNET
take patient to zone 3 to do full code
what is the MRI 5 G line?
5-gauss exclusion zone
risk to implanted devices is within that area
TRANSLATIONAL (pulling) forces could still occur outside of that zone (different issue)
how are medical devices graded in regards to MRI compatibility
what may cause patients to have arm/leg pain in an MRI?
how can you lessen these effects?
-worst with echoplanar imaging (high frequency switching) and high bandwidth readouts
1. narrower bandwidth readout
2. increase the TR
what is MRI SAR?
what factors determine the SAR?
what are the FDA limits for SAR
specific absorption ratio (heating)
proportional to field strength squared and flip angle squared
-lower field strength
-lower flip angle
4 W/kg over 15 mins
3 W/kg over 10 mins
what may cause a patient t oget a burn in MRI
touching the bore
medicated patches may heat up
"closed loops" - ex. hands touching each other
how frequently is MRI SNR QC performed?
required to have a door lock between zones 2 and 3
when are the slice select, phase, and frequency encoding gradients performed in each TR?
SSG at each RF pulse
FE at the echo
PE between RF pulse and echo
what is the typical precession frequency per Tesla in MRI
42.6 MHz per Tesla
regulatory amount of aluminum filtration required for x-ray units over 70 kVp
2.5 mm of aluminum