BME 311 Exam 1 Practice Questions

a CT image has a pixel size of 0.5 mm, but the true spatial resolution of the scanner is 2 mm. If the image is resampled to 0.25 mm pixels, what happens to the actual spatial resolution

it stays about 2 mm; pixels do not automatically mean better true resolution. pixel size/image does not always reflect actual resolution.

two imaging systems have spatial resolution of 1 mm and 5 mm. which is better at separating two very small nearby objects

1 mm; a smaller spatial-resolution value means higher/better resolution

an imaging system records a signal that changes at a maximum frequency of 100 Hz. what is the minimum sampling rate needed to properly represent it

200 Hz; nyquist says you need at least twice the signal frequency

why might an imaging system not simply sample at an infinitely high sampling rate

more sampling requires more storage and processing; unnecessary oversampling wastes data/storage/computation

an image is smoothed using a spatial filter. What tradeoff should you expect

noise decreases but spatial resolution worsens

a very small lesion occupies only part of a voxel, while the rest of the voxel contains normal tissue. how will the voxel most likely appear

it will have an intermediate value; partial volume effect

an EEG system detects changes over milliseconds, while an fMRI system detects changes over seconds. which has better temporal resolution

EEG; smaller interval time = better temporal resolution

a digital image goes from 8 bits per pixel to 12 bits per pixel. what primarily improves

contrast resolution; more bits = more possible gray levels

why can increasing bit depth improve an image even if pixel size remains exactly the same

because bit depth controls how many intensity values each pixel can represent, so it improves contrast resolution, not spatial resolution

a physician changes the windowing settings on a CT image and suddenly a soft-tissue structure becomes much easier to see. did the scanner acquire new information

no; windowing changes how existing pixel values are displayed, not the underlying acquired data

a photon has twice the frequency of another photon. which statement is true

it has twice the energy; E = hv, energy increases directly with frequency. the speed in vacuum remains c.

photon A has a shorter wavelength than photon B. which has higher energy

photon A; E = hc/λ, shorter wavelength = higher energy

a photon is absorbed by tissue. what happens to the energy of the remaining photons in the beam

their individual energies do not necessarily change; there are fewer photons; absorption removes photons and reduces intensity

a photon enters tissue and leaves traveling in a different direction but with the same energy. which interaction most likely occurred

rayleigh = direction changes, energy does not

a photon interacts with an outer-shell electron. the electron is ejected, and the photon continues in a different direction with less energy. What occurred

compton scattering’ outer-shell electron, ionization, scattered photon survives, scattered photon has less energy

a photon strikes an inner-shell electron and disappears completely. what interaction occurred

photoelectric absorption; then an electron cascade can produce characteristic x-rays or auger electrons

which interaction is more likely to reduce image contrast because scattered photons may still reach the detector

compton scattering; changes photon direction, and scattered photons reaching the detector and unwanted signal, reducing contrast

a photon has an energy of 500 keV. can it undergo pair production

no; threshold is about 1022 keV, 500 is too low

which photon is more likely to undergo photoelectric absorption in soft tissue: a 15 keV photon or a 100 keV photon

15 keV; photoelectric interactions dominate more strongly at lower photon energies; compton becomes more important at higher diagnostic energies

why are characteristic x-rays different for different elements

each element has different electron binding energies. when electrons transition between shells, the emitted photon energy equals the energy difference between those levels, so the x-ray energy is characteristic of that element

if the linear attenuation coefficient increases, what happens to the number of photons transmitted through the same thickness

fewer photons are transmitted

N_out = N_ine^^-μx; increasing μ makes the exponent more negative

two materials have attenuation coefficients of 0.1 cm^-1 and 1 cm^-1. which attenuates x-rays more strongly

1 cm^-1 ;larger μ = greater probability of interaction per unit distance

a beam passes through 4 half-value layers. approximately what fraction remains

1/8; (1/2)³=1/8

material A has a larger HVL than material B for the same photon beam. which material is less effective at attenuating the beam

A is less effective at attenuating photon beam; larger HVL means more thickness of a material needed to cut beam intensity in half; larger HVL, lower attenuation ability

why does filtration reduce patient dose

it removes low-energy photons likely to be absorbed without helping form the image

a 100-kVp x-ray tube produces bremsstrahlung photons. could it produce a 150-keV x-ray photon

no; maximum photon energy is limited by the electron kinetic energy: E_max=qV; 100 kVp gives a maximum of about 100 keV

a 100-kVp x-ray beam has an average photon energy of 100 keV. true or false?

false; 100 keV is the max possible photon energy. the bremsstrahlung spectrum contains photons from near zero up to that maximum, so the average is lower

why does an x-ray tube require substantial cooling

because only 1% of electron energy becomes x-rays, while approximately 99% becomes heat

an electron passes near the nucleus of the tungsten target, slows down, and releases an x-ray. what type

bremsstrahlung x-ray

an electron knocks a K-shell electron out of tungsten. an L-shell electron then falls into the K-shell vacancy. what kind of x-ray can be emitted

characteristic x-ray; because its energy comes from a specific shell-energy difference

why does bremsstrahlung produce a continuous spectrum while characteristic radiation produces discrete peaks

bremsstrahlung electrons can lose varying amounts of kinetic energy when interacting with nuclei, producing many possible photon energies.

characteristic radiation comes from fixed electron-shell energy differences, so only specific photon energies occur

if mAs is doubles while everything else stays constant, what happens primarily

number of x-ray photons approximately doubles; mAs primarily controls x-ray quantity/fluence

if kVp increases, which of the following generally occurs

maximum photon energy increases, beam penetration increases, x-ray fluence increases; kVp² dependence for x-ray fluence and the tube voltage sets the maximum electron/x-ray energy

why is absorbed dose not enough by itself to describe biological risk

absorbed dose only measures deposited energy per mass. different radiation types can produce different amounts of biological damage, which is why equivalent dose applies a radiation weighting factor

two patients receive the same absorbed dose, but one receives radiation with a larger radiation weighting factor. who has the larger equivalent dose

the one exposed to the radiation with the larger weighting factor

H=Dw_R

why is effective dose different from equivalent dose

equivalent dose accounts for radiation type

effective dose additionally accounts for the fact that different organs/tissues have different sensitivities and combines them into an estimate of whole-body risk

a researcher decreases voxel size in an image and observes that the boundaries now look smoother. They conclude that the scanner’s resolution improved. what is wrong with that conclusion

smaller voxels can change image appearance or sampling, but they do not necessarily change the intrinsic spatial resolution. resampling cannot recover information that the scanner never got

why might increasing spatial resolution make an image noisier if acquisition conditions are otherwise unchanged

smaller voxels generally collect less signal per voxel, so random variations become more prominent relative to the signal. imaging often involves tradeoffs rather than getting every quality parameter better at once

a detector receives photons that did not interact with the patient plus several Compton-scattered photons. why do the scattered photons hurt image quality even though they still provide detector signal

their direction no longer accurately represents where they originated in the body, so they add signal to incorrect locations and reduce contrast

why are low-energy x-rays often undesirable in diagnostic imaging even though lower energy can increase interaction probability

many are absorbed in the patient before reaching the detector. they therefore increase patient dose without contributing useful transmitted information, which is why filtration removes them

a student says “a harder x-ray beam must contain more photons” is that necessarily true

no; “hardness” refers mainly to photon energy/penetrating ability, not the number of photons. filtration hardens the beam while reducing the total number of photons

a student says, “if a beam is attenuated, every photons in it loses some energy.’ why is this wrong

attenuation refers to photons being removed from the original beam through absorption or scattering, the photons that pass through without interaction can retain their original energy

why does increasing atomic number Z tend to make a material a stronger x-ray absorber

Higher-Z materials have greater interaction probabilities for relevant mechanisms, especially photoelectric absorption. This is why high-Z materials are useful as x-ray targets, shielding, and contrast-related materials

what modality is generally best for distinguishing differences among soft tissues

MRi; provides great soft-tissue contrast

a brain image shows the head from the side, dividing the body into left and right portions. the image is a ____ slice

sagittal

a PET scan is aligned with a CT scan so functional information can be compared with anatomical position. this alignment is called

coregistration

a patient moves their head during an MRI acquisition. the unwanted image feature produced by this motion is best described as

artifact

a structure is farther from the trunk than another structure on the same limb. it is described as

distal; further from the point of attachment

a 75 kHz signal must be sampled a a frequency of at least ____ to satisfy Nyquist

150 kHz; twice the highest signal frequency

a signal is sampled every 4 ms. what is the sampling frequency

4 ms = 0.004 s; 1/0.004 = 250 Hz

a detector samples at 800 Hz. what is the highest signal frequency that can be represented without violating nyquist

400 Hz; nyquist frequency is half the sampling frequency

an image changes from 8 bits per pixel to 12 bits per pixel. which property is most directly improved

more bits provide more possible intensity levels; digital contrast resolution

how many possible intensity levels are represented by a 12-bit image

2¹² = 4096

why might a CT system use 4096 intensity levels rather than storing vastly more levels that contain no useful additional information

to optimize file size/data storage; unnecessary bit depth increases storage without useful contrast information

a small lesion and surrounding normal tissue occupy the same voxel, producing the intermediate voxel value. this is the

partial volume effect

which change can reduce partial-volume effects

smaller pixels/voxels

an image is spatially smoothed. what combination is most likely

less noise; worse spatial resolution

an image is resampled from 2 mm pixels to 0.5 mm pixels without acquiring new data. which statement is most accurate

pixel size changes, but intrinsic resolution does not improve

a physician changes the CT window to better visualize soft tissue. what happened to the acquired image data

the underlying data stayed the game; displayed mapping changed

which has better temporal resolution: a system resolving changes in 5 ms or one resolving changes in 2 s

5 ms; smaller time window means better temporal resolution

photon A has a higher frequency than photon B. compared with B, photon A has

higher energy and shorter wavelength; E=hv, c=λv

visible light wavelengths are on approximately which scale

400-700 nm

the approximate speed of light in vacuum is

3 × 10⁸ m/s

which photon is more energetic

200 keV is ten times the photon energy of 20 keV

ionizing an atom requires the incoming energy to be at least approximately equal to the electon’s

binding energy; remove electron from bound state

an outer electron moves to a higher shell but remains bound to the atom. this is

excitation; raises an electron to a higher bound state without removing it

an electron drops into an inner-shell vacancy and the energy difference leaves as a photon. the emitted photon can be a

characteristic x-rays; shell energy differences

which events can create an inner-shell vacancy that later leads to characteristic x-ray emission

an incident electron ejecting an inner-shell electron

an incident photon ejecting an inner-shell electron

a photon changes direction after interacting with an atom but retains essentially the same energy and causes no ionization. this is

rayleigh scattering; direction changes, energy doesn’t

a 60 - keV photon ejects an outer-shell electron carrying 18 keV. neglecting binding energy, what interaction occurred and what is the scattered photon energy

compton; 42 keV

a photon ejects an inner-shell electron and the incident photon disappears. this is

photoelectric absorption; absorbs incident photon

which interaction commonly produces scattered photons that can reach the detector and reduce image contrast

compton scattering; change direction and can add unwanted detector signal

pair production requires photon energy on approximately which scale

1000 keV; threshold is 1022 keV

the probability of photoelectric absorption is approximately proportional to

Z³ and 1/E³

if photon energy increases substantially while Z stays constant, photoelectric absorption probability generally

decreases strongly

if atomic number Z increases while photon energy stats constant, photoelectric absorption probability generally

increases strongly

as the linear attenuation coefficient μ increases for the same material thickness, transmitted x-rays

decreases exponentially

a beam passes through 4 half-value layers. what fraction of the original intensity remains

(1/2)⁴ = 1/16

material A has a smaller HVL than material B for the same beam. which statement is true

A attenuates more strongly

a low-energy, continuous portion of an x-ray spectrum is produced primarily by ____ and can be preferentially removed using ____

bremsstrahlung; beam filtering

why are very low-energy x-rays commonly filtered from a diagnostic beam

they usually increase dose without contributing useful transmitted information

after preferential removal of low-energy photons, the remaining x-ray beam has a higher average energy. this is called

beam hardening

which statement correctly distinguishes beam filtering from collimation

filtering removes selected low-energy photons; collimation restricts beam direction/area

an electron is deflected and decelerated by a target nucleus, producing an x-ray photon. this is

bremsstrahlung

which statement about bremsstrahlung is correct

it produces a continuous spectrum up to a maximum energy

a tube operates at 120 kVp. what is the approximate maximum possible x-ray photon energy

120 keV

if mAs is doubled while other settings remain unchanged, the most direct effect is

approximately twice more x-ray photons are produced

most of the kinetic energy of electrons striking the x-ray tube anode becomes

99% becomes heat; 1% becomes x-rays

which dose quantity measures energy deposited per unit mass without accounting for radiation type or tissue sensitivity

absorbed dose; energy/mass Gy

which pairing of dose quantity and unit is correct

equivalent and effective dose use Sv; absorbed dose uses Gy

a patient receives an absorbed dose D from a radiation type with weighting factor wR. which expression gives equivalent dose

H = Dw_R

why is effective dose different from equivalent dose

effective dose incorporates tissue sensitivity across irradiated organs; overall risk

the unit of the linear attenuation coefficient μ is most appropriately

cm^-1