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KL Boards > Rad + CT + MRI + US Physics > Flashcards

Rad + CT + MRI + US Physics Flashcards

1
Q

How is exposure defined? What are its units?

A

Exposure = total electrical charge per unit mass of dry air generated by x-rays or gamma rays (up to 3 MeV) Units: R or C/kg

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2
Q

How is absorbed dose defined? What are its units?

A

Absorbed dose = amount of energy per unit mass absorbed by the irradiated object Units: Rad or Gy

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3
Q

What is the quality factor? What are its units?

A

QF = weighting factor The factor by which the absorbed dose must be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage to the exposed tissue. It’s unitless!

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4
Q

What are the two values for QF and what types of ionizing radiation fall under each category?

A

QF 1 = x-rays, gamma rays, electrons/beta particles QF 20 = fast neutrons, protons, alpha particles

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5
Q

What is equivalent dose and how is it calculated? What are its units?

A

ED = AD x QF Calculated measurement of biological damage that factors in the type of radiation encountered. Units: REM or Sv

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6
Q

Conversion between R and C/kg

A

1 R = 2.58 x 10^ -4 C/kg

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7
Q

Conversion between rad and Gy

A

100 rad = 1 Gy

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8
Q

Conversion between REM and Sv

A

100 rem = 1 Sv

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9
Q

What is a scintillator?

A

A material that emits visible light or UV light after interaction with ionizing radiation

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10
Q

What is the major difference between fluorescence and phosphorescence? How is this exploited in imaging?

A

Fluorescence: light is emitted immediately upon encountering incident radiation –> used for screen film Phosphorescence: light emission is delayed over the course of minutes to hours, can be released by shining light of a specific color onto the phosphor –> used for CR

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11
Q

What is the inverse square law?

A

As distance from the source increases, beam strength (intensity) decreases At twice the distance there are one quarter the photons per area

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12
Q

What is the direct square law and why is it important?

A

Calculation of the mAs needed to maintain optimal density at varying SID (source-to-image distance)

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13
Q

What mAs must be used at 30 cm SID to maintain the same radiographic density obtained at 40 cm with 100 mAs?

A
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14
Q

What is the speed of light?

A

c = 3 x 10^ 8 m/sec

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15
Q

How do you calculate the energy of a photon?

A
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16
Q

What is an angstrom?

A

1 A = 10-10 m

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17
Q

What is the range of wavelengths of diagnostic xrays?

A

0.01 - 1 nm

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18
Q

How is the intensity of an x-ray beam defined?

A

Intensity = NUMBER OF PHOTONS in the beam multiplied by the ENERGY of each photon

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19
Q

How do the following factors influence the intensity of an x-ray beam?

  1. Increase kV
  2. Increase mA
  3. Add filtration
  4. High atomic number target material
  5. Increased distance from the source
A
  1. Increase kV –> increase in intensity (more photons)
  2. Increase mA –> increase in intensity (more electrons drawn across the tube and available to interact with the target)
  3. Filtration –> decrease in intensity (removes unwanted low energy photons, thus fewer photons)
  4. High atomic number target material –> increase in intensity (higher characteristic radiation energy)
  5. Increased distance from the source –> decrease in intensity (Inverse square law)
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20
Q

What is the half-value layer? How is it calculated?

A

A measure of penetrating power of the xray beam. It is the amount of matter required to attenuate the beam to half its initial energy.

HVL = 0.693/μ

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21
Q

What is the linear attenutation coefficient? How is it calculated?

A

LAC (μ) = the fraction of photons removed (attenuated) from a monoenergetic beam of xrays or gamma rays per unit thickness of material (x).

Unit = cm-1

N = N0e-μx

or (solving for μ)

μ = ln(No/N)/x

Where N0 = number of incident photons

N = number of photons transmitted through material

x = thickness of material

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22
Q

What is the mass attenuation coefficent (MAC)?

A

The LAC normalized for tissue density.

MAC = LAC/density

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23
Q

ACVR Guidelines for Imaging

What is the minimum standard for spatial resolution of a digital radiographic device?

A

2.5 lpm

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24
Q

What is the equation for HU?

A
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25
Q

Voxel size is dependent on what factors?

What is the equation for voxel size?

A
  • Matrix size
  • FOV
  • Collimator width

Voxel size (mm3) = FOV/matrix * slice thickness

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26
Q

What is the primary type of x-ray interaction in CT?

A

Compton

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27
Q

Name the generation(s) that correspond to each of the following characteristics/technologies:

  • Rotate/translate
  • Rotate/rotate
  • Stationary/stationary
  • Helical
  • Rotate/stationary
  • 360* array of detectors
  • Slip-ring technology
A
  • Rotate/translate: 1, 2
  • Rotate/rotate: 3
  • Stationary/stationary: 5
  • Helical: 6, 7
  • Rotate/stationary: 4
  • 360* array of detectors: 4
  • Slip-ring technology: 6, 7
28
Q

What is the effect of increasing slice thickness on the following parameters:

  • SNR
  • Spatial resolution
  • Partial volume averaging
  • Contrast resolution

If you decrease slice thickness, what technique parameter is usually changed to compensate?

A
  • Increasing slice thickness
    • SNR – increased
    • Spatial resolution – decreased
    • Partial volume averaging – increased
    • Contrast resolution – increased
  • When using thin slices, mAs is usually increased to compensate for the decreased SNR
29
Q

Which of the following will reduce patient dose from CT:

  • Use of a higher kVp
  • Use of a higher mA
  • Use of a higher pitch
  • Use of a narrow collimation in a single slice CT
  • Use of 4 x 2.5mm collimated slices to create a 10mm reconstructed slice instead of 2 x 5mm collimated slices in a multiple detector CT.
A

Higher pitch

30
Q

Describe each of the following scenarios for a single-detector system:

  • Pitch = 1
  • Pitch < 1
  • Pitch = 2
  • Pitch > 2

What is the ideal pitch for the balance of image quality and beam efficiency?

A
  • Pitch = 1 – no gaps
  • Pitch < 1 – overlap among helices
  • Pitch = 2 – 180* rotation brings the table a distance of one collimator width
  • Pitch > 2 – gaps in reconstruction

Ideal = 1.4

31
Q

If you see a large thin ring artifact on an image generated by a third generation scanner, what does this most likely indicate?

A

A nonfunctioning detector near the outside of the detector row.

Smaller ring = near the center

32
Q

What are the relative advantages and disadvantages of helical CT vs. axial CT?

A
  • Advantages
    • After acquisition of a spiral volume of data set, the images can be reconstructed at arbitrarily determined positions and intervals along the z-axis.
    • Faster
    • Uniform sampling along the z-axis of the patient
    • Optimizes the enhancement effect of contrast material due to short scan time
  • Disadvantages
    • Inherent inconsistency in the acquired projections because of the constant patient translation leading to image artifacts
    • Increases heat loading; limited by the power of the system.
    • Increased processing time due to z-interpolation
    • High pitch results in decreased z-axis resolution.
    • Slightly decreased resolution on axial images
33
Q

Define window level and window width

A

WL: central value of the window

WW: range of HU displayed

34
Q

In what scenarios would you use a wide window? A narrow window?

A
  • Wide window = inherently high contrast tissues
    • Lung & bone
  • Narrow window = inherently low contrasted tissues
    • Soft tissue
    • Allows you to see smaller differences in attenuation
35
Q

What is the optimal window width and level for examination of CT images of the following:

  • Canine pituitary gland
  • Lungs
  • Abdomen
A
  • Pituitary: WL: 80; WW: 250
  • Lungs: WL: -600; WW: 1600
  • Abdomen: WL: 20; WW: 320
36
Q

With increased matrix size, what happens to:

  • Pixel size
  • Spatial resolution
  • SNR
A
  • Pixel size: decreases
  • Spatial resolution: increases
  • SNR: decreases
37
Q

What is the main effect of a change to the exposure technique (kVP and mAs) in CT?

How do you reduce tube cooling time with respect to these parameters?

How do you adjust mAs for changes in slice thickness?

A
  • No over- or underexposure in CT; main effect of changing kVP and mAs technique is on noise and penetration
  • Reduce cooling time by reducing mAs and kVP
  • For thinner slices, must increase mAs
38
Q

What is noise? How does it vary with changes in mAs and slice thickness?

A

Noise: statistical variation in photons results in the same tissue appearing to have different HU measurements.

Noise increases when mAs and slice thickness are decreased

39
Q

Sharp reconstruction algorithms improve _________ resolution, but also enhance ________.

Fill in the blanks with: contrast resolution; spatial resolution; axial resolution; noise; artifact; blurring

A

Sharp reconstruction algorithms improve spatial resolution, but also enhance noise and artifacts.

40
Q

What is the factor of increased radiation dose relative to 3v CXR when performing a thoracic CT?

A

50x dose

41
Q

The effective dose to a large dog undergoing a pre- and post-contrast thoracic CT would most likely be in the range of….?

What about a head CT?

A

Thorax: 5-10 mSv

Head: 2 mSv

42
Q

Define diamagnetic, paramagnetic, and superparamagnetic. Give some examples of each.

A
  • Diamagnetic: substance with no intrinsic magnetic moment and thus negative magnetic susceptibility.
    • Ex: water, most organic matter
  • Paramagnetic: material with unpaired electrons, which allows it to magnetize structures around it; small magnetic susceptibility.
    • Ex: Gadolinium, deoxyhemoglobin, methemoglobin
  • Superparamagnetic: material that can substantially augment the magnetic field and exhibit self-magnetism; large magnetic susceptibility
    • Ex: iron, cobalt, nickel
43
Q

What is the Larmour equation?

What is the gyromagnetic ratio of hydrogen?

A

ωØ = γBØ

  • Where:
    • γ = gyromagnetic ratio (MHz/T)
    • BØ = magnetic field strength (T)
    • ωØ = angular frequency (#rotations/sec)
  • γ of hydrogen = 42.6 MHz/T
44
Q

What is T1 relaxation?

What is T2 relaxation?

What is T2* relaxation?

A
  • T1 = spin-lattice = longitudinal relaxation
    • Time it takes for spins to recover 63% of longitudinal magnetization (Mz)
  • T2 = spin-spin = transverse relaxation
    • Time it takes for spins to dephase to 37% of the maximal transverse magnetization (Mxy)
    • This is an idealized version of what happens
    • T2* is a slightly faster version of this due to
45
Q

True/False:

  • T1 is significantly shorter than T2 relaxation
  • B0 significantly influences T1 but not T2 decay
  • Images acquired with a TE = 10 msec and TR = 2,000 msec will be T1-weighted
  • Images acquired with a TE = 10 msec and TR = 500 msec will be T1-weighted
  • PD images are acquired with a short TR and a long TE
A
  • T1 is significantly LONGER than T2 relaxation
  • B0 significantly influences T1 but not T2 decay (TRUE)
  • Images acquired with a TE = 10 msec and TR = 2,000 msec will be T1-weighted (FALSE)
  • Images acquired with a TE = 10 msec and TR = 500 msec will be T1-weighted (TRUE)
  • PD images are acquired with a short TE and a long TR
46
Q

How can you calculate acquisition time?

A

AT = TR x Np x NEX

Where:

  • Np = number of phase encoding steps
  • NEX = # signal averages
47
Q
  • How is slice thickness related to bandwidth and gradient strength?
  • How does this relate to SNR?
A
  • With a fixed gradient:
    • Narrow bandwidth = thin slice
    • Wide BW = thicker slice
  • With a variable gradient:
    • High gradient = thin slice
    • Low gradient = thicker slice
  • SNR is proportional to 1/√BW
    • Thus narrow BW increases SNR
48
Q

K-space

  • The periphery of K-space corresponds to ________ and the center of K-space corresponds to ________.
  • Each phase encoding step contributes to what portion of the k-space?
A
  • The periphery of K-space corresponds to spatial resolution and the center of K-space corresponds to contrast resolution and SNR.
  • Each phase encoding step completes one row of the k-space matrix
49
Q

The signal generated by a given tissue after a 90* RF pulse is dependent on three intrinsic factors. What are they?

A
  1. Proton density
  2. T1 relaxation
  3. T2 relaxation
50
Q

What pulse sequence is this diagram illustrating?

A

FSE

51
Q

Name the pulse sequence

A

SE

52
Q

Name the pulse sequence

A

GRE

53
Q
  • Image contrast in gradient echo pulse sequences depend on what factors?
  • How do you produce a T1w, PDw, or T2w image with a GRE pulse sequence (FA, TE)?
A
  • Image contrast in GRE sequences depends mainly on FA (flip angle) and TE; also depends on proton density, T1, T2*, and TR.
  • T1w: large FA, short TE
  • PDw: medium FA and short TE
  • T2w: small flip angle and long TE –> this is the T2*w GRE used to amplify susceptibility artifact for imaging hemorrhage
54
Q

In MRI, spatial resolution depends on what factors (7):

A
  • FOV - determines the voxel size - ↓ FOV - ↑ SR 

  • Gradient field strength - less partial volume averaging 

  • Receiver coil characteristics (head coil, body coil, surface coils) 

  • Sampling bandwidth: ↓ BW - ↑ SR 

  • Image matrix: ↑ matrix - ↑ SR 

  • Slice thickness 

  • Strength of magnetic field 

55
Q
  • An air core magnet is found in ______ and ______ types of magnets; a solid core is found in _______ and ________ magnets.
  • In an air core magnet, B0 runs in which direction? In a solid core magnet?
A
  • An air core magnet is found in resistive and superconducting types of magnets; a solid core is found in resistive and permanent magnets.
  • In an air core magnet, B0 runs parallel to the long axis of the cylinder (horizontal magnet). In a solid core magnet, B0 runs in a vertical direction between poles of the magnet.
56
Q

For each of the following parameters, determine which would be associated with a horizontal magnet or a vertical magnet:

  • Solid-core magnet
  • Resistive or superconducting
  • Higher SNR
  • Permanent
  • More susceptible to external RF interference
A
  • Solid-core magnet: vertical
  • Resistive or superconducting: horizontal
  • Higher SNR: horizontal
  • Permanent: vertical
  • More susceptible to external RF interference: vertical
57
Q

What type of coil is illustrated by this diagram? What is the advantage of this coil type?

A

Quadrature coil: circularly polarized, so functions as two coils at right angles to one another. This increases SNR.

58
Q

What is the 5 gauss line?

A

0.5 mT line that should be marked and should not be passed by anyone with a pacemaker

59
Q

MRI Safety:

  • What is specific absorption rate and upon what factors does it depend?
  • What is the FDA limit for permissible increased in body temperature during an MRI?
  • What are some contraindications to MRI?
A
  • SAR = energy absorbed by the patient during a scan
    • Increases by the square of the magnet strength (3T has 9x higher SAR than 1 T)
    • Increases with flip angle and size of the patient
  • FDA limit = 1* C
  • Contraindications:
    • Active electronic device 

    • Cerebral aneurysm clip 

    • Intraocular metal fragments 

    • Ferromagnetic foreign bodies 

    • Any unfamiliar device 

60
Q
  • What factors affect axial resolution in CT?
  • Longitudinal resolution?
  • Contrast resolution?
A
  • Axial resolution (MFPS = Mother Fucking Snakes on this Plane)
    • Matrix size
    • FOV
    • Pixel size
    • Slice thickness
  • Longitudinal resolution (DCPR)
    • Detector array thickness
    • Collimation
    • Pitch
    • Reconstruction interval
  • Contrast resolution
    • mAs
    • FOV
    • Slice thickness
    • Tube voltage
    • Reconstruction algorithm
    • Size of patient
    • Noise
    • Rotation speed
61
Q
  • What equation relates wavelength, frequency, and velocity of sound?
  • What is the constant used for the speed of sound through tissue?
  • What factors affect propagation speed?
A
  • c = λ f
  • c = 1540 m/s
  • Propagation speed is influenced by tissue density and stiffness
62
Q
  • At what angle is maximum doppler shift detected?
  • What is the Nyquist limit?
A
  • Max when probe is parallel to flow; 0 if perpendicular
  • Nyquist limit = 1/2 PRF
    • PRF = (# pulses/sec)
63
Q

How can you correct aliasing?

A
  • Increase PRF
  • Move baseline
  • Decrease transmitted frequency –> decreased resolution
  • Reduce the depth of the range gate
  • Switch to CW Doppler
  • Increase Doppler angle
64
Q
  • Describe each of the following:
    • Pulse wave doppler
    • Continuous wave doppler
    • Color flow doppler
    • Power doppler
A
  • Pulse wave doppler:
    • Sampling flow in a specific, gated area
    • Describes pattern of flow (laminar, etc)
  • Continuous wave doppler
    • Information from every point along the length of the beam
    • Not subject to aliasing
  • Color flow doppler
    • Flow directionality and magnitude
  • Power doppler
    • Flow amplitude/magnitude without directionality
    • Less sensitive to angle and can display flow perpendicular to the beam
    • More sensitive to low flow velocities
65
Q

Umbilical US of calves and foals:

  • What are the normal sizes of the umbilical vein in calves and foals?
  • What is the normal size of the umbilical artery in foals? What does it look like in calves?
  • What is the normal size of the combined umbilical arteries and urachus in the foal?
A
  • Umbilical vein
    • Calves: 10-25 mm
    • Foals: < 10mm
  • Umbilical arteries
    • Calves: won’t extend cranial to the bladder apex after 1 week of age
    • Foals: < 12 mm
  • Combined urachus and arteries (foals only)
    • < 25mm

Magri, M. (2018) Ultrasonography of the umbilical remnant in foals. In Practice 40, 301–305

Steiner, A. & Lejeune, B. (2009) Ultrasonographic Assessment of Umbilical Disorders. Veterinary Clinics of North America - Food Animal Practice 25, 781–794

66
Q
A

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