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Radiology Core > Physics > Flashcards

Physics Flashcards

1
Q

How can you shorten the spatial pulse length (SPL)?

A

By increasing the pulse frequency.

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2
Q

What determines SAR?

A
  • Field strength.
  • Flip angle. -

TR.

Doubling Field strength or flip angle will cause 4x increase in SAR.

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3
Q

What’s an advantage for STIR over T1 fat sat?

A

No need for higher field magnet.

STIR can be obtained in low magnet.

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4
Q

Which direction has the BEST resolution is US?

A

AXIAL; about 1mm. followed by Lateral; about 4 mm.

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5
Q

In X-ray, Scatter DOES NOT affect

A

Spatial resolution or image mottle.

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6
Q

What’s the unit for Linear attenuation coefficients?

A

inverse centimeters (cm –1 ).

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7
Q

What does Linear attenuation coefficientsof 0.1 cm –1 mean?

A

means that 10% of the incident photons are lost (i.e., absorbed or scattered) in 1 cm.

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8
Q

What’s the most important determinant of HVL?

A

The atomic number.

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9
Q

Quality of an x-ray beam is

A

HVL expressed as a thickness of aluminum (mm).

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10
Q

What does filtration do to the beam quality and quantity?

A
  • Increases Quality (more penetrating beam).
  • Decreases Quantity (weaker photons are filtered out).
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11
Q

Does beam hardening occur with monochromatic xray?

A

NO. Because there is no differential energy filtration.

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12
Q

What’s grid ratio?

A

the ratio of the strip height (H) along the x-ray beam direction to the gap (D) between the lead strips (i.e., H/D).

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13
Q

What’s contrast improvement factor?

A

is the ratio of contrast with a grid to contrast without a grid.

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14
Q

What’s the percentage for grid transmission of primary xrays?

A

70%

Primary xrays are the USEFUL xrays.

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15
Q

What’s the percentage of grid absorption of scattered xrays?

A

90%

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16
Q

What’s “average” film density in most film radiography?

A

1.5

The higher the density, the balcker the film.

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17
Q

Film blackening is normally measured using

A

Optical Density (OD).

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18
Q

Unexposed film has an base plus fog level of?

A

0.2 OD

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19
Q

What’s “fog” in film density?

A

Fog is the level of blackening in the absence of any radiation exposure

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20
Q

What’s the maximum film OD?

A

Maximum film OD is 3 OD units which occur when all the grains in the film have been sensitized and reduced to silver grains during development.

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21
Q

What’s the purpose for screens in screen-film set-up?

A
  • They absorb 50 times more photons (intensifying).
  • They convert the x-ray pattern to a light pattern, which is subsequently recorded on radiographic film.
  • They decrease exposure times and patient doses, by 50 times than film alone.
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22
Q

Why Scintillators such as CsI are excellent x-ray absorbers?

A

because of the K-shell binding energies of cesium (36 keV) and iodine (33 keV)

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23
Q

What’s Baud rate?

A

describes the rate of information transfer in bits per second.

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24
Q

How many shades of grey can be coded in 1 byte (8 bit)?

A

256 (2^8)

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25
Q

A monitor where the horizontal dimension is longer is called

A

Landscape display.

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26
Q

A monitor where the vertical dimension is longer is called?

A

Portrait display.

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27
Q

Which type of detector is most likely to offer the lowest patient dose in chest x-ray imaging performed at 120 kV?

A

Scintillator (CsI).

CsI scintillator will absorb most of the incident x-rays generated at 120 kV (chest radiography), and more than the other types of detectors.

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28
Q

Which image processing algorithm is most likely to improve the visibility of tubes, lines, and catheters on bedside chest x-rays?

A

Unsharp mask enhancement.

  • Unsharp masking involves subtraction of a smoothed version from the original which is then added to a replicate original.
  • Unsharp masking also increases noise and may introduce artifacts.
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29
Q

Subject contrast can be positive if

A

the lesion absorbs fewer x-rays compared to the surrounding tissues. (Darker lesion).

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30
Q

Image contrast in screen-film radiography is primarily dependent on

A

film density.

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31
Q

What’s Latitude?

A

Latitude is the range of radiation intensity (K air ) values that result in a satisfactory image contrast.

latitude is K air(max) minus K air(min).

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32
Q

What’s Imaging dynamic range?

A

is the ratio K air (max) : K air (min).

Range is a Ratio

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33
Q

Quantum mottle is quantified as

A

the percentage fluctuations about the mean value

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34
Q

What is the dominant source of random noise in most of x-ray imaging?

A

Quantum mottle

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35
Q

What’s the effect of binning in fluoro?

A

will reduce interpixel fluctuations (noise), but also reduce the spatial resolution.

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36
Q

What’s Sampling frequency for pixles?

A

the number of pixels in each millimeter.

Example: Sampling frequency for 0.5-mm pixels is 2 pixels/mm.

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37
Q

At low spatial frequencies, the MTF is

A

Excellent. Close to 1.0

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38
Q

Image quality is always

A

Task-dependent.

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39
Q

The distribution of pixel values in a uniformly exposed digital x-ray detector is best described as being:

A

Gaussian

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40
Q

Which is the best indicator of the overall visibility of a lesion in a radiograph?

A

SNR

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41
Q

Adding 3 mm aluminum to a typical radiographic x-ray beam (80 kV) is likely to reduce K air by what percentage?

A

50%

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42
Q

What’s entrance air KERMA?

A

is the amount of radiation incident on the patient required to generate a satisfactory image.

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43
Q

What’s The median KAP in radiographic imaging?

A

1 Gy-cm2 .

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44
Q

The median KAP in interventional radiology

A

200 Gy-cm2 .

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45
Q

For fluoroscopy-guided GI studies and urologic procedures, the median KAP is

A

20 Gy-cm2

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46
Q

What should be taken into account when comparing KAP from different imaging systems?

A

must ensure patient sizes are similar, and x-ray beam qualities (penetrating power) are taken into account.

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47
Q

Why An entrance K of 1 mGy can result in a superficial skin dose of up to 1.5 mGy?

A

Tissues absorb 10% more than air (higher Z), and the presence of backscatter can increase superficial tissue doses by up to 40%.

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48
Q

Which type of dose best predicts the likelihood of the relevant bioeffects (i.e., burns, epilation, cataracts)?

A

PEAK SKIN DOSE.

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49
Q

Which type of dose is used to predict the stochastic (cancer) risk?

A

Average dose.

When half the lung is exposed to 3 mGy, and the other half to 1 mGy, the average lung dose (i.e., 2 mGy) is used to predict the lung cancer risk.

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50
Q

What’s the distribution of interacting xray with the patient?

A
  • 2/3 is absorbed.
  • 1/3 is scattered.
  • Less than 1% reached the detector and form the image.
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51
Q

For an abdominal radiograph in a standard-sized patient (AP projection), the embryo dose is about

A

1/3 of the air Kerma.

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52
Q

What’s the main source of embryo dose during chest CT?

A

Internal scatter.

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53
Q

What’s Integral Dose (Energy Imparted)?

A

The integral dose measures the total energy (J) imparted to a patient.

Measured in Joules.

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54
Q

How is energy deposittion of alpha particle different from xray?

A

alpha particles result in a more concentrated pattern of energy deposition than x-rays which produce a more diffuse pattern.

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55
Q

Radiation weighting factor (WR) which depends on which value?

A

Linear Energy Transfer (LET).

Higher LET values generally result in higher WR values.

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56
Q

Higher radiation weighing factors indicate more biologic damage at the same radiation dose. Repeat …

A

Higher radiation weighing factors indicate more biologic damage at the same radiation dose.

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57
Q

Which has higher radiation weighting factor, protons or neutrons?

A

Neutrons; ALWAYS.

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58
Q

Population-averaged incidence of fatal cancer from radiation is currently estimated to be

A

4% per Sv.

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59
Q

Effective doses cannot be indicators of patient risk because these do not account for:

A

Patient demographics. You always need age and sex to determine risk.

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60
Q

Health consequences of cell death occur on time scale measured in

A

Hours and weeks.

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61
Q

What’s is used to triage the severity of acute radiation exposures?

A

Peripheral lymphocyte count. Becaue they are highly sensitive to radiation.

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62
Q

Whole-body doses of higher than 10 Gy would likely kill everyone in 5 to 10 days due to

A

loss of epithelial lining of the GI tract (i.e., GI syndrome).

” you have a 10 meter long GI”.

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63
Q

Whole-body dose of 100 Gy would likely kill everyone in 1 to 2 days from

A

permeability changes in brain blood vessels (i.e., cerebrovascular syndrome).

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64
Q

What’s the time scale for onset of skin deteministic effects?

A
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65
Q

Full recovery is generally expected below what dose?

A

Less than 10 Gy.

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66
Q

Long-term effects of skin doses include:

A
  • telangiectasia,
  • dermal atrophy or induration,
  • with possible late skin breakdown,
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67
Q

When does epilation occur?

A
  • Scalp dose 3-5 Gy. Temporary.
  • Onset 2-3 weeks.
  • Hair regrowing starts 2 months after radiation.
  • Hair might be grey.
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68
Q

Fractionated exposure is better in term of dose and effect EXCEPT in:

A

Male gonads, fractionated exposure to the gonads produces more damage than acute exposure.

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69
Q

Which type of radiation-induced cancer is more likely to occur in children than in adults?

A

Thyroid cancer.

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70
Q

Stochastic risk is based on

A

Linear No Threshold (LNT) model.

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71
Q

What’s The doubling dose?

A

is the absorbed dose to the gonads of the whole population that would double the spontaneous mutation incidence.

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72
Q

carcinogenic effects of fetal exposure is most important in which trimester?

A

3rd timerster.

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73
Q

The hematopoietic syndrome most likely occurs at acute whole-body doses (Gy) of about:

A

4 Gy.

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74
Q

Threshold dose introduced by ICRP (2011) for cataract induction is (Gy):

A

0.5 Gy

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75
Q

Uncertainties in current radiation risk estimates are most likely:

A

Factor of three (e.g., ± x3)

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76
Q

When a 25-year old undergoes a TIPS procedure (effective dose 100 mSv), his/her cancer risk increases from 40% to approximately (%):

A

41%.

Average cancer risk to a 25 year old from 100 mSv is about 1% (a risk of 0.1% that is normally assumed for a dose of 10 mSv).

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77
Q

The National Committee Council on Radiological Protection and Measurements (NCRP) recommends that the operator effective dose be taken to be

A

0.18 of the dose recorded by a dosimeter worn on the collar (H collar ).

Neck size is 18.

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78
Q

How frequent lead aprons should be tested?

A

Annually; by fluoroscopy.

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79
Q

The minimum regulatory lead equivalent lead apron thickness (mm) in the US is currently

A

0.25 mm

Most lead aprons are now 0.5 mm thickness

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80
Q

Minimum thickness of aluminum filtration in xray room walls operating above 70 kvp:

A

2.5 mm

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81
Q

What’s tube loading?

A

Heat energy deposited in the focal spot

82
Q

Heat is transferred from the focal spot into the anode by which method?

A

by conduction

83
Q

Magnitude of the heel effect depends on

A
  • anode angle (inverse),
  • Source to Image Distance (SID) (inverse),
  • field size (proportional)
84
Q

What’s the grid ratio commonly used?

A

10:1

85
Q

What’s exposure index (EI)?

A

to provide a universally understood quantitative measure of Kair at any image receptor.

86
Q

A Kair of 1 microGy at the image receptor corresponds to what exposure index (EI)?

A

an EI of 100.

87
Q

What’s deviation index (DI)?

A

quantifies how closely any given Kair at an image receptor matches the target value.

88
Q

A deviation index DI value of +3 (or −3) indicates what?

A

an exposure that is double (or half) the target K air at the image receptor.

89
Q

What’s AEC?

A

Ionaization chamber placed between the patient and the detector. Functions to terminate the exposure.

90
Q

How bedside CXR are different from the ones done in the department?

A
  • Shorted SID; 100 cm instead of 180 cm.
  • No Grid; difficult to align.
  • No AEC, tech relies on EI for exposure.
  • Lower kvp; 80 instead of 120 to reduce scatter.
91
Q

What additional filter is used in peds xray units and IR?

A

Additional copper filter; to make the beam more prenetrating.

92
Q

What’s the anode heat dissipation rate?

A

10 kW

93
Q

What controls the average x-ray beam in mammography?

A

The K-edge filters; Not the tube voltage.

94
Q

How’s noise reduced in mammography?

A

By using very high radiation intensities at the image receptor Kair.

|In other words; Mammo had much larger mAs compared to radiography.

100 times compared with CXR.

95
Q

What filters are used with Tungsten target in mammo?

A

ONLY Rh (23) and Ag (25) (silver) are used; Mo is NOT used.

96
Q

What’s the grid ratio in mammogram?

A

5:1

Less compton scatter and more PE interaction, hence less ratio needed.

97
Q

In which stage mammogram images are ideal as input for CAD?

A

“For Processing”; After application of detector corrections.

98
Q

How frequent MQSA recertification should be?

A

Every 3 years.

99
Q

What’s the ACR breast phantom composed of?

A
  • 6 fibers
  • 5 speckles
  • 5 masses
100
Q

What’s Tissue HVL in mammography?

A

10 mm.

101
Q

What’s Flux gain?

A

is the number of light photons emitted at the output phosphor for each photon emitted at the input phosphor, which is typically about 50

102
Q

What’s Minification gain?

A

is the increase in image brightness that results from reduction in image size from the input phosphor to the output phosphor. Typically about 100 times.

103
Q

What’s brightness gain?

A

is the product of the flux gain (∼50) and minification gain (∼100), or about ~5,000.

104
Q

Last Image Hold is required by which regulatory body in th US?

A

FDA!!

105
Q

What’s the other name for frame averaging in fluoro?

A

Temporal filtering

106
Q

What’s frame averaging?

A

A technique of adding and averaging pixel values in successive images.

  • Temporal filtering occurs in real time and reduces the effect of random noise.
107
Q

How much of the xray beam is attenuated by the fluoro table?

A

1/3 of the beam. A third

108
Q

How’s Kair chnage with Electronic Magnification?

A
  • Doubled in Mag 1
  • Quadruples in Mag 2
  • Eightfold in Mag 3
109
Q

How does the ABC work during electronic magnification\?

A

Since the irradiated area on the II is smaller during elec mag (reduced minification gain, i.e. image brightness); the ABC increases the beam intensity (Kair) at the receptor to maintain image brightness.

110
Q

What’s the effect of Electronic magnification on KAP?

A

No change

111
Q

How the stochastic effect in Electronic magnification is different from regular fluoro?

A

Unchanged; because the total radiation incident on the patient is not changed.

112
Q

Replacing continuous fluoro by pulsed fluoroscopy (30 pulses/second) results in

A

- Sharper images (i.e., reduced motion blur).

  • Patient doses are unchanged sice frame rate is unchanged (i.e., same mAs per frame).
113
Q

Switching from 30 to 15 fps would thus reduce patient doses by about

A

35%.

Pulsed fluoroscopy with reduced frame rates uses a higher dose per frame to reduce the perceived level of random noise.

114
Q

How does collimation and magnifiation change resolution?

A
  • Collimation: No change.
  • Electronic Mag: Improved.
115
Q

How does collimation change Kair and KAP?

A
  • Kair: Unchanged.
  • KAP: Reduced.
  • Image contrast: Improved; due to reduced scatter.
116
Q

How’s peak skin dose different between electonic mag and collimation?

A
  • Electronic Mag: Increased. (higher risk for burns).
  • Collimation: No chnage.
117
Q

In fluoro, II resolution is determined by

A

input phosphor (CsI) thickness

118
Q

Which image intensifier component absorbs light and emits low-energy electrons?

A

Photocathode

119
Q

When switching from normal mode to Mag 1 (electronic magnification), the area exposed is most likely reduced to:

A

50%

120
Q

Heel effect depends on

A
  • Field size
  • Anode angle
  • SID
121
Q

What happens to Kair in FPD with smaller FOV?

A

Increases. To reduce mottle perceived mottle by radiologists.

Increases in K air with reduced FOV are programmed in by the vendor.

122
Q

Difference between Kair in II and FPD when reducing FOV?

A
  • II: When halving the FOV (by electronic mag); K air will be quadrupled.
  • FPD: When halving the FOV (by electronic mag); K air will be doubled.
123
Q

Difference in resolution between II and FPD?

A

FPDs have slightly better resolution; 3 lp/mm without binning.

Standard fluoroscopy resolution is about 1 lp/mm using a 500 line TV.

124
Q

What additional cover FPDs have over II?

A

Detectors in IIs and FPDs are similar (400 micrometer CsI), but FPDs have a carbon cover which transmits slightly more x-rays than the 1.5 mm Al used in image intensifiers.

  • At 70 kV, an FPD detects more photons than an II (e.g., 90% vs. 75%).
125
Q

What’s interventional reference point (IRP)?

A

is an imaginary point, 15 cm closer to the focal spot than the system isocenter.

  • IR gantries rotate around the isocenter, located close to the center of the patient.
126
Q

How’s interventional reference point (IRP) measured?

A

is always measured free in air (not tissue) and excludes patient backscatter.

127
Q

How’s peak skin dose relate to intervenvtional reference point\?

A

PSD is always less than IRP; likely because it includes table attenuation and backscatter while IRP doesn not.

Therefore, Dose Index is between 0.5-0.8

Dose Index: PSD/IRP Kair

128
Q

The Joint Commission (JC) defines an unintended skin dose in excess of 15 Gy as

A

Sentinel event.

  • A sentinel event requires a full root cause analysis, and a visit by a JC Inspector.
129
Q

in FPDs, what happens to skin dose and effective dose when FOV is halved?

A
  • Skin Dose: +50% (to correct for mottle)
  • Effective dose: -25%
130
Q

Best method to reduce eye lens dose in IR?

A

By placing a hang-down transparent leaded glass barrier between the operator and the patient.

Leaded glasses only reduces 65% of the dose while lead shield reduces 90%.

131
Q

in CT, the distance from the focal spot to the isocenter is

A

60 cm.

This results in 2-fold geometric magnification for object at the isocenter.

132
Q

Benefits of Bow tie filters?

A
  • minimize beam hardening differences with tissues.
  • ensure all detectors receive similar exposures
  • reduced dynamic range
133
Q

What’s the fan beam angle in CT?

A

50 degrees.

134
Q

What’s the beam width in 64-MDCT scanner?

A

40 mm

135
Q

in CT, what’s the effective mAs?

A

true mAs/pitch

136
Q

CT HU values generally depend on

A

kV, filtration and reconstruction algorithm.

137
Q

Compared to radiographic x-ray tubes, those used in CT most likely have much higher:

A

Anode capacity; 10 times higher (better dealing with heat).

138
Q

1 HU corresponds to an increase of

A

0.1% in attenuation.

139
Q

What determines in plane spatial resolution in CT (along beam axis)?

A
  • Focal spot size.
  • Detector size.
140
Q

What determines the spatial resolution in the longitudinal direction (along the long patient axis) in CT?

A

Detector thickness

141
Q

Beta particle range increases with

A
  • increasing beta particle energy (e.g., 82 Rb)
  • low-density tissues such as lung.
142
Q

After an isomeric transition, both parent and daughter nuclei have

A

the same mass number and atomic number

143
Q

After 10 half-lives, how much of the initial activity remains?

A

0.1%

144
Q

What’s Magnetophosphenes?

A

Magnetophosphenes are flashes of light (phosphenes) that are seen when one is subjected to a changing magnetic field.

More with higher fields

145
Q

After how long full net longitudinal magnetization is achived?

A

after 4 × T1.

146
Q

What happens to T1 time when the field strength is quadrupled?

A

It doubles

147
Q

Increasing spin–lattice interactions results in

A

Shorter T1. (more relaxation)

148
Q

How to reduce problems associated with eddy currents?

A

By placing Actively shielded gradient coils

149
Q

What material lines Faraday cage?

A

Copper.

150
Q

in GRE, what happens at TE/2?

A

Reversal of the dephasing gradient.

151
Q

What direction is the reversed gradient applied in GRE?

A

Frequency encoding

152
Q

In GRE, which weighting has larger flip angle?

A

T1: 45 degree angle

T2: 10 degree

T2*: 10 degree

153
Q

What’s the main determinant of image contrast with GRE sequences?

A

T2*

154
Q

What’s the name of the RF pulse applied at TI (inversion)?

A

Read-out pulse.

155
Q

Effects of diffusion gradient depend on

A
  • gradient strength (mT per meter),
  • gradient duration,
  • time between gradients.
156
Q

How are diffusion gradient applied in SE?

A

DWI gradients are applied on either side of the 180-degree refocusing pulse in an SE sequence

157
Q

How the signal from TOF is obtianed?

A

Unsaturated “fresh” blood comes into the slice and gives signal.

158
Q

How the phase contrast signal is obtained?

A

By applying a bipolar gradient to detect moving spins; and then this gradient is reversed on the next excitation.

Black represents the maximum flow in one direction, white corresponds to the maximum flow in the opposite direction, and gray indicates stationary tissues.

159
Q

What are the 3D MRI planes?

A
  • Two sets of orthogonal phase-encoding gradients are applied along z and y directions. (in-pane and thru-plane)
  • Echoes are sampled with frequency-encode gradient along x direction.
160
Q

How’s Echo planar imaging (EPI) done?

A

90-degree RF pulse followed by Rapidly switched gradients are applied in the frequency-encode direction.

Each echo is preceded by a different phase-encode gradient.

161
Q

How does O2 affect T2* effect?

A

It shields the hemoglobin iron atoms and reduces dephasing of adjacent protons. Therefore, it REDUCES T2* effect.

162
Q

What’s fMRI sequence?

A

EPI with T2* weighting.

163
Q

What’s magnetization transfer?

A

Magnetization transfer. An specially designed RF pulse (called an MT Pulse) is applied which selectively injects energy into the bound pool of protons (macromolecules and bound water). This energy is then transferred (primarily by dipolar-dipolar interactions) to the free water pool, partially saturating it.

This results in lower T2 signal from the free water molecules in subsequent images.

164
Q

What are some applications for magnetization transfer contrast?

A
  • improving contrast especially in MR angiography.
  • MS protocol.
165
Q

in MRS, Frequency differences of metabolites are measured in

A

parts per million (ppm).

166
Q

What are the nuclei most often used for in vivo localized spectroscopy?

A
  • 1H - voxel size: 1 cm3
  • 31Phosphorus - voxel size: 8 cm3
167
Q

What’s the most important determinant of image quality in MRI?

A

SNR

168
Q

Four acquisitions of any image will quadruple the (deterministic) signal, but will only double the (random) image noise. Repeat …

A

Four acquisitions of any image will quadruple the (deterministic) signal, but will only double the (random) image noise.

169
Q

What’s the Limit for whole-body heating in normal mode limit? First and 2nd levels?

A
  • Normal mode: 0.5 degrees Celsius
  • First-level controlled mode (medical supervision): 1 Cْ
  • second-level controlled mode (temperatures exceeds 1°C) requires IRB approval.
170
Q

MR spectroscopy most likely requires the use of exceptionally:

A

Uniform magnetic fields

171
Q

What’s Acoustic impedance?

A

product of the density (r) and the sound velocity (v) in the material, which is expressed in rayls.

172
Q

When ultrasound passes from one tissue to another having a different speed for sound,

A

the frequency remains the same but the wavelength changes.

173
Q

Resonance frequency in US is determined by

A
  • the piezoelectric element thickness.
  • High-frequency transducers are thin, and vice versa.
  • thickness (t) is equal to one-half of the wavelength
174
Q

What’s the purpose of damping material in US probe?

A

to reduce vibration (ring-down time) to shorten pulses. Shorter SPL.

Placed behind transducers.

175
Q

What’s the purpose of the matching layer in the US probe?

A
  • To improve the efficiency of energy transmission into (and out of) the patient.
  • Matching layer material(s) has an impedance value that is intermediate between that of the transducer and that of the tissue.
176
Q

What’s the thickness of the matching layer in US probe?

A

one-fourth the wavelength of sound in that material (quarter wave matching).

177
Q

The length of the near field in US is proportional to

A

the effective transducer size.

Doubling the transducer effective size will quadruple the length of the near field.

178
Q

The major benefit of focusing ultrasound beams is

A

to improve lateral resolution.

179
Q

in US, electronic focusing introduces flexibility that allows image quality to be markedly improved. Repeat…

A

in US, electronic focusing introduces flexibility that allows image quality to be markedly improved.

180
Q

Which frequency (MHz) is most likely a harmonic frequency of a 2.5-MHz transducer?

A

5 MHz is twice the transducer frequency (2.5 MHz), and would be the transducer’s first harmonic frequency (i.e., needs to be an exact multiple of the fundamental frequency).

181
Q

The thickness of an ultrasound transducer that generates sound with a wavelength λ is most likely:

A

λ/2

Transducer thickness is generally half the wavelength, so the thickness of a 1.5-MHz transducer (λ = 1.0 mm) is 0.5 mm.

182
Q

When a transducer element diameter is doubled, the near field length is likely:

A

(Quadrupled). Doubling the transducer element diameter will quadruple the transducer near-zone distance.

183
Q

Which focusing method is likely to offer variable focal depths?

A

Phased array.

Multielemental transducers (e.g., phased arrays) can adjust the focal depth by varying the time delays of electrical pulses to individual elements.

184
Q

What’s line density (LD) in US?

A

is the number of lines per image divided by the corresponding field of view (FOV)

185
Q

How can line density (LD) be increased?

A
  • reducing the frame rate (affects temp resol),
  • reducing the FOV,
  • or increasing the PRF (depth will decrease).
186
Q

What’s Pulse repetition frequency (PRF)?

A

Pulse repetition frequency (PRF) refers to the number of separate pulses (i.e., lines of sight) sent out every second.

  • Common PRF value is ~4 kHz (i.e., or 4,000 pulses per second).
187
Q

How’s 1.5D arrays transducer different than 2D arrays?

A
  • 1.5D arrays have a large number of transducer elements in the scan plane (e.g., 192) and a small number (e.g., 6) in the slice thickness direction
  • Focusing the small number (e.g., 6) transducer elements can be used to reduce the slice thickness and improve elevational resolution.
188
Q

Benefits of Spatial compound imaging?

A
  • reduces angle-dependent artifacts and clutter,
  • providing improved contrast and margin definition.
189
Q

Which harmonic is mostly used?

A

The first harmonic (twice the fundamental frequency) is most frequently used.

Harmonic imaging reduces artifacts and clutter.

190
Q

Which patient group will benefit the most from harmonic imaging?

A

Patients with thick and complicated body wall structures

191
Q

What’s the principle of CEUS?

A

large difference in acoustic impedance between the gas and surrounding fluids/tissues creates signal.

Microbubbles are encapsulated and smaller than RBC size.

192
Q

How does Pulse inversion harmonic imaging work in CEUS?

A
  • Pulse inversion harmonic imaging uses two pulses, consisting of a standard plus inverted (phase reversed) along the same beam direction.
  • These two cancel out for soft tissues, but not for microbubbles, improving the sensitivity of ultrasound to contrast agents.
193
Q

What’s the relationship between Doppler frequency shift and reflector velocity?

A
  • At a given Doppler angle, the Doppler frequency shift is directly proportional to the reflector velocity.
  • Doubling the reflector velocity generally doubles Doppler frequency shifts.
194
Q

How PRF values for Doppler compared to B-mode?

A

PRF values are generally higher in Doppler than B-mode imaging (e.g., 8 kHz).

195
Q

What’s the most important determinant of lateral resolution in US?

A

Ultrasound beam width ( the narrower the better).

196
Q

Which US resolution is independent of depth?

A

AXIAL.

Both lateral and elevational resoltuions are affected by depth.

197
Q

What are the QC tests recommended by the ACR Accreditation program?

A

Performed semi-annually.

198
Q

Representative intensities in B-mode ultrasound?

M-Mode?

Doppler?

A
  • B-mode: 10 mW/cm2.
  • M-Mode: 40 mW/cm2.
  • Doppler: 500 mW/cm2.
199
Q

What’s the most important factor for tissue heating in MRI?

A

surface area-to-volume ratio

The higher the ratio (neonates), the more effective exccess heat is released.

200
Q

What’s the explanation for “ants crawling on the skin” feeling during MRI?

A

Peripheral nerve stimulation

201
Q
A

Radiology Core (9 decks)

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