Physics Flashcards
1
Q
How can you shorten the spatial pulse length (SPL)?
A
By increasing the pulse frequency.
2
Q
What determines SAR?
A
- Field strength.
- Flip angle. -
TR.
Doubling Field strength or flip angle will cause 4x increase in SAR.
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.
4
Q
Which direction has the BEST resolution is US?
A
AXIAL; about 1mm. followed by Lateral; about 4 mm.
5
Q
In X-ray, Scatter DOES NOT affect
A
Spatial resolution or image mottle.
6
Q
What’s the unit for Linear attenuation coefficients?
A
inverse centimeters (cm –1 ).
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.
8
Q
What’s the most important determinant of HVL?
A
The atomic number.
9
Q
Quality of an x-ray beam is
A
HVL expressed as a thickness of aluminum (mm).
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).
11
Q
Does beam hardening occur with monochromatic xray?
A
NO. Because there is no differential energy filtration.
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).
13
Q
What’s contrast improvement factor?
A
is the ratio of contrast with a grid to contrast without a grid.
14
Q
What’s the percentage for grid transmission of primary xrays?
A
70%
Primary xrays are the USEFUL xrays.
15
Q
What’s the percentage of grid absorption of scattered xrays?
A
90%
16
Q
What’s “average” film density in most film radiography?
A
1.5
The higher the density, the balcker the film.
17
Q
Film blackening is normally measured using
A
Optical Density (OD).
18
Q
Unexposed film has an base plus fog level of?
A
0.2 OD
19
Q
What’s “fog” in film density?
A
Fog is the level of blackening in the absence of any radiation exposure
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.
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.
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)
23
Q
What’s Baud rate?
A
describes the rate of information transfer in bits per second.
24
Q
How many shades of grey can be coded in 1 byte (8 bit)?
A
256 (2^8)
25
A monitor where the horizontal dimension is longer is called
Landscape display.
26
A monitor where the vertical dimension is longer is called?
Portrait display.
27
Which type of detector is most likely to offer the lowest patient dose in chest x-ray imaging performed at 120 kV?
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.
28
Which image processing algorithm is most likely to improve the visibility of tubes, lines, and catheters on bedside chest x-rays?
**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**.
29
Subject contrast can be positive if
the lesion absorbs **fewer** x-rays compared to the surrounding tissues. **(Darker lesion).**
30
Image contrast in screen-film radiography is primarily dependent on
**film density.**
31
What's Latitude?
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).**
32
What's Imaging dynamic range?
is the ratio K air (max) : K air (min).
**R**ange is a **R**atio
33
Quantum mottle is quantified as
the **percentage fluctuations** about the mean value
34
What is the dominant source of **random noise** in most of x-ray imaging?
Quantum mottle
35
What's the effect of binning in fluoro?
will **reduce** interpixel fluctuations (**noise**), but also **reduce** the **spatial resolution.**
36
What's Sampling frequency for **pixles**?
the number of **pixels** in each **millimeter**.
Example: Sampling frequency for 0.5-mm pixels is 2 pixels/mm.
37
At low spatial frequencies, the MTF is
Excellent. Close to 1.0
38
Image quality is always
Task-dependent.
39
The distribution of pixel values in a uniformly exposed digital x-ray detector is best described as being:
Gaussian
40
Which is the best indicator of the overall visibility of a lesion in a radiograph?
SNR
41
Adding 3 mm aluminum to a typical radiographic x-ray beam (80 kV) is likely to reduce K air by what percentage?
50%
42
What's entrance air KERMA?
is the amount of radiation incident on the patient required to generate a **satisfactory image**.
43
What's The median KAP in radiographic imaging?
1 Gy-cm2 .
44
The median KAP in interventional radiology
200 Gy-cm2 .
45
For fluoroscopy-guided GI studies and urologic procedures, the median KAP is
20 Gy-cm2
46
What should be taken into account when comparing KAP from different imaging systems?
must ensure **patient sizes are similar**, and **x-ray beam qualities** (penetrating power) are taken into account.
47
Why An entrance K of 1 mGy can result in a superficial skin dose of up to 1.5 mGy?
**Tissues** absorb 10% **more than air** (higher Z), and the presence of backscatter can increase superficial tissue doses by up to 40%.
48
Which **type of dose** best predicts the likelihood of the relevant **bioeffects** (i.e., burns, epilation, cataracts)?
PEAK SKIN DOSE.
49
Which **type of dose** is used to predict the **stochastic** (cancer) risk?
**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.
50
What's the distribution of interacting xray with the patient?
* 2/3 is absorbed.
* 1/3 is scattered.
* Less than 1% reached the detector and form the image.
51
For an **abdominal radiograph** in a standard-sized patient (AP projection), the **embryo dose** is about
1/3 of the air Kerma.
52
What's the main source of embryo dose during chest CT?
Internal scatter.
53
What's Integral Dose (Energy Imparted)?
The **integral** dose measures the total energy (J) **imparted** to a patient.
Measured in **Joules**.
54
How is energy deposittion of alpha particle different from xray?
**alpha** particles result in a more **concentrated pattern** of energy deposition than x-rays which produce a more diffuse pattern.
55
Radiation weighting factor (WR) which depends on which value?
Linear Energy Transfer (LET).
Higher LET values generally result in higher WR values.
56
**Higher radiation weighing factors** indicate **more biologic damage** at the same **radiation dose**. Repeat ...
**Higher radiation weighing factors** indicate **more biologic damage** at the same **radiation dose**.
57
Which has higher radiation weighting factor, protons or neutrons?
**Neutrons**; ALWAYS.
58
Population-averaged incidence of **fatal cancer** from radiation is currently estimated to be
4% per **Sv**.
59
Effective doses cannot be indicators of patient **risk** because these do not account for:
Patient demographics. You always need **age** and **sex** to determine **risk**.
60
Health consequences of cell death occur on time scale measured in
Hours and weeks.
61
What's is used to triage the severity of acute radiation exposures?
Peripheral lymphocyte count. Becaue they are highly sensitive to radiation.
62
Whole-body doses of higher than **10 Gy** would likely kill everyone in 5 to **10 days** due to
loss of epithelial lining of the GI tract (i.e., GI syndrome).
" you have a 10 meter long GI".
63
Whole-body dose of **100 Gy** would likely kill everyone in **1 to 2 days** from
permeability changes in brain blood vessels (i.e., **cerebrovascular** syndrome).
64
What's the time scale for onset of skin deteministic effects?
65
Full recovery is generally expected below what dose?
Less than 10 Gy.
66
Long-term effects of skin doses include:
* telangiectasia,
* dermal atrophy or induration,
* with possible late skin breakdown,
67
When does epilation occur?
* Scalp dose 3-5 Gy. Temporary.
* Onset 2-3 weeks.
* Hair regrowing starts 2 months after radiation.
* Hair might be grey.
68
Fractionated exposure is better in term of dose and effect EXCEPT in:
Male gonads, fractionated exposure to the gonads produces **more** damage than acute exposure.
69
Which type of radiation-induced cancer is more likely to occur in children than in adults?
Thyroid cancer.
70
Stochastic risk is based on
Linear No Threshold (LNT) model.
71
What's The doubling dose?
is the **absorbed dose to the gonads** of **the whole population** that would **double the spontaneous mutation incidence**.
72
carcinogenic effects of fetal exposure is most important in which trimester?
3rd timerster.
73
The hematopoietic syndrome most likely occurs at acute whole-body doses (Gy) of about:
4 Gy.
74
Threshold dose introduced by ICRP (2011) for **cataract** induction is (Gy):
0.5 Gy
75
Uncertainties in current radiation risk estimates are most likely:
Factor of three (e.g., ± x3)
76
When a 25-year old undergoes a TIPS procedure (effective dose 100 mSv), his/her cancer risk increases from 40% to approximately (%):
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).
77
The National Committee Council on Radiological Protection and Measurements (NCRP) recommends that the operator effective dose be taken to be
0.18 of the dose recorded by a dosimeter worn on the collar (H collar ).
Neck size is 18.
78
How frequent lead aprons should be tested?
**Annually**; by fluoroscopy.
79
The minimum regulatory lead equivalent lead apron thickness (mm) in the US is currently
0.25 mm
Most lead aprons are now 0.5 mm thickness
80
Minimum thickness of aluminum filtration in xray room walls operating above 70 kvp:
2.5 mm
81
What's tube loading?
Heat energy deposited in the **focal spot**
82
**Heat** is transferred from the **focal spot** into the **anode** by which method?
by conduction
83
Magnitude of the **heel effect** depends on
* anode angle (inverse),
* Source to Image Distance (SID) (inverse),
* field size (proportional)
84
What's the grid ratio commonly used?
10:1
85
What's exposure index (**EI**)?
to provide a **universally** understood quantitative measure of Kair at **any image receptor.**
86
A Kair of **1 microGy** at the image receptor corresponds to what exposure index (EI)?
an EI of 100.
87
What's deviation index (DI)?
quantifies **how closely** any given Kair at an image receptor matches the **target value.**
88
A deviation index **DI** value of +3 (or −3) indicates what?
an exposure that is **double (or half)** the target K air at the image receptor.
89
What's AEC?
Ionaization chamber placed between the patient and the detector. Functions to **terminate the exposure**.
90
How bedside CXR are different from the ones done in the department?
* **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
What additional filter is used in peds xray units and IR?
Additional **copper filter**; to make the beam more prenetrating.
92
What's the anode heat dissipation rate?
10 kW
93
What controls the average x-ray beam in mammography?
The **K-edge filters**; Not the tube voltage.
94
How's noise reduced in mammography?
By using v**ery 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
What filters are used with Tungsten target in mammo?
ONLY **Rh (23)** and **Ag (25) (silver)** are used; Mo is NOT used.
96
What's the grid ratio in mammogram?
5:1
Less compton scatter and more PE interaction, hence less ratio needed.
97
In which stage mammogram images are ideal as input for CAD?
“**For Processing”**; After application of detector corrections.
98
How frequent MQSA recertification should be?
Every 3 years.
99
What's the ACR breast phantom composed of?
* 6 fibers
* 5 speckles
* 5 masses
100
What's Tissue HVL in mammography?
10 mm.
101
What's Flux gain?
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
What's Minification gain?
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
What's brightness gain?
is the product of the **flux gain** (∼50) and **minification gain** (∼100), or about ~5,000.
104
Last Image Hold is required by which regulatory body in th US?
**FDA**!!
105
What's the other name for frame averaging in fluoro?
Temporal filtering
106
What's frame averaging?
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
How much of the xray beam is attenuated by the fluoro table?
**1/3** of the beam. A third
108
How's Kair chnage with Electronic Magnification?
* **Doubled** in **Mag 1**
* **Quadruples** in **Mag 2**
* **Eightfold** in **Mag 3**
109
How does the ABC work during electronic magnification\?
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
What's the effect of Electronic magnification on KAP?
**No change**
111
How the stochastic effect in Electronic magnification is different from regular fluoro?
**Unchanged**; because the total radiation incident on the patient is not changed.
112
Replacing continuous fluoro by **pulsed** fluoroscopy (30 pulses/second) results in
**- Sharper images** (i.e., reduced motion blur).
- Patient doses are unchanged sice frame rate is unchanged (i.e., same mAs per frame).
113
Switching from 30 to 15 fps would thus reduce patient doses by about
35%.
Pulsed fluoroscopy with reduced frame rates **uses a higher dose per frame** to **reduce** the perceived level of random **noise**.
114
How does collimation and magnifiation change resolution?
* Collimation: No change.
* Electronic Mag: **Improved**.
115
How does collimation change Kair and KAP?
* Kair: Unchanged.
* KAP: Reduced.
* Image contrast: Improved; due to reduced scatter.
116
How's **peak skin dose** different between electonic mag and collimation?
* Electronic Mag: Increased. (higher risk for burns).
* Collimation: No chnage.
117
In fluoro, II resolution is determined by
input phosphor (CsI) thickness
118
Which image intensifier component absorbs light and emits low-energy electrons?
Photocathode
119
When switching from normal mode to Mag 1 (electronic magnification), the area exposed is most likely reduced to:
50%
120
Heel effect depends on
* Field size
* Anode angle
* SID
121
What happens to **Kair** in **FPD** with **smaller** FOV?
**Increases**. To reduce mottle perceived mottle by radiologists.
Increases in K air with reduced FOV are programmed in by the vendor.
122
Difference between Kair in II and FPD when reducing FOV?
* **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
Difference in resolution between II and FPD?
FPDs have slightly better resolution; **3 lp/mm** without binning.
Standard fluoroscopy resolution is about **1 lp/mm** using a 500 line TV.
124
What additional cover FPDs have over II?
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
What's interventional reference point (IRP)?
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
How's interventional reference point (IRP) measured?
is **always** measured **free in air** (not tissue) and **excludes patient backscatter.**
127
How's peak skin dose relate to intervenvtional reference point\?
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
The Joint Commission (JC) defines an **unintended skin dose** in excess of **15 Gy as**
**Sentinel event.**
- A sentinel event requires a **full root cause analysis**, and a visit by a JC Inspector.
129
in FPDs, what happens to skin dose and effective dose when FOV is halved?
* Skin Dose: +50% (to correct for mottle)
* Effective dose: -25%
130
Best method to reduce eye lens dose in IR?
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
in CT, the distance from the focal spot to the isocenter is
60 cm.
This results in **2-fold geometric magnification** for object at the isocenter.
132
Benefits of Bow tie filters?
* minimize beam hardening differences with tissues.
* ensure all detectors receive similar exposures
* reduced dynamic range
133
What's the fan beam angle in CT?
50 degrees.
134
What's the beam width in 64-MDCT scanner?
40 mm
135
in CT, what's the effective mAs?
true mAs/pitch
136
CT HU values generally depend on
kV, filtration and reconstruction algorithm.
137
Compared to radiographic x-ray tubes, those used in CT most likely have much higher:
Anode capacity; 10 times higher (better dealing with heat).
138
1 HU corresponds to an increase of
0.1% in attenuation.
139
What determines **in plane** spatial resolution in CT (along beam axis)?
* Focal spot size.
* Detector size.
140
What determines the spatial resolution in the longitudinal direction (**along the long patient axis**) in CT?
Detector **thickness**
141
Beta particle **range** increases with
* increasing beta particle **energy** (e.g., 82 Rb)
* low-density tissues such as **lung**.
142
After an **isomeric transition**, both **parent** and **daughter** nuclei have
the same **mass number** and **atomic number**
143
After 10 half-lives, how much of the initial activity remains?
0.1%
144
What's Magnetophosphenes?
Magnetophosphenes are flashes of light (phosphenes) that are seen when one is subjected to a changing magnetic field.
More with higher fields
145
After how long full net longitudinal magnetization is achived?
after 4 × T1.
146
What happens to **T1 time** when the **field strength** is **quadrupled**?
It **doubles**
147
Increasing spin–lattice interactions results in
Shorter T1. (more relaxation)
148
How to reduce problems associated with **eddy** currents?
By placing **Actively** shielded gradient coils
149
What material lines Faraday cage?
Copper.
150
in GRE, what happens at TE/2?
Reversal of the dephasing gradient.
151
What direction is the reversed gradient applied in GRE?
Frequency encoding
152
In GRE, which weighting has larger flip angle?
T1: 45 degree angle
T2: 10 degree
T2\*: 10 degree
153
What's the main determinant of image contrast with GRE sequences?
T2\*
154
What's the name of the RF pulse applied at TI (inversion)?
Read-out pulse.
155
Effects of diffusion gradient depend on
* gradient strength (mT per meter),
* gradient duration,
* time between gradients.
156
How are diffusion gradient applied in SE?
DWI gradients are applied on either side of the 180-degree refocusing pulse in an SE sequence
157
How the signal from TOF is obtianed?
Unsaturated "fresh" blood comes into the slice and gives signal.
158
How the phase contrast signal is obtained?
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
What are the 3D MRI planes?
* 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
How's Echo planar imaging (**EPI**) done?
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
How does O2 affect T2\* effect?
It **shields the hemoglobin iron atoms** and reduces dephasing of adjacent protons. Therefore, it **REDUCES T2\* effect.**
162
What's fMRI sequence?
**EPI** with T2\* weighting.
163
What's magnetization transfer?
Magnetization transfer. An specially designed RF pulse (called an MT Pulse) is applied which selectively **injects energy into the bound poo**l 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
What are some applications for magnetization transfer contrast?
* improving contrast especially in **MR angiography**.
* MS protocol.
165
in MRS, Frequency differences of metabolites are measured in
parts per million (ppm).
166
What are the nuclei most often used for in vivo localized spectroscopy?
* 1H - voxel size: 1 cm3
* 31Phosphorus - voxel size: 8 cm3
167
What's the most important determinant of image quality in MRI?
SNR
168
Four acquisitions of any image will quadruple the (deterministic) signal, but will only double the (random) image noise. Repeat ...
Four acquisitions of any image will quadruple the (deterministic) signal, but will only double the (random) image noise.
169
What's the Limit for whole-body heating in normal mode limit? First and 2nd levels?
* 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
MR spectroscopy most likely requires the use of exceptionally:
Uniform magnetic fields
171
What's Acoustic impedance?
product of the **density** (r) and the **sound velocity** (v) in the material, which is expressed in **rayls**.
172
When ultrasound passes from one tissue to another having a different speed for sound,
the frequency remains the same but the wavelength changes.
173
Resonance frequency in US is determined by
* the piezoelectric element thickness.
* High-frequency transducers are thin, and vice versa.
* thickness (t) is equal to one-half of the wavelength
174
What's the purpose of damping material in US probe?
to reduce vibration (ring-down time) to shorten pulses. **Shorter SPL**.
Placed behind transducers.
175
What's the purpose of the matching layer in the US probe?
* 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
What's the thickness of the matching layer in US probe?
**one-fourth** the wavelength of sound in that material (quarter wave matching).
177
The length of the near field in US is proportional to
the effective transducer size.
Doubling the transducer effective size will quadruple the length of the near field.
178
The major benefit of focusing ultrasound beams is
to improve **lateral resolution.**
179
in US, electronic focusing introduces flexibility that allows image quality to be markedly improved. **Repeat**...
in US, electronic focusing introduces flexibility that allows image quality to be markedly improved.
180
Which frequency (MHz) is most likely a **harmonic** frequency of a 2.5-MHz transducer?
**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
The thickness of an ultrasound transducer that generates sound with a wavelength λ is most likely:
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
When a transducer element diameter is doubled, the near field length is likely:
(**Quadrupled**). Doubling the transducer element diameter will quadruple the transducer near-zone distance.
183
Which focusing method is likely to offer variable focal depths?
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
What's line density (LD) in US?
is the number of **lines per image** divided by the corresponding field of view (**FOV)**
185
How can line density (LD) be increased?
* reducing the frame rate (affects temp resol),
* reducing the FOV,
* or increasing the PRF (depth will decrease).
186
What's Pulse repetition frequency (PRF)?
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
How's 1.5D arrays transducer different than 2D arrays?
* 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
Benefits of Spatial compound imaging?
* reduces angle-dependent artifacts and clutter,
* providing improved contrast and margin definition.
189
Which harmonic is mostly used?
The **first** harmonic (**twice the fundamental frequency**) is most frequently used.
Harmonic imaging reduces **artifacts** and **clutter**.
190
Which patient group will benefit the most from harmonic imaging?
Patients with thick and complicated body wall structures
191
What's the principle of CEUS?
large difference in acoustic impedance between the gas and surrounding fluids/tissues creates signal.
Microbubbles are encapsulated and smaller than RBC size.
192
How does Pulse inversion harmonic imaging work in CEUS?
* Pulse inversion harmonic imaging uses **two pulses**, consisting of a **standard** plus **inverted** (phase reversed) along the same beam direction.
* T**hese two cancel out for soft tissues**, but not for microbubbles, improving the sensitivity of ultrasound to contrast agents.
193
What's the relationship between Doppler frequency shift and reflector velocity?
* 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
How PRF values for Doppler compared to B-mode?
PRF values are generally **higher** in Doppler than B-mode imaging (e.g.**, 8 kHz**).
195
What's the most important determinant of lateral resolution in US?
Ultrasound beam width ( the narrower the better).
196
Which US resolution is **independent of depth**?
**AXIAL**.
Both lateral and elevational resoltuions are affected by depth.
197
What are the QC tests recommended by the ACR Accreditation program?
Performed **semi-annually**.
198
Representative intensities in B-mode ultrasound?
M-Mode?
Doppler?
* B-mode: 10 mW/cm2.
* M-Mode: 40 mW/cm2.
* Doppler: 500 mW/cm2.
199
What's the most important factor for **tissue heating** in MRI?
**surface area**-to-**volume** ratio
The higher the ratio (neonates), the more effective exccess heat is released.
200
What's the explanation for "ants crawling on the skin" feeling during MRI?
Peripheral nerve stimulation
201
