X-rays

Most adult CXRs are around 120 kVp. Good technique for babies is ___ kVp with around the same or lower mAs (___).

60 kVp. 2-4 mAs.

X-ray quantity can be increased by using material with higher ___. Only those x-rays produced via ___ are increased.

Z (atomic number; number of protons). Bremsstralung (not ionization/characteristic x-ray production).

Photoelectric x-ray interactions with matter occurs throughout the diagnostic energy range (20-120 keV) but dominates ___ relative to Compton.

At lower energy. <30 keV: photoelectric. >30 keV: Compton.

Relationship between negative potential energy of an electron and proximity to the nucleus.

Directly related. Thus, if an electron moves from the L-shell to the K-shell, energy is emitted in the form of a "characteristic" x-ray photon in order for the electron to live in the K-shell (which has a high negative potential energy).

Graphic representation of the dynamic range of digital vs film.

Digital is linear with larger dynamic range while film is curvilinear with small dynamic range.

This method of digital radiography has a fill factor of nearly 100% (measure of efficiency; area of detector which is sensitive to x-rays).

Direct (photoconductor).

A major difference between plain film and digital is the primary factor influencing contrast. With plain film it's ___; with digital it's ___.

Plain film: kVp. Digital: look up table (LUT).

Collimating without increasing the mAs will reduce scatter but the overall ___ may worsen due to mottle.

SNR. Increase the mAs when collimating (usually automatic).

Isotopes.

Same Z (number of protons); different A (protons plus neutrons).

Off-focal radiation is ___ from the anode outside the focal area leading to increase in patient exposure and blurring. Tubes constructed with ___ enclosures have less off-focal radiation.

Scatter (can be reduced with small lead collimator near tube output). Metal (metal attracts scattered electrons).

The S:P ratio refers to ___.

Secondary (scatter) x-rays to primary x-rays. Lower ratios are better.

X-ray quality is affected by using material with different ___. Only those x-rays produced via ___ are affected.

Z (atomic number; number of protons). Ionization/characteristic x-ray production (not Bremsstralung).

X-ray wavelengths.

0.01-10 nm.

What percent of electrons striking the anode actually produce x-rays?

1% (most are converted to heat via excitation).

How does the probability of the photoelectric effect occurring relate to the energy of the incident photon and the atomic number?

1/(energy of incident photon)^3. Z^3. Ideally, the binding energy of the inner shell electron and incident photon are about the same (assuming IP is still > BE); a tighter bound K shell electron - i.e. those in higher Z material are more likely to undergo photoelectric effect.

The average bremsstrahlung energy is equal to ___ of the kVp selected.

1/3rd.

Classical/coherent/Thompson x-ray interaction with matter is kind of like excitation when an electron hits another electron but is not ejected. The wimpy incident x-ray, usually <___ keV, is not absorbed nor does it hit the target, instead it ___.

10 keV. Scattered (deflected in random direction). *No net transfer of energy; useless dose is added to patient.

Energy required to remove an electron from an atom.

15 eV. A photon with energy greater than 15 eV will be ionizing, while one with less will be non-ionizing.

To double the x-ray exposure, double the mAs or increase the kVp by ___.

15%. Increasing kVp by 15% will double the intensity of the spectrum.

Whereas fiberglass casts require no technique adjustments, plaster casts require an increase in mAs, ___ when dry and ___ when wet.

2x when dry. 3x when wet.

The typical digital display is ___ pixels.

3 mega pixels.

K-shell electrons prefer being ionized by energy that is fairly close to the binding energy (above, not below). Above ___ and there is virtually no characteristic x-ray production using tungsten.

300 kVp (K-shell binding energy for tungsten is 69.5 keV).

About ___ cm of tissue reduces the x-ray intensity to one-half its original value (HVL). To overcome this, crank up the ___ by two.

4 cm (1 cm for breast). mAs (double for every 4 cm of tissue; or 1 cm in breast).

Digital detector types: (1) storage phosphor (computed radiography) which uses phosphor. (2) flat panel detectors (digital radiography) which uses scintillators (indirect) or photoconductors (direct). Basic concept of the former.

A "latent" x-ray image is produced on a cassette with trapped electrons which are later liberated by a laser sweeping back and forth, releasing blue-green light. This light is then digitized. The plate is reset by exposure to bright white light (failure to do this before the next x-ray will give you ghosting artifact).

What is the function of a focusing cup in x-ray production?

A negatively charged cup that keeps the electrons in check before zooming across to the anode.

Qualities of alpha vs beta particles.

Alpha can't travel or penetrate far. Beta can.

Direct (photoconductor) method of digital radiography takes advantage of ___ which releases electrons. These electrons travel to the other side and neutralize a portion of the applied charge, which is picked up by a TFT.

Amorphous selenium, a type of photoconductor.

If the energy released from the filling of an inner shell vacancy by an outer shell electron is imparted to another electron (instead of being emitted as a photon), the second electron is referred to as ___. How many x-rays are produced here?

Auger electron. *The auger electron is ejected but no x-rays are emitted in this process; biological tissue is more likely to emit auger electrons than heavy metals like tungsten.

Beam intensity and kVp relationship.

Beam intensity = (kVp)^2.

Electron emitted from the nucleus.

Beta particle.

Benefit and drawback of a small anode. Focal spot of mammo vs general x-ray.

Better spatial resolution but poor heat dispersion (hence angling anode and using a rotating anode). Mammo: 0.3 and 0.1 mm. General: 0.6 and 1.2 mm.

Benefit and drawback of small effective focal spot.

Better spatial resolution but worse Heel effect.

Only Bremsstrahlung and ionization produce x-rays, excitation does not. What type of x-rays are produced with the two interactions? Which one produces the majority of x-rays?

Bremsstrahlung: "general" x-rays. Ionization: characteristic x-rays. *Bremsstrahlung produces 80% of x-rays.

Window level (or center) is the thing you change for ___ while window width is the thing you change for ___.

Brightness. Contrast.

More intense electrons are produced on the cathode side of the x-ray tube due to Heel effect. To take advantage of this effect, how is the x-ray tube oriented in a CXR and mammo?

CXR: anode-cathode oriented vertically with cathode over abdominal contents (more penetration). Mammo: anode-cathode oriented horizontally with cathode over chest and anode over nipple.

Indirect (scintillators) method of digital radiography takes advantage of ___ which emits light in response to an absorbed x-ray. The light is converted by a photodiode into an electric charge and captured by a ___.

Cesium iodide (CsI), a type of scintillator. Thin-film transistor (TFT).

___ x-ray interaction with matter is bad because it is the dominant factor behind scatter/fog and major source of occupational exposure.

Compton.

Imaging processing software is capable of producing an image with a specified level of ___ even when the kVp are high.

Contrast. Achieved via LUT.

How do you turn off characteristic x-rays on the keV curve?

Decrease kVp below threshold for k shell electrons.

The higher the detective quantum efficiency (DQE), the less ___ is needed to maintain a signal. What is the DQE of storage phosphor (CR) relative to flat panel detectors (DR)?

Dose/radiation. DR >> CR (0.45 vs 0.25). DQE is a measure of efficiency.

What is thermionic emission?

Electrons being boiled off a tungsten filament at high temperature (over 2000 deg. C. via applied current).

X-rays originate from ___.

Electrons; fast moving electrons interacting with atoms can emit x-rays.

Noise increases as the distance between the tube an detector increases by the inverse square law.

Energy twice as far from the source is spread out over four times the area.

Free electrons moving from a tungsten cathode (negative) and striking the tungsten anode (positive) lose the energy they have gained (keV) by 3 different methods.

Excitation (just makes heat), ionization, and Bremsstrahlung.

Detective quantum efficiency (DQE) is an estimate of the required ___ necessary to produce an optimal image. It is directly proportional to ___.

Exposure. MTF. In general, DQE of DR is 0.45 and CR or plain film is 0.25.

Using a single-source with fast KV switching for DEXA has the advantage of ___.

Good temporal resolution since two data sets obtained simultaneously. Possibility of spectral overlap is a drawback.

___ is an undesirable absorption of primary x-ray beams by grid strips, which prevents the useful x-rays from reaching the image receptor, causing quantum mottle.

Grid cutoff. Reposition the grid if this happens, don't just increase mAs.

An air gap is an alternative to using a ___. Primarily used in magnification radiography, it increases the OID which reduces scatter but at the cost of ___.

Grid. Loss of detail and sharpness.

Half value layer (HVL) is the amount of material required to attenuate an x-ray to ___. Soft x-rays will be absorbed first, thus increasing the overall average photon energy of the beam. Multiple layers can be added.

Half the original output (# of photons).

What are "hard" vs "soft" x-rays?

Hard: wavelength below 0.1 nm. Soft: wavelength above 0.1 nm. *Spectrum of x-ray wavelengths is 0.01-10 nm. The smaller the wavelength, the "harder" the x-ray (small wavelength translates to high energy). Imaging is done with hard x-rays; soft x-rays are useless (just add unesessary dose).

How does an alpha particle generate ionization?

Have 2 protons and 2 neutrons (net charge +2; equivalent to to helium nucleus) which can pull off electrons from atoms creating ions.

The grid ratio is ___ over ___.

Height of lead over distance between them; higher ratio means less scatter (but more dose).

What affects focal spot "blooming"? What affects focal spot "thinning"?

High mA, low kVp. High kVp.

Lateral dispersion of light is a problem with this type of digital detector, made worse by increasing the thickness of the crystal. It does this to the image.

Indirect (scintillator) method. Loss of spatial resolution. *Having a thin crystal will decrease ability to capture x-rays, so it's a tradeoff. Using a "lead pipe" like matrix or going to direct method solves the issue.

How does the binding force of electrons relate to distance away from the nucleus?

Inversely sqaured.

The characteristic radiation that is produced by the photoelectric effect in the human body is irrelevant because ___.

K shell photon emitted from heaviest element in human body, calcium, is only 4 keV. These secondary x-rays are too wimpy to make it to the film and typically aborbed within a few mm. However, barium and iodine contrast agents will make it to the image but only contribute to fog.

Photoelectric effect peaks just above the K-shell binding energy, for iodine that is 33 keV, which is a sweet spot referred to as ___. What should the kVp be set at to facilitate this?

K-edge. About 70 keV.

How should mA and kVp be adjusted if low contrast objects are being imaged.

Keep kVp constant and increase mA. Don't increase kVp because you'll loose photoelectric effect important for tissue contrast.

Fundamental x-ray interaction with matter that produces an image with contrast.

Photoelectric effect. More photoelectric effect means more absorbed photons, so fewer photons hitting the image receptor. The end result? Bones are white on x-ray.

Scatter is increased with high kVp, ___, and ___.

Large FOV. Thick parts (or people).

Three factors that decrease Heel effect.

Larger anode angle (less material for electrons to travel through), longer focus-to-film distance (more uniform gradient), and smaller film.

If general diagnostic x-rays use 50-120 kVp, breast uses relatively ___ to accentuate differences of soft tissues via more photoelectric effect.

Less (25-30 kVp) to achieve ideal energy of 16-23. *Remember that probability of PE is proportional to Z^3 and 1/E^3.

Automatic exposure control (AEC) replaces the timer of yesterday to shut off the x-ray when sufficient signal is produced. It is basically an ionization chamber. Practically, it limits ___ when making the x-ray.

Limits mAs, not kVp. Ionization chamber placed between the patient and the image receptor.

Linear attenuation is the number of photons interacting per ___ whereas mass attenuation is the number of photons interacting per ___.

Linear: per unit thickness. Mass: per gram tissue. Linear attenuation is different for ice, water, and water vapor but the same for mass attenuation.

How does kVp relate to tissue contrast?

Low kVp: high contrast. High kVp: low contrast. *Scatter is the enemy of contrast.

Typical kVp for CXR is 120 while the mAs is 5. How should this be adjusted for AP abdomen?

Lower kVp (because subject has low intrinsic contrast and therefore you want to rely more on photoelectric effect) and increase the mAs to maintain a similar density/blackness on the film.

Factoring in effects from photoelectric effect, Compton scatter, and coherent scatter, to increase attenuation you can increase density, have higher Z material, dial in kVp near K-edge, and ___.

Lower the kVp.

A (lower/higher) kVp will accentuate contrast differences just based on thickness alone.

Lower. Lower kVp -> smaller attenuation coefficients (low subject contrast areas like the abdomen) -> more contrast.

The purpose of modulation transfer function (MTF) is to maintain ___ as a function of ___. What is the relationship between MTF and spatial resolution (frequency)?

Maintain contrast as a function of spatial resolution.

The probability of Compton scatter is dependent on (Z/material density).

Material density, not Z (the energy of out shell electrons is universally low so Z is not a factor; elements contain around the same amount of electrons per gram).

Increasing mAs and kVp can decrease quantum mottle, however if increasing kVp this may lead to more ___.

Noise. Better option is increasing mAs

Gamma rays originate from ___; they are excess energy given off as an atom decays in an attempt to become more stable.

Nucelus of atom.

What is represented by the atomic number, Z? Mass number, A?

Number of protons in an atom. Number of protons plus neutrons.

Lead gowns protect the user by this type of x-ray interaction with matter.

Photoelectric effect (lead has high Z so most photons are aborbed).

(Photoelectric effect/Compton scatter) generates higher dose to the patient.

Photoelectric effect > Compton scatter < classical. In general, select the highest kVp possible that will still yield a diagnostic image. Increasing kVp beyond a certain point will decrease the likelihood of photoelectric effect.

A (mono/poly)-energetic beam has a higher HVL if kVp is constant.

Poly.

The glass enclosure around the anode and cathode maintains a vacuum for the purpose of controlling ___.

Quantity and speed of electrons produced (if glass enclosure were filled with a gas, secondary electrons could come about, but at variable reduced speeds).

X-ray beam is not perfectly uniform and at low doses you notice it more is the basic concept behind ___.

Quantum mottle. Increasing FOV decreases quantum mottle.

How should mA and kVp be adjusted if a lower dose is desired with exposure remaining constant.

Raise kVp by 15% and lower mA by 50%.

What is the "line focus principle"?

Reducing target angle will create tighter effective focal spot (usually good).

When an electron which is ejected from the atom has enough kinetic energy to cause additional ionization events (i.e. eject additional electrons) this process is called ___ and the ejected electrons are sometimes called ___.

Secondary ionization. Delta rays.

In CR, the following will improve spatial resolution: laser spot size, phosphor plate density/thickness, sampling frequency, imaging plate.

Small laser spot size. Less plate thickness. Higher sampling frequency. Small imaging plate.

Leakage (x-rays transmitted through housing) plus scatter (x-rays deflected in direction once leaving the tube) is referred to as ___.

Stray. *Secondary radiation refers to characteristic radiation made from electron interactions with materials other than the target (glass, housing, etc.)

What can be said about an x-ray tube with 50 mA and one with 100 mA?

The 100 mA tube has twice the flow of electrons (current/quantity of electrons) as the 50 mA tube.

What is the "space charge effect"?

The electron cloud that is produced at the tungsten cathode, called the space charge, which in turn limits the emission of electrons.

Whereas classical/coherent/Thompson x-ray interaction with matter occurs at low keV, Compton occurs at high keV, usually >___ keV. It is sort of like the former where the photon is scattered (deflected in random direction) with the addition of ___.

The keV is sufficient to knock an outer shell electron loose (Compton electron), which in turn can cause harm and the atom becomes ionized.

Generally, when x-raying the body, a grid is used if the object is ___.

Thicker than 10 cm (the foot for instance does not get a grid, unlike CXR and PA abdomen). Remember that the grid adds dose! Grids are not used on extremities or peds!

If contrast between bone and soft tissue is greatest at lower keV, why would you purposefully increase keV?

To reduce dose to patient (remember that photoelectric effect dominates at lower keV allowing for good tissue contrast as seen in the graph but also contributes the most to dose).

Anything that increases photoelectric effect increases absorption (and therefore decreases ___). The important ones to remember are ___, ___, and ___.

Transmission. kVp near k-edge, higher Z, higher density.

Graphic representation of characteristic x-rays being produced.

Via ionization (Bremsstrahlung is different). Key: K-shell binding energy of tungsten is 69.5 keV.

Variation between maximum and minimum voltage.

Voltage ripple. Increasing voltage ripple will decrease x-ray quantity and quality.

The highest energy x-ray leaving the x-ray tube will depend on the selected kVp whereas the lowest energy x-ray will depend on ___.

What filter is used.

One of the advantages of digital over film is post-processing of contrast and brightness, relying less on ___.

kVp. So with digital you can go up on kVp by 15% and down on mAs by 50%. Thus, decreasing dose or "higher dose efficiency".

The amount of bremsstrahlung interactions is proportional to ___ and ___.

keV and atomic number (Z; # of protons of anode). In nuclear medicine, low Z materials (plastic) is used to shield beta emitters (eg, Y-90) in order to minimize bremsstrahlung.

Difference between keV and kVp.

keV: energy of single electron. kVp: peak potential energy (voltage applied between cathode and anode). *Electrons strike the target with different energies because the electron stream is not uniform.

An electron with energy x keV can only produce an x-ray up to ___ kVp.

x. Max energy = max kVp.