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Diagnostic Imaging Midterm Review: X-Rays

Notes here on basic vocabulary and physics of radiography, as well as a few positioning tips for visualizing common pathologies.


PHYSICS

What effect does the 15% kVp rule have on radiographic blackness?
Blackness increases 100% with a 15% increase in kVp.

Radiographic contrast?
Higher kVP lowers contrast, gives longer gray scale.

Patient dosage?
Higher kVp reduces patient dose because there is more penetration / less absorption of ray.
--?? I have questions about this. If the photons are going so fast through the body that they aren't absorbed, do they disturb anything on the way through? What is the basis for saying the "dose" is lower when the penetration is higher?

mAs usage?
mAs is main control of blackness, is beam intensity per second, change blackness by either variable or by changing source to film distance (inverse square law). If too dark, decrease mAs. Change mAs by halving or doubling, need at least 25% change to see difference.

What is kVp and what is its function?

It is the force that drives electrons toward its target. Increasing kVp causes electrons to travel faster. Faster traveling electrons produce a higher frequency shorter wavelength photon that is more penetrating and therefore less harmful to pt.

kVp controls contrast. Low kVp -> high contrast and higher dose. High kVp -> low contrast and lower dose. The 15% kVp rule describes change in black dt change in kVp, but kVp should not be used routinely to change radiographic blackness, it's for contrast. The rule: 15% increase in kVp causes 100% increase in blackness, 10%-->66%, 5%-->33%.

--15% increase in kVp will double blackness
--15% decrease in kVp willl half blackness
--to maintain same blackness and lower kVp, mAs must go up
--contrast can't be evaluated if blackness is incorrect
--fixed kVp system designates ranges of kVp for specific body areas
--ranges are highest possible that will penetrate and give diagnostic contrast
--purpose: minimize pt dose

Can you alter factors given to change contrast (make higher or lower) and maintain radiographic blackness?
Yes, adjust kVp, higher for less contrast/more grays, lower for more contrast.
70kVp @ 20 mAs= enough radiation to cook 2 eggs
80kVp @ 20 mAs= cook 4 eggs
80kVp @ 10 mAs= cook 2 eggs

What is radiographic blackness?
Blackness/darkness on film

What controls it?
mAs

What effect would doubling mA have on radiographic blackness? Increase darkness.

Doubling exposure time? Increase darkness.

Cutting mA by half? Decrease darkness.

Cutting exposure time by half? Decrease darkness.

What is the likely effect of long exposure time on resolution?
Decrease resolution due to pt movements.

--resolution is detail visibility
--improved by use of small focal spot, screens with small crystals (detail screens)
--distance has major effect on resolution: close is better
--OID causes magnification (far from screen) which decreases resolution
--always place part as near as possible to film
--increased SID increases resolution, but also requires increase of mAs
--SID = source to image-receptor distance
--other causes of poor resolution: scatter, motion blur, poor film-screen contact
--penumbra is geometrical unsharpness around useful xray beam

--image distortion = change of size or shape
--elongation dt xray beam not perpendicular to plane of film
--foreshortening dt body part not parallel to plan of film

What is the effect of distance on intensity? Double distance quarters intensity. Half distance quadruples intensity. If distance is doubled what happens? Intensity is quartered.

--inverse square law: when dist from source of xray is doubled, intensity of beam is quartered
--when distance is halved, intensity is 4x
--tube is closer to film, film gets darker proportionally
--to maintain blackness, increase mAs when dist increases, decrease mAs when dist decreases
--40" to 72" increase mAs by 3.24 times
--72' to 40" decraese mAs by 3.24 times

What is the anode heel effect? Xray beam is less at anode side of film, greater at cathode side. Only visible on larger films.

What would be a practical application of it? Useful to equalize exposure on body parts that are thicker/thinner at one end, put the thick end of the body part at the cathode side of the film.

What are grids?
Strips of metal to absorb some of the xrays and decrease scatter, most commonly constructed of lead strips, space between may be aluminum? A high ratio grid requires more exposure, produces higher contrast and allows less positioning latitude. Detail screens should not be used with grids.

Why are they used?
To reduce scatter and remove low energy photons from primary radiation. Bucky is a device that moves grid so strips aren't seen on film.

When are they used?
When xraying a part thicker than 10cm and mostly soft tissue, or when over 70kVp (Robert says 60kVp) will be used. May not be needed for foot or knee.

What is beam restriction? Collimation = narrowing beam to only expose necessary areas

What is scatter? Bounced photons, most in soft tissue.
--scatter radiation causes films to appear gray
--soft tissue scatters more than bone

Good? no
Bad? YES

How is scatter minimized? Collimation and other beam restrictors reduce scatter before beam enters pt, radiographic grids reduce it as it exits pt before it enters film

--Compton scatter results when photons interact with loosely bound outer-shell electrons in body
--lead impregnated rubber "scatter strips" may also be used to mask an area
--air-gap is another method of scatter reduction

What is attenuation?
How many/much photons are absorbed by body tissues

How does pathology affect it? May increase or decrease it.
--pathologies that increase attenuation: more air: pleural effusion, pneumonia, soft tissue tumor, ascites, sclerosis, calcified stones, paget's dz
--pathologies that decrease attentuation: more fluid or harder tissue: emphysema, pneumothorax, bowel obstruction, osteoporosis, osteolytic metastases

How are attenuation and radiographic blackness related?
More attentuation-->more white, less attenuation-->blackness.

Which body substance attenuates most? Bone.
Least? Air.
Fat/water/muscle are similar.

--subject contrast low when adjacent structures absorb similarly
--subject density refers to how much a body substance absorbs
--photoelectric absorption occurs at lower kVp levels
--accounts for higher contrast images
--cases higher dosage to pt because photons are absorbed
--incident photon dislodges an inner shell electron and is then completely absorbed
--space created in inner shell is filled by next outermost shell electron, causing characteristic radiation to be emitted, secondary radiation

What is the half-value layer?
a layer that will absorb half the radiation dose (usu aluminum is used)

What is filtration?
Filtration eliminates low energy “soft” photons that are more harmful, improves the quality of the x-ray beam, “less harmful and less skin dosage to the pt”

What is inherent filtration?
Built into the tube. Produced by the Port (where X-rays exit tube) and insulation oil of the tube.

Added filtration? what's outside of the tube
"Aluminum plate is inserted after the Port and the sliver of the columnators mirror create added filtration." collimators?

What is the legal requirement for total filtration in x-ray tubes using over 70 kVp?
LEGAL LIMIT: 2.5 mm Al/eq
two and a half milimeters of aluminum per equivalent?

What is REM?
radiation equivalent in man, measure of bio change.
--there is no "maximum permissible dose" anymore
--there are occupational dose limits, annual body (50mSv or 5rem)
--eye lens (150mSv or 15rem), public, fetus (hardly any), etc.
--general rule: 5 x (current age - 18) = rem dose limit for you

What is RAD?
radiation absorbed dose (any absorbing material)
100 RAD --> death
25 RAD --> hematologic depression
5 RAD --> xsome damage
10 RAD --> gonad dysfx
300 RAD --> skin damage
USED:
200-620mR -->23 cm into abdomen
95-135mR --> 13 cm to C-spine
70-145mR--> 15 cm into skull

What is ROENTGEN? measure of ionization of gas

What are their corresponding SI names?
Rem converts to Severts (1 REM = 10 mSv)
RAD converts to Grays (1 RAD = 10 mGy)
ROENTGEN is a SI unit and relates to charge/kg

What is the “10-day rule"? First 10 days of female's menstrual cycle best time to image if non-emergency, due to lower odds of pregnancy, no non-emergency pelvic studies of females

ANATOMY, POSITIONING AND PRESCRIBING/PERFORMING RADIOGRAPHIC STUDIES

Which joint space(s) are seen well on routine shoulder (internal & external rotation) views?
A/C joint seen well but often over exposed so need filter

A/C = acromioclavicular joint

Which joint space is not seen well on those views?
Glenohumeral joint not seen on A/P view

Which view(s) will demonstrate what is not seen on routine views?
Grashey view

When to order PA/lateral chest exam?
Respiratory and/or heart complaints

When to order thoracic spine exam?
When you want to see the spine.

Rib exam?
When you want to see the ribs. Usu done on inhalation.

Why include an upright chest film with a rib study?
It may be the most comfortable position.
You might be able to visualize damage to soft tissue the ribs have done (pneumothorax, etc)

What are the differences between chest, thoracic spine and rib exams?
--Higher contrast for thoracic spine, lower kVp ~60 to 80.
--Higher kVp for chest study (100 to 125 kVp), fast exposure so pt doesn’t have to hold breath as long. Do it on inhalation.
--Rib study needs two views A/P or P/A AND oblique view. Must include 1st rib.
--Always want as small focal spot as possible.

What is a scaphoid view? Which side of the wrist is the scaphoid on?
ulnar deviation AP, scaphoid is lateral (in anatomical position!)

How to image lumbar spine instability?
Flexion and extension studies.

What is a pars interarticularis fracture? What condition might it result in?
Pars interarticularis on posterior spine holds lumbar vertebrae in ant/post position, and when they break off (spondylolysis) discs can slide forward (anterior displacement) relative to each other (spondylolisthesis), commonly occurs at L5 and L4 because of the angles of the vertebrae and poor posture

In addition to the routine three-view which cervical spine views are indicated when radicular symptoms are noted?
--AP lower C3-C7
--AP Open Mouth C1-C2
--Neutral Lateral C1-C7
--Flexion/Extension
--Obliques

The best view for sacroiliac joints and lumbosacral area?
The AP axial view gives a good look into the lumboscacral joint which is not seen well in standard AP view due to overlap of L5 & S1. Looks into S-I joints and in many cases eliminates need for S-I oblique views.

A common fracture site in cases of inversion ankle sprain is the base of the 5th metatarsal. Which additional views demonstrate the area?
Medial oblique (in addition to standard AP foot) studies

What study demonstrates pneumoperitoneum and/or bowel obstruction?
An acute abdomen series consists of supine and upright or decubitus films of the abdomen and an upright chest. Dorsal decubitis projections shows air-fluid levels (pneumoperitoneum), gas accumulation and abdominal aortic aneurysm.

decubitus = reclining

dorsal decubitus lying on the back, supine

lateral decubitus lying on one side, designated right lateral decubitus when the subject lies on the right side and left lateral decubitus when on the left side

ventral decubitus lying on the stomach, prone

What plain film study will be useful in determining whether a patient has a leg-length inequality? AP weight bearing lumbar spine (three exposures on one film without moving pt.- hips, knees and ankles are imaged) and scanogram

OTHER NOTES

Salter Harris fracture = fracture at growth plate in child's longbone

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