Ch 15-18 Flashcards
2 factors contributing to Compton scatter
kVp (affects beam penetrability)
Volume of irradiated material (FS and patient thickness)
3 affects on interactions increasing kVp has
Increased transmission
Decreased photoelectric absorption
Increased Compton scatter
Interactions that happen in the body when x-rays get absorbed
An interaction between x-rays and matter characterized by an incident electron with slightly greater energy than the binding energy of the electrons in the inner shells, ejecting an electron from the inner shell while being absorbed in the reaction, resulting in an ionized atom
Photoelectric absorption
3 affects on patient dose increasing kVp has
Decreased dose
Decreased photoelectric absorptions because x-rays have more energy to pass through the body
Increase in kVp typically accompanied by reduction in mAs
Affects on image quality increasing kVp has
Low/longer scale of contrast (many shades of gray)
2 affects on interactions decreasing kVp has
Decreased transmission and scatter
Increased photoelectric absorption
Affect on interactions decreasing kVp has
Shorter scale/increase in contrast (black and white)
Don’t have as much penetrating ability so x-rays absorbed
What does increasing field size affect?
Increases volume tissue irradiated and results in increased scatter and patient dose
What does decreasing field size affect?
Decreases beam quantity, scatter, and shortens scale of contrast image
4 beam restrictors
Aperture diaphragm
Cones
Cylinders
Collimators
Flat sheet of metal, usually lead, with an opening cut in the center and attached to the x-ray tube port
Simplest of all beam-restricting devices and different diaphragms are needed to accommodate different receptor sizes and distances
Fixed field size
Aperture diaphragm
Circular aperture diaphragms with metal extensions and has an extension that flares or diverges, with the upper diameter smaller than the bottom flared end
Most effective means of scatter control
Fixed field size
Cones
Circular aperture diaphragms with metal extensions and has an extension that flares or diverges, with the upper diameter smaller than the bottom that doesn’t flare
Fixed field size
Cylinders
A set of lead shutters at right angles to one another that move in opposing pairs
Collimator
Devices tailored for a specific use during a given procedure, designed to restrict the beam to a specific shape for a particular examination
Used to absorb scatter produced by patient
Must check vendor for information for digital systems before using lead blockers or masks
Ancillary devices
Get rid of off-focus radiation
Upper collimators
Radiation produced in tube someplace other than the anode
Photons that were not produced at the focal spot; extrafocal radiation
Off-focus radiation
Uses light reflected off mirror to project coverage of x-ray beam
Proper adjustment of mirror necessary to accurately display location of exposure field
X-ray beam coincide testing should be part of quality control (QC) program
Needs to be accurate within 1/2 inch (2% of SID)
Light field
Reduce penumbra
Bottom shutters
Unsharp shadow around the sharp shadow
A geometric unsharpness around the periphery of the image
Penumbra
An automatic collimator that adjusts to the size and placement of the cassette
Possible to override (can reduce beam to smaller field size than cassette size)
Positive Beam Limitation (PBL) devices
Adjusts to the size and placement of the cassette
Automatic collimator
2 ancillary devices
Lead blocker
Lead mask
A sheet of impregnated rubber that can be cut to any size or shape
Shields
Lead blocker
Usually cut to correspond to the particular field size desired and is then secured to the end of the collimator
Lead mask
The reduction in the number of x-ray photons in the beam, and subsequent loss of energy, as the beam passes through matter
Increased part thickness results in increase in this
Result of photoelectric absorption (provides radiologic significant information) and Compton scatter
Dependent on thickness and composition of patient’s tissues
Attenuation
An interaction between x-rays and matter characterized by an incident x-ray photon interacted with a loosely bound outer-shell electron, resulting in removal of the electron from the shell, which then proceeds in a different direction as a scattered photon
Provides no useful information
Contributes to personnel exposure
The picture on the finished image is never put there by this because it only adds to the exposure the image receptor gets
Compton scatter
Patient is radiographer’s greatest variable (thickness and pathology)
Composition of human body determines its radiographic appearance
The human body as an attenuator
4 major substances account for variable attenuation
Air
Fat
Muscle
Bone
Effective atomic number: 7.78 (greater than fat or muscle)
Lowest tissue density
Absorbs few photons (results in increased area of exposure on image receptor)
Air
Soft tissue
Effective atomic number and tissue density similar to water
Effective atomic number slightly less than muscle
Tissue density less than muscle
Fat
Soft tissue
Slightly higher atomic number and tissue density than fat
Muscle
Calcium among highest atomic number of elements found in body
Greatest tissue density of four basic substances
Absorbs a lot of photons (decreased area of exposure on image receptor)
Bone
Image receptor exposure will be altered by changes in amount or type of tissue being irradiated
How dense patient is, degree at which different parts of body absorbs
Radiographic density = blackness/variations of gray
Subject density
Degree of differential absorption resulting from differing absorption characteristics of tissues in body
Subject contrast
Recorded detail of structures dependent on:
Position within body
Body’s placement in relationship to image receptor
Ex: heart lies anterior in body, want anterior part against IR to get it as true to size as possible
Movement affects detail
Subject detail
Unless patient is positioned specifically to demonstrate a particular structure, may not be accurately represented on image receptor
Subject distortion
Radiographer’s 4 responsibilities
Read requisition
Take accurate patient history
Observe patient closely
Adjust technical factors when necessary
Increase tissue thickness, effective atomic number, and/or tissue density (increase attenuation
Inversely related to image receptor exposure)
Require increase in technical factors to properly expose image receptor
Thicker, denser part requires more penetration
General compensation: increase in kVp
Additive conditions
2 increased attenuation (additive) conditions in multiple systems
Abscess
Edema
Tumors
11 increased attenuation (additive) conditions in the chest
Atelectasis Bronchiectasis Cardiomegaly Congestive heart failure (CHF) Empyema Pleural effusions (hemothorax and hydrothorax) Pneumoconiosis Pneumonia (pneumonitis) Pneumonectomy Pulmonary edema Tuberculosis (advanced and miliary)
4 increased attenuation (additive) conditions in the abdomen
Aortic aneurysm
Ascites
Cirrhosis
Calcified stones
7 increased attenuation (additive) conditions in the extremities and skull
Acromegaly Chronic osteomyelitis Hydrocephalus Osteoblastic metastases Osteochondroma Paget’s disease Sclerosis
Decrease tissue thickness, effective atomic number, and/or tissue density (decrease attenuation)
Directly related to image receptor exposure
Require decrease in technical factors to properly expose image receptor
General compensation: decrease mAs
Destructive conditions
3 decreased attenuation (destructive) conditions in multiple systems
Anorexia nervosa
Atrophy
Emaciation
2 decreased attenuation (destructive) conditions in the chest
Emphysema
Pneumothorax
2 decreased attenuation (destructive) conditions in the abdomen
Aerophagia
Bowel obstruction
11 decreased attenuation (destructive) conditions in the extremities and skull
Active osteomyelitis Aseptic necrosis Carcinoma Degenerative arthritis Fibrosarcoma Gout Hyperparathyroidism Multiple myeloma Osteolytic metastases Osteomalacia Osteoporosis
An encapsulated infection increases tissue thickness and may alter composition, particularly in lungs; additive
Abscess
Swelling causes an increase in tissue thickness and may alter composition if it occurs in the lungs, additive
Edema
An abnormal growth in tissue results in an increase in tissue thickness and may alter composition, particularly in the lungs or bones or when calcification results; additive
Tumor
The chronic dilatation of the bronchi can result in peribronchial thickening and small areas of atelectasis causing an increase in lung tissue density, additive
Bronchiectasis
An enlargement of the heart causes an increase in thickness of the part, additive
Cardiomegaly
When the heart is in failure, the cardiac output is diminished resulting in backward failure or increased venous congestion in the lungs; lung tissue density is increased and the heart is enlarged as well, additive
Congestive Heart Failure
Pus in the thoracic cavity causes an increase in tissue density, additive
Empyema
When the pleural cavity fills with either blood or serous fluid, it displaces normal lung tissue resulting in an increased tissue density within the thoracic cavity; additive
Pleural effusions (hemothorax and hydrothorax)
The inhalation of dust particles can cause fibrotic (scarring) changes; when healthy lung tissue becomes fibrotic the density of the tissue increases, additive
Pneumoconiosis
The removal of a lung will cause the affected side to demonstrate an increase in density because normal air-filled lung tissue is removed, additive
Pneumonectomy
Inflammation of the lung tissues causes fluid filled in the alveolar spaces, fluid has much greater tissue density than the air normally present; additive
Pneumonia (pneumonitis)
When fluid fills the interstitial lung tissues and the alveoli, tissue density increases; this is a typical complication of congestive heart failure, additive
Pulmonary edema
An infection by mycobacteria causes the inflammatory response, which results in an increase in fluid in the lungs
If the mycobacteria were inhaled, it begins as a localized lesion (usually upper lobes), which can spread to a more advanced stage
If the infection reaches the lungs by the bloodstream, it has a more diffuse spread (miliary)
Increased tissue density results in both advanced and miliary, additive
Tuberculosis (advanced and miliary)
A large dilation of the aorta will result in increased thickness of the affected part, additive
Aortic aneurysm
Fluid accumulation within the peritoneal cavity causes an increase in tissue thickness; the free fluid has a unique “ground glass” appearance radiographically, additive
Ascites
Most commonly found throughout the abdomen in such organs as the gallbladder and kidney
Calcium may be deposited which causes an increase in the effective atomic number of the tissue, additive
Calcified stones
Fibrotic changes in the liver cause the liver to enlarge and ascites can result resulting in an increase in the thickness of the liver and the entire abdomen, additive
Cirrhosis
An overgrowth of the hands, feet, face and jaw as a result of hypersecretion of growth hormones in the adult will result in an increase in bone mass; additive
Acromegaly
A chronic bone infection results in new bone growth at the infected site, additive
Chronic osteomyelitis
A dilation of the fluid-filled cerebral ventricles causes an enlargement of the head resulting in an increased thickness, additive
Hydrocephalus
The spread of cancer to bone can result in uncontrolled new bone growth, additive
Osteoblastic metastases
A tumor arising in the bone and cartilage will result in an increased thickness of the bone, additive
Osteochondroma
An increase occurs in bone cell activity, which leads to new bone growth resulting in increased bone thickness with the pelvis, spine and skull most often affected; additive
Paget’s disease (osteitis deformans)
An increase in hardening as a result of chronic inflammation in bone increasing the density of the bone tissue, additive
Sclerosis
A psychological eating disorder that results in extreme weight loss, overall body thickness is reduced; destructive
Anorexia nervosa
A wasting away of body tissue, destructive
Atrophy/emaciation
The overdistension of the lung tissues by air will result in a decrease in lung tissue density, destructive
Emphysema
Free air in the pleural cavity displaces normal lung tissue and results in decreased density within the thoracic cavity, destructive
Pneumothorax
A psychological disorder resulting in abnormal swallowing of air; the stomach becomes dilated from the air and overall tissue density decreases, destructive
Aerophagia
An obstruction in the bowel results in the abnormal accumulation of air and fluid
If a large amount of air is trapped in the bowel, the overall density of the tissues is decreased; destructive
Bowel obstruction
With a bone infection there is initially a loss of bone tissue (containing calcium) resulting in a decrease in the thickness and composition of the part, destructive
Active osteomyelitis
Death of bone tissue results in a decrease in composition and thickness of the part, destructive
Aseptic necrosis
Malignancies in bone can cause an osteolytic process resulting in decreased thickness and composition of the part, destructive
Carcinoma
Inflammation of the joints results in a destruction of adjoining bone tissue which decreases the composition of the part, destructive
Degenerative arthritis
This malignant tumor of the metaphysis of bone causes an osteolytic lesion with a “moth-eaten” appearance resulting in reduced bone composition, destructive
Fibrosarcoma
During the chronic stages of this metabolic disease, areas of bone destruction result in punched-out lesions that reduce the bone composition; destructive
Gout
Oversecretion of the parathyroid hormone causes calcium to leave bone and enter the bloodstream; the bone becomes demineralized and composition is decreased, destructive
Hyperparathyroidism
This malignant tumor arises from plasma cells of bone marrow and causes punched-out osteolytic areas on the bone; often many sites are affected and reduced bone tissue composition results, destructive
Multiple myeloma
When some malignancies spread to bone they produce destruction of the bone resulting in reduced composition, destructive
Osteolytic metastases
A defect in bone mineralization results in decreased composition of the affected bone, destructive
Osteomalacia
A defect in bone production due to the failure of osteoblasts to lay down bone matrix results in decreased composition of the affected bone, destructive
Osteoporosis
Device used to improve contrast of the radiographic image
Absorbs scattered radiation before it reaches image receptor
Allow primary radiation to reach image receptor
Primary disadvantage of use: lines on film
Grid
Responsible for dark areas
Transmission
Responsible for light areas
Absorption
Creates fog and lowers contrast
The interaction of x-ray photons and matter that causes a change in direction of the photons
Scatter
Scatter increases as…(4)
kVp increases
Field size increases
Thickness of part increases
Atomic number decreases
2 indications for grid use
Part thickness greater than 10 cm
kVp greater than 60
Radiopaque lead strips (x-rays can’t get through them)
Separated by radiolucent (x-rays can get through) interspace material (typically aluminum because it’s cheap and has higher atomic number which will absorb scatter)
Basic grid construction
Who created grids?
Dr. Gustav Bucky (1913) - crosshatched design
2 improvements Dr. Hollis Potter made to use of grids
Realigned lead strips to run in one direction
Moved grid during exposure to make lines invisible on image
Potter-Bucky Diaphragm
Height of radiopaque strips
h in grid dimensions
Distance between strips/thickness of interspace material
D in grid dimensions
Height of lead strips divided by thickness of interspacing material, always to 1
The ratio of the height of the lead strips to the distance between the strips
Higher one more efficient in removing scatter because there’s more lead content and less angle for scatter to get to IR
Lower ones allow more angled x-rays to get to IR
Typical range = 5:1 to 16:1
Grid ratio
Grid ratio formula
G=h/D
Why don’t we usually use 16:1 grid ratio?
Don’t use 16:1 becaure they’re very picky; have to adjust everything just right or you’ll have grid cutoff
Number of lead strips per inch or centimeter
Range: 60-200 lines/in and 25-80 lines/cm
Typically, higher ones have thinner lead strips
Less lead strips are less frequent so they need to be thicker because keeping same ratio absorbs the same amount of radiation
Grid frequency
Very high-frequency grids: 78-200 lines/in and 70-80 lines/cm
Recommended for use with digital systems (minimizes grid line appearance)
Digital imaging systems
Most important factor in grid’s efficiency
Measured in mass per unit area: g/cm2
High ratio, low frequency grids tend to have highest lead content bc strips are thicker
Greater in grid with high ratio and low frequency
As this increases, removal of scatter increases therefore contrast increases (black and white)
Lead content
2 grid patterns
Criss-cross or cross-hatched
Linear
Has horizontal and vertical lead strips
Primary beam must be centered perpendicular to grid
Grid must remain flat (can’t angle tube)
Two linear grids placed on top of one another so that the lead strips form a criss-cross pattern
Criss-cross or cross-hatched grid
A grid with lead strips running in only one direction
Allows primary beam to be angled along directions that lines are running
In typical x-ray table, strips run along long axis of table
Allows for angling tube toward head or feet of patient
Linear grid
Grid lines run across short axis of grid
Useful for portable chest procedures when cassette place crosswise
Decreased chance of grid cut-off
Short-axis grid
2 types of linear grids
Focused
Parallel
Lead strips angled to match divergence of beam
Primary beam will align with interspace material
Scatter absorbed by lead strips
Convergence line (anode) goes with this
A grid created with the central grid strips parallel with the strips and becoming more inclined as they move away from the central axis; the lines would intersect along a point in space called the point of convergence
Narrow positioning latitude
Focused linear grid
Area between convergence line and lead strips
Grid radius
All lead strips parallel to one another, lines never intersect
Absorb large amount of primary beam resulting in some cut-off
Parallel linear grids
Grids that can be attached to cassette for use
Grid cassettes
Stationary grids
Motor drives grid back and forth during exposure
Reciprocating
2 types of grid movement
Reciprocating
Oscillating
Electromagnet pulls grid to one side
Releases it during exposure and spins
Oscillating
Grid conversion factor
Required increase in technique can be calculated
mAs1/mAs2 = GCF1/GCF2
2 criteria International Commission on Radiologic Units and Measurements (ICRU) evaluates grid performance by
Selectivity
Contrast improvement ability
Describes grid’s ability to allow primary radiation to reach image receptor and prevent scatter to see how well grid determines what is scatter and what is primary
Grids designed to absorb scatter and sometimes absorb primary radiation
Highly selective grids better at removing scattered radiation
High lead content grids more selective
Selective = % of primary rad transmitted/% scatter
Selectivity
“K” factor (contrast improvement)
Compares radiographic contrast of image with grid to radiographic contrast of image without grid
Typically ranges between 1.5–3.5
K= radiographic contrast with the grid / radiographi
Contrast improvement ability
Proper alignment between x-ray tube and grid (very important)
Improper alignment results in cut-off
Grid errors
5 grid errors
Off-level Off-center Off-focus Upside-down Moire effect
Occurs when the tube is angled across the long axis of the grid strips
When this error occurs there is an undesirable absorption of primary radiation, which results in a radiograph with a decrease in exposure across the entire image
Off-level
The result of the primary beam being angled into the lead strips
Grid cut-off
X-ray tube not centered along the central axis of focused grid resulting in a decrease in exposure across the entire image
Off-center
A grid is used at a distance other than that specified as the focal range
Results in grid cut-off along the peripheral edges of the image
Off-focus
A focused grid has an identified tube side based on the way the grid strips are angled
If the grid is used wrong side up, severe peripheral grid-cutoff will occur
Radiation will pass through the grid along the central axis where the grid strips are most perpendicular and radiation will be increasingly absorbed away from the center
Upside-down
See grid lines on image
Digital systems: grid lines parallel to scan lines
Can be prevented by high-frequency grids
Moire effect
Alternative to low ratio grid use
10” air gap has similar clean-up of 15:1 grid
Increase OID, scatter produced from body has area in middle where it can go out and off IR = less scatter
The air-gap technique
