Radiographic Grids: Scatter Control

Grids: Scatter Control

• Grids improve radiographic image contrast by absorbing scatter radiation before it reaches the image receptor (IR).
• They are positioned between the patient and the IR.
• Grids are the most effective way to limit scatter radiation reaching the IR.
• Grids enhance image contrast, leading to more diagnosable images.
• Primary photons interact in three ways: direct transmission, photoelectric absorption, or Compton scatter.
• Grids are found in the table and erect bucky, placed after the patient.
• Use a grid when the body part thickness exceeds 10 cm, or when using kVp above 60.
• Grids can be used with any type of IR.
• Grids are rectangular devices with radiopaque lead strips separated by radiolucent interspace material. These act as beam attenuators.
• Lead strips absorb the X-ray beam.
• Interspace material is usually aluminum, carbon fiber, or plastic.
• Strips are encased in plastic or aluminum for protection.
• The first grid was made in 1913 by Gustav Bucky.
• In 1920, Dr. Hollis Potter improved the design by realigning lead strips, making them thinner, and introducing the Potter-Bucky diaphragm, which moves the grid during exposure blurring the lead strips.

Grid Frequency

• Grid frequency means the number of grid lines per inch.
• Higher frequency grids have thinner but more lead strips.
• Increased grid frequency improves contrast.
• Grids range from 60-200 lines/inch (25-80 lines/cm).
  • Low frequency: 40-50 lines/cm (100-120 lines/inch).
  • Medium frequency: 50-60 lines/cm (120-150 lines/inch).
  • High frequency: 60-70+ lines/cm (150-170+ lines/inch).
• Higher frequency grids clean scatter better.

Grid Materials

• Radiopaque strips are made of lead.
• Radiolucent interspace materials are aluminum, carbon fiber, or plastic.
• Grid ratio is lead strip height (h) divided by interspace width (D).
• Grid ratios can go up to 16:1, but usually 10:1 is used.

Grid Ratio

• Grid ratio is the height of lead strips divided by the width of the interspace.
• Decreasing the interspace increases grid ratio.
• There is an inverse relationship between lead strips and grid ratio when height remains the same.
• As grid ratio increases, contrast and scatter absorption increase, but greater positioning accuracy is needed.
• High grid ratios require increased technical factors to account for receptor exposure loss.
• Increasing mAs when using a grid will increase patient exposure but provides a more diagnostic image.

Grid Patterns

• Grid patterns refer to the linear arrangement of lead lines.
• Linear Grid:
  • Lead strips run in one direction.
  • They never intersect.
  • This is the most common pattern.
  • They can be used with tube angulation along the long axis of the table.
• Crossed-Grid (Cross-Hatched):
  • Two linear grids are on top of each other at right angles.
  • These remove more scatter but tube cannot be angled.
• Parallel:
  • Non-focused; lead lines run parallel.
  • They never intersect.
  • Best used with long SID.
• Focused:
  • Lead lines are angled to match the x-ray beam divergence.
  • Central grid strips are parallel, with increasing inclination away from the central axis.
  • Strips converge at the grid-focusing distance.
  • This allows more transmitted photons to reach the IR.
  • Imaginary lines from lead lines meet at convergence point.
  • Convergent line is the line where convergence points intersect.
  • These lines determine the focal distance (SID).

Grid Errors

• Poor images can result from improper grid use.
• Grid cutoff occurs when lead strips absorb an undesirable amount of the primary beam.
• Off-Level:
  • Most common type of cutoff.
  • Occurs with parallel or focused grids.
  • Happens when angling the x-ray tube across the grid lines or angling the grid itself.
  • Results in decreased exposure on one side or overall.
• Off-center or Lateral decentering:
  • Must be centered along the central axis of a focused grid.
  • Off-center CR means the beam will not correspond to the grid.
  • Results in decreased exposure across the entire image.
• Off-Focus:
  • Focused grids must be at specific SID ranges.
  • Occurs at distances other than the specified focal range.
  • Cutoff occurs at the peripheral edges.
• Upside-down:
  • Focused grids have an identifiable tube side.
  • Using the grid upside-down causes severe peripheral grid cutoff on both sides.
• Moire Effect:
  • Occurs with digital systems when grid lines are not parallel to the CR scanning laser reader.
  • Creates a zebra pattern (wavy) on the image.

Stationary vs. Reciprocating Grids

• Stationary grids can show grid lines on the image.
• Moving the grid during exposures blurs grid lines.
• Moving (reciprocating) grid = Bucky (Potter-Bucky diaphragm).
• Movement is activated by the x-ray exposure switch.
• Reciprocating grids move back and forth (lateral direction) of the IR during exposure.
• Table and wall bucky use moving grids.
• Stationary grids are portable.
• Stopped Grid (Error):
  • A reciprocating grid that stopped during exposure shows uniform grid lines.

Conversions

• No grid is equal to 1.
• Bucky factor or grid-conversion factor is used to convert from non-grid to grid.
  • Non-grid to grid: GCF= (mAs WITH grid) / (mAs WO grid).
  • Or mAs(2)= mAs(1) x (GCF2)/(GCF1).
  • Grid to grid: (mAs(1)) / (mAs(2))= (GCF(1)) / (GCF(2)).
  • Or mAs(2)= mAs(1) x (GCF2)/(GCF1).
• Air Gap Technique:
  • Increased OID creates an air gap between patient and IR.
  • Reduces scatter reaching the IR.
  • Increases contrast but decreases sharpness due to OID.
  • A 10” air gap cleans up scatter like a 15:1 grid for 10 cm body part.

AEC: Automatic Exposure Control

• Generic terms include: AEC, AED, Phototiming.
• In 1942, Russell Morgan made the 1st photo timer for use with mass chest screening photofluorographic units.
• Hugh Hodges used ionization chamber type and automatic density control.

AEC

• AEC measures exit radiation before it reaches the IR.
• It eliminates the need to set an exposure time.
• AEC provides consistent optical density/signal-to-noise ratio between images, regardless of patient size or density.
• AEC units are in the table or wall bucky, not in the IR.
• It helps the IR receive perfect exposures for every size compensating for differences.
• AEC only terminates exposure time.
• Systems other factors are preprogrammed using Anatomically Programmed Radiography (APR).
• Precise positioning over the ionization chamber cell is need.
• The exposure terminates when the radiation chamber receives a predetermined signal.
• AEC devices expose structures positioned directly above the ionization chambers.

Anatomically Programmed Radiography

• APR is a radiographic system where the user can select an anatomic area via a button.
• Console allows for selection of an anatomical region and position.
• Preprogrammed exposure factors are displayed.
• APR combines AEC with computerized exposure chart for anatomical structures/positioning criteria.
• The user can adjust pre-selected exposure values
  • kVp, mA, FSS.
  • AEC chamber selection.

Path of the Beam

• Tube
• Patient
• Table
• Grid
• AEC ionization chamber
• IR

What happens? Ionization chamber type AEC

• Air in the chamber is ionized, creating an electrical charge proportional to radiation.
• Current travels to the timer circuit to terminate the exposure time.
• It measures electric charges produced by ionization of air before reaching the IR which is used to turn off switch.

Determining configurations

• Various chamber combinations can be used to control the exposure.
• The cell receiving the most radiation contributes the greatest electrical signal.
• The averaging is of the signals from the cells amplified and summed and divided by number of chambers activated.
• Proper chambers and positioning must be used .

Photocell arrangement

• The radiographer decides which photocells to activate.
• Typical arrangement includes:
  • Two outside photocells activated when the density of the body part over the two photocells is close to the same for each photocell (PA chest).
  • Middle photocell activated for a body part whose center section is to be demonstrated (lateral chest, AP or lateral spine, AP hip, AP shoulder).
  • All three sensors (large part).
• Activating the wrong photocell can cause the radiograph to turn out too light or too dark.

Positioning- barium studies

• For stomach studies, center cell should be positioned in that region.
• If all 3 cells were activated with barium over one cell, the normal tissue over the other 2 cells will result in a slightly overexposed image.
• If only 1 cell is activated, barium positioned over the cell, image would be greatly overexposed because insufficient radiation was received to terminate the exposure.

C-spine positioning

• Using AEC is not ideal for flexion and extension views.

Lateral anatomic positioning

• Center cell is used.

Prosthetic positioning

• Prosthetic devices such as total hip hardware can also cause the selected ionization chamber to overexpose the image receptor.
• Selecting the chamber over the prosthetic device may lead to saturation.

Collimation considerations

• If sensors are collimated off, exposure will continue until the chamber reaches a pre-determined charge or the backup time has been reached.
• If collimators are left open, too much scatter, creates premature exposure termination.
• When shielding, if the beam intercepts the shield, exposure of the unshielded anatomy will be increased.

AEC- Optical density controls

• The AEC system permits the adjustment of the amount of radiation necessary to send the exposure termination signal.
• Density controls may be used when the ionization chamber configuration is not producing the desired image.
• Density controls are not used to compensate for patient thickness or kVp changes.
• Setting the density one step lower than normal (0) causes the exposure time to be about 30% shorter than normal.
• Setting the density one step higher than normal (0) causes the exposure time to be about 30% higher than normal.

Minimum response time

• Minimum response time is the length of time necessary for the AEC to respond to the ionization and send a signal to terminate the exposure.
• Modern AECs have a minimum response time = 0.001 second (1 ms).

Backup timer

• Backup timer establishes the maximum exposure time for the system in order to prevent overexposure.
• The backup timer is a safety measure for both the patient and the tube.
• Should be set at least 150% (1.5 times) of the anticipated manual exposure time.
• U.S. law requires that generators automatically terminate AEC exposures at 600 mAs above 50 kVp.
• When the backup timer is set too short, underexposure will be the result.

Quality control standards

• The AEC system should be able to adjust the exposure time to maintain receptor exposure and image quality with any changes in milliamperage selected by the radiographer
• Must ensure accuracy and consistency of the machines AEC
• Federal mandates require that any variation cannot exceed +/-10%.

kVp

• AEC affects only the exposure time, and time controls receptor exposure.
• kVp controls contrast.
• If the kVp is changed, the AEC will compensate with an adjustment in exposure time to adjust density.
• If the kVp is decreased, density decreases, so the AEC will increase the time to compensate for the decrease in kVp.
• If the kVp is increased, density increases, so the AEC will decrease the time to compensate for the increase in kVp.
• An increase or decrease in kVp when using AEC only changes the contrast on the radiograph, not density, so the kVp should not be varied when using AEC
• kVp determines the penetration of the body part, the contrast on the radiograph, and the amount of scattered radiation produced.

Milliamps (mA)

• Theoretically, any mA can be selected when using AEC and the radiograph will have adequate receptor exposure.
• If a low mA is selected, the length of the exposure time will be increased by the photocell to produce adequate receptor exposure.
• If a high mA is selected, the length of the exposure time will be decreased by the photocell to produce adequate receptor exposure.
• Typically, it’s best to set a high mA. This results in the exposure being as short as possible, which helps control motion.
• If a small focal spot is required, the mA must be low enough to be in the small focal spot range, and the exposure time will be longer.

Upside to using AEC

• It takes less time to set technique for an exam.
• Improves exposure accuracy, as long as proper positioning is executed.
  • Lowers repeat rates.
  • Reduces patient dose.
• There is more consistency in receptor exposures.
  • From patient to patient.
  • From tech to tech.
  • From room to room.

Downside to using AEC

• Technologists come to rely upon the system.
• Overconfidence in using the system may cause techs to become neglectful & commit errors.
• If you are not centered over the AEC cell properly, exposure may not be correct.
• It cannot be used on portables or C-arm procedures.
• AEC may work differently from room to room.

Recap

• When using AEC the technologist must set:
  • kVp
  • mA - FSS
  • Photocell arrangement
  • Automatic Exposure Control density
  • Position the body part correctly over the designated cell
  • Back up timer

Over exposure/ Under exposure effects

Spatial Resolution

• Photographic image factors include receptor exposure and contrast.
  • Receptor exposure: the overall amount of radiation that reaches the IR (density)
  • Contrast: the difference in densities on adjacent areas of the radiographic image.
    • High contrast= short scale=black and white, fewer grays.
    • Low contrast=long scale=many shades of gray.
• Geometric property: Spatial resolution.
  • Spatial resolution= the degree of geometric sharpness or accuracy of the structural lines.
  • The ability to perceive structures on the radiographic image receptor as being separate and distinct. It's dependent upon how well the edges of objects are recorded on the IR
  • Visibility – brightness/contrast.
  • Sharpness- spatial resolution/distortion.

Geometric blur

• Penumbra which means Geometric unsharpness, area of unsharpness surrounding the image (peripheral).
• Umbra which means area of image sharpness.

Sharpness of image

• Image cannot be an exact reconstruction of the anatomic structure
  • Always some magnification.
  • Some information get lost during the process of image formation (PSF).
• Anode side of the tube produces less penumbra than cathode (line focus principle).

Low spatial resolution

• Images will appear blurry.
• Edges of anatomical structures will not appear sharp.
• It is a low detail image.
• Low visibility of structures and spaces in between structures.
• Motion may be present.
• Poor alignment of the x-ray photons to the part.
• Large object size.

High spatial resolution

• Images will be clear without blurred areas.
• Edges of anatomical structures will appear sharp and easily visualized.
• Is a high detail image.
• High visibility structures and spaces in between structures.
• No motion present.
• Accurate alignment of the x-ray photons to the part.
• Small object size.

Factors affecting spatial resolution

• Spatial resolution tends to improve on images through factors that can increase patient dose.

Motion and spatial resolution

• When fine detail is lacking, the image will often appear blurred.
• Voluntary motion=patient can control.
  • May need immobilization or support to keep still.
  • Communication in key to reducing voluntary motion.
• Involuntary motion=patient is unable to control.
  • Cardiac motion, an intubated patient will not be able to control respiration so decrease time.
• Increases unsharpness on the image.

OID and spatial resolution

• Short OID brings the object close to the IR and improves spatial resolution.
• When a large OID is used, photons will diverge and cause image blurring.
• Increased OID will cause magnification of the part, decreasing the visualization of small details.

Focal spot size and spatial resolution

• Focal spot (focal tract) is the area on the anode where x-ray photons are created.
• Small fss (narrow) is better-photons leave the x-ray tube more aligned and will show clean edges of structure detail. -Large fss (wide or broad)- photons leave the x-ray tube not. Aligned as well which anatomica structures will be blurred.
• Focal spot size is directly affected by the filament size and anode angle.

SID and spatial resolution

• Greater SID improves spatial resolution. The more central part of the beam penetrates the part
• When a short sid is used, photons will diverge more and cause image blurring.

Width of penumbra

• P = FSS x OID / SOD
• SOD= SID-OID.

Patient size and spatial resolution

• A larger pt size increased OID.
• Increase in pt size or thickness results in.
  • Magnification (OID).
  • Increased scatter radiation.
  • Decreased spatial resolution.

Angulation and spatial resolution

• Tube, body part, or IR angulation will result in distortion.
• Increased OID = decreased spatial resolution.
  • Trauma situations where the part cannot be close to the IR.
• Ideally, tube, part, and IR are always aligned.

Pixel size

• Digital imaging spatial resolution is determined primarily by matrix size, pixel size, and gray scale bit depth.
• Described in terms of spatial frequency.
• High resolution – shorter-wavelength signal (higher frequencies) = pairs of lines that can be visualized very close together.
• Low resolution – longer wavelength (lower frequency) = pairs of lines further apart.
• Trabecula of bone is a good guide in distinguishing adequate image resolution.

Assessing resolution

• Assessing spatial resolution = imaging systems’ ability to display objects accurately in two dimensions.
  • Point spread function (psf) / line spread function (lsf) / edge spread function.
  • Spatial frequency.
  • Modulation transfer function (MTF).
  • Noise.
• Spatial resolution is increased when two objects can be distinguished when they are smaller or closer together.

Assessing spread function

• Point spread function – using a point.
• Line spread function – using narrow slit in a sheet of lead.
• Edge spread function – using a sharp edge instead of a line or point.
• Used to express boundaries of an image.

Assessing spatial frequency

• Defined by the unit of line pairs (includes a line and a space) per millimeter (lp/mm).
• High spatial resolution represents a high-frequency signal that is capable of imaging smaller objects which will have lines closer together.
  • Lines closer together.
  • Reduces contrast.
  • Reduces modulation. -Low spatial resolution represents a lower-frequency signal that can only image larger objects & will increase MTF.
• It is determined by measuring the distance between pairs of lines that can be imaged as distinct from one another.

Resolution test pattern (will record and measure lp/mm)

• Test tools are pairs of lines that are different distances from one another.
• We want to see more lines close together.
• The resolution tool is read by selecting the point at which the finest lines are still visible as separated from one another and that point is then compared to the scale to determine the lp/mm reading.
• The most lp/mm has the highest spatial resolution.

Assessing modulation transfer function

• Measures the accuracy of an image compared to the original object on a scale of 0-1.
• DR has a higher MTF at low spatial resolution frequencies because of the expanded dynamic range (range of values a digital imaging system can accurately detect & represent) of DR and its higher Detective Quantum Efficiency (DQE) – how well an imaging system converts an input signal to an output signal.

Assessing image signal (exposure related) and noise

• Signal – deposit of energy in a detector IR (image data) that an analog or digital.
• Image noise contributes no useful diagnostic information.
• Noise decreases image quality which limits the ability to visualize objects.
• Types of noise:
  • Quantum noise.
  • Scatter.
  • Electronic.

Signal- noise ratio (SNR)

• Depends on amount of exposure to IR (signal).
• Evaluates overall quality of image.
• Combines the effects of contrast, resolution, and noise.
• Ratio between “signal” (meaningful information) and “noise” (background information).
• SNR – as noise increases – visualization decreases.
• Desirable to have a high SNR where higher signal makes for a lower noise and better image quality.

Contrast-to-noise ratio (CNR)

• Ratio = the difference of signal intensities of two regions of interest to the imaging noise.
• Assesses visibility of a specific feature against its background.
• SNR measures the overall signal strength relative to noise, while CNR specifically focuses on the contrast between different parts of an image.

Low contrast resolution

• Type of contrast resolution - the ability to distinguish between objects with slightly different densities or signal intensities (ex. Soft tissues).
• Determined by the system’s ability to visualize small objects of low contrast.

Temporal resolution (TR)

• Relationship between duration of data acquisition & motion of structures such that Shorter acquisition times leads to better TR due to minimized motion.
• TR increases – spatial resolution decreases (inversely proportional).

Digital sampling

• Digital systems sample incoming signals (collect information) at discrete points.
• Nyquist criterion = DR systems require spatial resolution frequency signals to be sampled twice from each cycle.
• Aliasing occurs when the Nyquist criterion is violated (high frequencies are represented as low frequencies & the incoming data are sampled less than twice per cycle) – MRI / CT image artifact.
• Processing algorithm that averages the incoming analog data by using the distance between the imaging detector elements.

Review Points:

• Spatial resolution = Detail on structural lines that help us view image overall better.
• Synonymous terms include Sharpness detail definition.
• Effects of distance on spatial resolution should include inc sid while dec oid.
• Factors that affect penumbra are Motion, oid, fss, sid.
• Techniques to prevent patient motion Communication for voluntary motion and Short time for involuntary motion.
• Clinical protocol for improving resolution = Bend in joints that cant help, angle IR, Ap chest vs pa chest, Increased sid on larger pt.