“Helping educational programs make digital radiography easy to understand.”

How Digital Changes Grid Use and Scatter Clean-up

Quinn B. Carroll, MEd, RT

Atlanta Society of Radiologic Technologists 2018 Conference

 

 

  1. Virtual Grid Software

Announced in 2014 by Fuji as “Virtual Grid”

Followed by Philips “Skyflow”, Samsung’s “Sim Grid” and others

“Can provide scatter correction without the use of grids in many cases

Rave reviews from departments that have tested it

  1. Testimonials in articles:

-“In fact, our neurosurgeons now specifically request the FDR Go

interoperatively for any surgical lateral spine cases”

-“We are using virtual grid in 90% of all  portable chests and 75% of

abdomens”

  1. Recent Articles:

– Fuji 1: “Fast, Reliable Mobile Capabilities”:

Reduced artifacts from misalignment

Reduced repeat rates “from 7-8% to 4.2%”

Reduced patient dose by 70% (?)

Fuji 2: “Reduces patient dose by up to 50%”

“Can be applied to all body parts”

                        Philips SkyFlow: “Without any user interaction or implications on workflow,

 SkyFlow enhances image contrast as a grid would … For images

 of different patients, contrast enhancement is stronger for obese

 patients than for slim patients”

“Skyflow …provides a consistent grid-like image impression for a

 wide range of patient types, which is available instantly”

                        FDR D-EVO GL Detector with VG (Fuji) “Extra Long (125 cm / 48”) DR

Detector”: “1/2 patient dose with equivalent image quality”

                                    -Disadvantage: Gentle gradient of image density, therefore for

 mobile procedures, misalignment must be minimized”

  1. Best White Paper: “Improvement in Image Quality and Workflow of X-Ray Examinations using a New Image Processing Method, “Virtual Grid Technology”:

Conclusion: Comparing human chest radiographs:

Using ½ mAs, the Virtual Grid image “has nearly the same level of contrast as the image … exposed with a grid. In the enlarged view comparison, we can confirm that details of anatomical structures such as pulmonary vessels, ribs, and the heart are close to those of grid exposure images, and that even in terms of sharpness and granularity, the Virtual Grid provides image quality comparable to that acquired with grid exposure.”

            “We anticipate broad applications of this technology”

 

  1. STEPS (c/o Fuji):
  2. Scattered X-Ray Estimation

-An estimate of the dose at the IR is derived from pixel values

-This amount is subtracted from typical raw-beam dose for the procedure

             to find an estimated absorbed dose ratio

-From this percentage, an estimated thickness for the subject is derived

-Based on this thickness and the actual technique used, the expected

 amount of scatter radiation produced is estimated

  1. Grid Effect Calculation

-Based on the estimate of scatter production, the intensity of contrast

 improvement needed is determined

-This serves the same purpose as selecting a grid ratio used to serve

-As a marketing ploy, manufacturers refer to this process as selecting the

 virtual grid ratio, which the radiographer can change at the console

  1. Granularity Improvement Processing

-This is CNR (Contrast-Noise Ratio) image analysis, after which noise

 reduction algorithms are applied to the image  

  1. Important to avoid confusion for the student:

-It is not possible to “go back in time” and actually change the amount of

 scatter produced at the IR

-It is not possible to “select a grid ratio” to use after the exposure has been

 taken. (This process simply selects the intensity with which the

 above algorithms are applied.)

                  -These terms are used by manufacturers as marketing ploys, to make the

 software seem like magic

                  -Email from educator: Samsung’s ‘Sim Grid’ “a post-processing feature

 which removes scatter”

  1. Analyzing Statements from Manufacturers:

-Philips: “Grid-like Contrast Enhancement”

-Fuji 2: . “VG emulates a wide range of physical grid characteristics by

 grid ratio, density, and interspace material”

-Fuji 3: “VG intuitively recognizes and reduces scatter effect based on

 exam menu”

-Fuji 4: “VG replicates grid use, increasing visibility and contrast to …

 reduce window width and window leveling and improved motion

 unsharpness reduction

 

  1. STEPS SIMPLIFIED:
  2. Contrast Enhancement Algorithms
  3. Noise Reduction Algorithms

KEY POINT: The combination of these algorithms has become so effective that

they can compensate for the lack of a grid during exposure

  1. Samung’s Sim Grid: Test c/o Bob Grossman:

Conclusion: “Sim Grid does improve the image quality but does not

 compare with the image quality achieved through the use of a grid”

 Difference in manufacturers? Samsung < Fuji?

  1. Most VG software has demonstrated effectiveness approaching that of grid

 use  -While not quite matching it

  1. VG software has been clinically demonstrated to reduce patient dose by 50-

70% with NG techniques

 

  1. Exposure Latitude of Digital: How Wide Ranging?
  2. You can’t have it both ways:

-“Grids are as important as ever” (or more imp.)

-“Digital systems are more sensitive to scatter” (not just detectors)

Even though digital imaging has much wider exposure latitude

-In fact, a whole magnitude wider (10X) than F/S

  1. Film-based analog radiography required a subject contrast difference between

tissues of at least 10% to resolve details, (but could then do so with higher

sharpness [curve farther left]). Because of its contrast enhancement

capabilities, digital imaging can resolve tissues with only 1% subject

contrast.

  1. In Reality:
  2. Compensating Filters are less important
    1. Default digital processing tends to balance densities across img
    2. Filters needed only for “extreme cases”
    3. Demo: next slide
    4. Scatter is less important
    5. Higher kVp can be used on all procedures
    6. Grids are less important (next slide)
    7. Longer SIDs can be used without compensating

-Patient dose is reduced even with mAs compensation

-Patient dose is further reduced when SID is increased up to 15%

 without compensating mAs

  1. In Reality:
  2. Grid use is less important

            More procedures can be done non-grid

            Thicker anatomy (up to 13 cm) can be done NG

  1. Grid ratio is less important

            Lower ratios can be used for each anatomical part

      We can leverage these benefits to reduce patient dose

 

  1. Types of Noise & Scatter as Noise
  2. In the digital age, our focus is on ensuring good SNR (signal-to-noise ratio)

All forms of noise must be minimized

All forms of noise can potentially affect the shape of the histogram

                               Histogram analysis

                               Rescaling and gradient processing errors

  1. Our concern is the cumulative impact of all forms of noise combined
  2. Table: Types of Image Noise  

 

  1. Quantum mottle
  2. Material mottle
  3. CR plate phosphor crystals
  4. Indirect-conversion DR phosphor crystals
  5. Image intensifier input phosphor crystals
  6. Fiber-optic bundles
  7. Scatter radiation
  8. Off-focus Radiation
  9. Aliasing Artifacts
  10. Insufficient digital sampling
  11. Magnification (zoom) of displayed image
  12. Grid line frequency vs. sampling frequency
  13. Electronic noise
  14. Image intensifier
  15. Charge-coupled devices (CCDs)
  16. Direct-conversion DR active matrix arrays (AMAs)
  17. Computer hardware, including ADCs and DACs
  18. Display systems (LCD, CRT)
  19. Algorithmic (computer) noise
  20. Exposure artifacts
  21. Grid lines
  22. Extraneous objects
  23. Destructive superimposition of tissue

(or structures) within the patient

  1. False images (tomographic streaks, ring artifacts, etc.)
  2. In the digital age, the sole purpose of the electrical technique factors set for

 initial exposure is to assure that an adequate amount of useful

 information reaches the IR

  1. Focus has changed from avoiding scatter to ensuring good SNR (signal-to-noise

 ratio)

  1. For kVp, there is a trade-off between scatter and mottle, both forms of noise

 (end of presentation)

  1. All forms of noise must be minimized

Scatter is just one of these types of noise

  1. High kVp and Mottle:

-Can the 15% rule be applied to reduce mAs without causing significant

 mottle? 

-If so, how many steps may be applied before mottle becomes significant?

  1. Answer: YES – At least one 15% step
  2. We CAN make at least a single 15% step increase in kVp, with attendant halving

 of the mAs, “across the board”

            -Due to already low mAs values, chests may be an exception

            -2 steps may be possible for some manufacturers, with some anatomy

  1. How effective is the 15% rule at reducing patient exposure and patient dose?

 

  1. Cutting mAs to ½ reduces both ESE (Entrance Skin Exposure) and absorbed

 dose to appx. ½ (50%)

  1. Raising kVp 15% increases bremsstrahlung production in the anode by 25-

35% (approx. 1/3)

                        50% + 1/3 of 50% = 67%

  1. The net result is appx. 67% of the original skin exposure

        -This a savings of 1/3 exposure, well worth the effort!

  1. Absorbed dose is reduced still more due to increased beam penetration

 resulting in fewer interactions of all kinds

  1. When a radiologist’s report states, “Images for this patient demonstrate an

 acceptable amount of mottle,” the radiographer has used the best

 technique for minimizing patient dose

  1. The radiographer’s goal should not be to completely eliminate mottle from digital

 images, but rather to minimize patient dose until barely perceptible but

 acceptable levels of mottle become apparent, as defined by the

 radiologist.

  1. Departments must collaborate with radiologists to define acceptable amounts of

 mottle for each procedure

  1. Conventional film radiographs of the lateral lumbar spine often showed a fog

 pattern from table scatter, which obliterated the posterior portion of the

 spinous processes

  1. Digital processing is able to correct for this on most lumbar radiographs, such

 that the spinous processes are demonstrated  entirely

  1. About 3 out of 4 film images suffered from the fog pattern
  2. About 3 out of 4 digital images correct for it
  3. A Digital equipment is remarkably resilient to the effects of scatter radiation

 caused during an exposure

-The “garbage in, garbage out” philosophy implies that computer

 algorithms cannot compensate for “raw” data that has been

 affected by scatter radiation, i.e., “garbage.” This is largely false –

-The post-processing capability of digital imaging allows for correction of

 just such errors as excessive darkness or excessive gray scale

 that scatter can cause

  1. Most typical localized fog patterns do not alter the shape of the acquired image

 histogram enough to throw off landmark identification

-ADD typical FOG:

-If SMAX has been defined as the second time the threshold is reached,

 scanning from right to left, histogram analysis looking for the SMAX point

 within the anatomy on this histogram will still be able to locate it

 

  1. How Frequency Processing Eliminates Fog Patterns
  2. Frequency processing can even “clean up” specific fog patterns within the latent

 image

      -This is because frequency processing can target “structures” of a specific size to

 eliminate from the image, and fog patterns often have a characteristic size

 that is larger than any of the bone structures in the image

  1. Note that small-detail frequency layers of the digital image do not show the black

 “background density,” because it is too large. Like background densities,

 fog patterns are often much larger than bony structures. They are low

 frequency patterns

  1. Frequency Review:

-In the transverse, (horizontal) dimension, the vertical bars at the left have

 a frequency of 5 Hertz (5 pairs)

                        – The fog pattern at the right has a frequency of 1 Hertz

                                    (1 pair of densities fits across)

  1. By comparison, a lumbar fog density (left) typically has a frequency of about 2

 Hertz. Shown in C, two pairs of these densities would fit across a 25 cm

 image receptor

  1. A computer algorithm that simply removes frequency layers of 2 to 3 Hertz will

 delete this pattern from the image

  1. For example, conventional film radiographs of the lateral lumbar spine often

 showed a fog pattern from table scatter, which obliterated the posterior

 portion of the spinous processes

  1. Digital processing is able to correct for this on most lumbar radiographs, such

 that the spinous processes are demonstrated entirely

  1. Use of Grids with Digital Radiography

-Where grid use or grid ratios can be reduced, substantial patient dose

 can be saved

-Patient dose can be reduced to as low as 1/3 to 1/4

-In addition, much greater flexibility in positioning, centering, and angling is

 allowed – This is especially beneficial for mobile and trauma

 radiography

  1. The several ways in which digital processing compensates for the effects of

 scatter radiation are supplanting the traditional purpose for grids

  1. Grids now comprise one method of reducing the overall amount of noise the

 computer must deal with

  1. However, grids are now strictly necessary only in the most extreme cases of

 scatter production

  1. Anatomy smaller than 13 cm (the average thigh) should no longer be done with

 grids

-The noise-reduction capabilities of default digital processing make this

 possible

  1. In some departments, radiographers are leaving the grid in place for all DR

 procedures including distal extremities. This violates the ALARA principle,

 and results in 3 times (or more) the needed exposure, and must be

 discouraged

  1. On Reducing the Use of Grids

– All these approaches will save patient dose

-Added benefit of allowing more positioning flexibility

 

  1. Only use grids on anatomy exceeding 13 cm

Especially for pediatric procedures

  1. Do not use grids for procedures within the aerated thorax

 (unless patient is very large):

-Chest (unless larger than 26 cm)

-Thoracic Spine

-Sternum

-Ribs above the diaphragm

  1. Use lower grid ratios than for F/S radiography

-For all mobile procedures, use a 6:1 ratio grid

  1. Remember: It is the soft tissue thickness, NOT the procedure or

 even the part, that determines grid use

 

 

  1. Mottle or Scatter: Which is More Acceptable?

-All forms of noise increase the risk of digital processing errors

 

-Using a grid reduces noise in the form of scatter

-Removing a grid (if the technique is not compensated, reduces the

 probability of noise in the form of mottle

 

-With digital equipment, which is more likely to affect the final displayed

 image?

-Which is more important to remove?

  1. Reducing patient dose and the probability of mottle can both be achieved:
  2. Remove the grid

-A 10:1 or 12:1 grid have a bucky factor of 4

-Removing the table grid, one-quarter of the mAs could be used

-This would result in a 75% reduction in patient dose

 

  1. Reduce mAs only to ½ or 1/3 the original grid technique, (instead of ¼)

            -The image receptor is now receiving more radiation than the grid

 technique allowed, such that mottle is less likely, yet patient dose

 has also been cut by 50-66% from the grid technique!

 

  1. What We Should Absolutely Be Doing:

 Leveraging Digital to Reduce Patient Dose

  1. Reducing all grid ratios to 6:1 AND use virtual grid

-Especially for mobile procedures

-From 12:1 to 6:1 = 3/4 = 25% Reduction

-(From 15:1 to 6:1 = 3/5 = 40% Reduction)

 

  1. COMBINE this with one 15% step increase and half mAs across the

 board on technique charts

(Possible exception: Chests)  =  35% Reduction (25-40%)

Conservatively, this would result in 65% of 75% = 49% patient dose!

 

  1. Why and How We Should Reduce Grid Use
  2. On Reducing the Use of Grids:

– Grids should not be used when it is unnecessary

-Digital technology is allowing us as a profession to explore and adopt

 tools that will benefit the patient in various ways

 

  1. Noble & Culp Article (ASRT):

Discussion: Published grid conversion factors are not acceptable with PSP plates and can result in ovrerexposure to the receptor, which would result in overexposure to the patient … Traditional grid conversion factors do not apply in computed radiography because of the larger dynamic range. Based on this research, the continued publication of grid conversion factors in radiologic science  textbooks and literature should be reconsidered or discussed in the historical context of analog image. The discussion of antiscatter grids … should remain in the context of the type of receptor and digital imaging system being used …

 

Conclusion:

The authors do not recommend a standard set of grid conversion factors in digital imaging because exposure indicators vary among vendors, receptor technology, and institutions. Furthermore, the authors recommend that radiology department interdisciplinary teams that include physicists, radiologists, and technologists create technique charts and calibrated grid conversions based on the digital systems being used and on radiologist preference for image quality with the goal of maintaining the ALARA principle.

 

 

  1. Letter: Our radiologists have merged with a larger group who now use a grid for all chest mobile imaging … If we need to change, what grid ratio would be best to use for high kVp chest imaging. I know that a 10:1 ratio is good but is not forgiving.

My Response: See if the radiologists would settle for a minimum-ratio grid such as a 6:1. Having a grid at all makes a much greater difference than going from one ratio to the next higher, in other words, you may get 60% clean-up with a 6:1 and going to a 12:1 may only increase this to 70-75%, so the trade-off is not worth it when you have much less latitude for mobile positioning and also the increased patient dose.

 

Remember too that the air-insufflated lungs do not produce much scatter compared to soft-tissue body parts like the abdomen, and this is why mobile chests can be done non-grid unless that patient is very large. Therefore, compared to other body parts, for chests there should be the flexibility to use a low-ratio grid.

 

 

  1. If continuing grid use:

Smit Roentgen grid by Philips claims the “lowest bucky factor in the industry”

-Fiber interspacers (Vs. aluminum)

-“Uses 30-40% technique of regular grid (?)

 

  1. Disadvantages of conventional grids:
  2. Increased patient dose
  3. Inflexibility for mobile positioning

                  (just where flexibility is needed)

-Centering

-Angling   

  1. Potential for repeats
  2. Risk of damaging grid
  3. Financial incentive for department

 

  1. On Reducing the Use of Grids:

-Grids should not be used when it is unnecessary

 -Digital technology is allowing us as a profession to explore and adopt

 tools that will benefit the patient in various ways

 

 

 

 

 

Optional Material:

 

  1. Grid Radius Ranges?

-Is there a “sweet spot” mid-way between 40” and 72” for the best distance to 

      minimize grid cut-off from being out of radius?

                        Absolutely not for the table grid

Not at all likely for the wall bucky:

Sample of manufacturer published radii:

“Available:”

26-32”

34-44” (NOTE: 40” is mid-range)

48-72” (Not 40-72”)

62-72”

  1. Contrast-Noise Ratio:

Difference (in brightness) between two different densities or tissue areas

 divided by the background noise:            T1 – T2 / N

-Similar to SNR, but for two specific tissue densities

-Noise levels can be measured by imaging various test objects that

 present “just noticeable differences (JNDs)” between bars or spots.  -A numerical value can be assigned to the result

                        -Subtracting the CNR Image:

                                    – The predominant size of noise artifacts (granularity) is determined,

 then the frequency layer containing that granularity can be

 identified for filtering upon image reconstruction by reverse Fourier transform

                                    – Any layer can be left out upon image reconstruction)

 

  1. *kVp and Scatter:
  1. CONVENTIONAL RADIOGRAPHS of an elbow demonstrate that even when the

 kVp was increased by 25 kVp (from 65 to 90), there is no visible fogging

 from scatter radiation, only lengthening of the gray scale – Small anatomy

 does not generate substantial scatter radiation even at very high kVp’s

  1. CONVENTIONAL RADIOGRAPHS of the abdomen (with a grid) using A: 80 kVp,

 and B: 92 kVp demonstrate that a single 15% increase in kVp does not

 visibly fog the image from increased scatter, (though the gray scale is

 lengthened from higher penetration), even on the largest body part

  1. High kVp and Scatter Radiation:              

-The following 2 slides show series of 15% step increases in kVp for

 various manufacturers, each with a halving of the mAs. Note how

 slight the contrast loss is from one image to the next

-Although increasing kVp reduces contrast in the initially displayed image,

 this reduction is very slight, much less than it was for conventional

 radiography

            -For a single 15% step increase in kVp, the reduction in displayed contrast

 is generally negligible, and often indiscernible

  1.  Even with a 52-kVp increase, no fog patterns are apparent as was the case with

 conventional screen/film radiography

 

  1. On the High kVp Philosophy

– Even if all the forgoing were inaccurate, starting out with a high kVp approach:

            -Ensures adequate signal penetrating through to the detector, which is

 critical, and

            -Always allows some decrease in mAs that results in a net savings in

 patient dose

-Even a 10% reduction in patient dose is worth pursuing

  1. Minimizing Patient Exposure

Since the advent of digital imaging, many  departments that have not yet adopted an across the board 15% increase in kVp have identified procedures (collaborating with radiologists) for which a modest reduction of image quality (including a moderate amount of mottle) is deemed acceptable for a substantial reduction in patient dose

 

An important example is adolescent scoliosis series, which are performed every few months