OBJECTIVES
With the onset of digital imaging the process of assessing our images has expanded from not only being able to produce excellent images but to being able to view them in high definition. Doing away with high contrast single emulsion film and high quality thin phosphorescent screens has simplified our QC lives in many ways eliminating many tedious tasks. However, now that our images are beamed through the stratosphere and displayed on a distant monitor, the display unit becomes crucial.
This article will be reviewing the image evaluation using the good old RMI which is still an indispensable standard in quality image production. It will also demonstrate method and rationale for careful monitor evaluation using the SMPTE test pattern.
RMI Phantom Evaluation Review:
Objective
The purpose of performing and evaluating the 'phantom test' is to identify weaknesses in our imaging chain and correct them.
Frequency
Weekly immediately after processor QC
Equipment
RMI 156 or equivalent mammography testing phantom
Designated QC cassette, tested and cleaned
Mammography film from working film bin
The acrylic OD difference disc
Densitometer
Magnifying glass
Phantom worksheet or control chart
PROCEDURE
Orient the phantom in the correct manner.
Place your optical density acrylic disc on top of the phantom in the same manner every time. Mark the placement of the disc with an indelible marker.
Do NOT glue the disc to the phantom.
Lower the compression paddle onto the phantom and the disc.
Position the photocell in the middle of the wax insert.
Expose the phantom and acquire the image
PHANTOM IMAGE SCORING
Optical Density Difference of Phantom Image (for film/screen):
Using your densitometer, take the OD of the phantom image in the middle of the wax insert and record the value.
Take the OD in the center of the acrylic disc and record the value.
Subtract the OD value of the acrylic disc from the background OD value.
This represents your ' density difference'.
The density difference MUST be above 0.4 to be acceptable.
Scoring of Phantom Image:
RMI phantom images should be reviewed under optimal and identical viewing conditions by the same technologist each time.
A minimum of the four largest fibers (4/6) should be seen in their entirety. To receive a score of 1 fiber the entire fiber must be seen. If the whole fiber cannot be seen the fiber may be given a score of .5. Score the individual fibers counting from the largest to smallest fiber adding the results together until you encounter a partial score (0.5). This is your total score; no more fibers may be counted.
A minimum of the three largest speck groups (3/5) should be seen. Four to six specks in each group must be seen to result in a score of 1 speck group. If only two or three specks are seen in any group, that group may be given a score of .5. Score the speck groups from the largest to the smallest adding the results together until you encounter a partial score (0.5). This is your total score; no more specks may be counted.
A minimum of the three largest masses (3/5) should be seen. Both a density difference and an entire circumscribed edge will result in a score of 1 mass. If only the density difference or part of the edge is seen the mass may be given a score of .5. Score the masses form the largest to the smallest adding the results together until you encounter a partial score (0.5). This is your total score; no more masses may be counted.
Examine the image background for any artifacts that appear similar to a fiber, speck or mass. For a fiber-like artifact, subtract 1/2 a point for each artifact from your fiber score. For a mass-like artifact, subtract 1/2 a point for each artifact from your mass score. Subtract one speck for each speck-like artifact from the final group of specks scored.
Record individual scores for fibers, specks and masses on you control chart weekly. The scores for any individual group should not decrease by more than 0.5.
Evaluating the Image on the Monitor: SMPTE Test Pattern Tutorial
Brightness and contrast:
The brightness and contrast of your monitor are adequately set if the 5% squares at both ends of gray scale are visible.
Using the pattern below: The gray scale is shown as a series of squares in the center of the image that range from black (0%) to white (100%) in a semi-rectangle. The 0% and 100% squares (see arrows on image) each contain smaller squares within them that represent signal level steps of 5% and 95%, respectively. You should be able to visually differentiate the inner square from the larger square that contains it.
Be aware that it may be impossible to adjust your monitor to show both of these inner squares perfectly and equally. Most video monitors do better in showing the 95% square than the 5% square. However, you might see if reducing ambient light improves the visibility of the 5% square.
BRIGHTNESS AND CONTRAST
:
Spatial Resolution and Distortion:
The spatial resolution (linearity) and aliasing (distortion) of your monitor are within acceptable limits if the high contrast bar patterns in the test image are distinct as simple patterns of black and white pairs.
Using the pattern below: In each corner of the image as well as in the very center (see arrows on image), inspect the 6 squares filled with varying widths of alternating black/white horizontal and vertical lines. You should be able to differentiate all the lines, from fat to narrow (6 pixels, 4 pixels, and 2 pixels) and both horizontally and vertically.
SPATIAL RESOLUTION AND DISTORTION:
SUMMARY:
A lot of things are changing in the wonderful world of breast imaging, but one of the things that remains is producing and displaying sharp detailed images. As our technologies expand and our expertise grows we are able to assure excellence in our images both out of the unit and on the display screen. One day the radiologist may be viewing these pictures on his Blackberry and we will be up to that challenge also.
Keep on growing, knowledge is power!
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