Term
| What are the dimensions of the ACR MRI Accreditation phantom? |
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Definition
Length - 148 mm
Diameter - 190 mm |
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Term
| What is the ACR MRI phantom filled with? |
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Definition
| A solution of nickel chloride and sodium chloride. |
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Term
| What is marked on the outside of the ARC MRI phantom? |
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Definition
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Term
| What seven quantitative tests are made using the phantom? |
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Definition
| 1. Geometric accuracy, 2. high-contrast spatial resolution, 3. slice-thickness accuracy, 4. slice-position accuracy, 5. image intensity uniformity, 6. percent signal ghosting, 7. low-contrast detectability |
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Term
| Describe the structures used to determine slice position accuracy. |
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Definition
| 2 crossed 45° wedges at the top of slices 1 and 11. |
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Term
| Slice position accuracy - the magnitude of the bar length difference is _____ the actual slice displacement. |
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Definition
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Term
| What is the limit for the bar-length difference on the slice position accuracy test? |
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Definition
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Term
| Describe the structures used to measure the slice thickness. |
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Definition
| Two thin horizontal ramps in slice 1. They have a slope of 10 to 1 wrt to the plane of the slice. |
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Term
| How do you set W/L for slice thickness. |
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Definition
| Place a rectangular ROI over the middle of each ramp. Take the average signal value of the ROIs. Set the Level to 1/2 the average signal value. Set the Window to its minimum. |
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Term
| After measuring the two ramps, how is slice thickness calculated? |
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Definition
| Thickness = 0.2 * (top x bottom) / (top + bottom) |
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Term
| What is the criteria for measured slice thickness? |
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Definition
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Term
| How many geometric accuracy measurements are made and where? |
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Definition
| 7 total: 1 end-to-end on the localizer; top-to-bottom and right-to-left on slice 1; and top-to-bottom, right-to-left and both diagonals on slice 5 |
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Term
| How do you set the W/L for the geometric accuracy tests? |
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Definition
| Set the W to its minimum. Lower the level until the water only regions are all white. Increase the level until half the water turns dark. This is the average signal of the water only regions. Now lower the level to half the mean signal and set the window equal to the mean signal. |
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Term
| What is the criteria for the geometric accuracy tests? |
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Definition
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Term
| Describe the high contrast spatial resolution inserts. |
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Definition
| In slice 1 there are 3 sets of bright dots. Each set contains a pair of 4 dots by 4 dots. One square is the upper left (UL) and the other square is the lower right (LR). The left pair is 1.1 mm hole diameters, center is 1.0 mm, and right is 0.9 mm. |
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Term
| Describe how to assess the high contrast resolution holes. |
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Definition
| Look for a row of distinguishable dots in the UL square and a column of distinguishable dots in the LR square. |
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Term
| What is the pass criteria for the high contrast spatial resolution test. |
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Definition
| Should be able to resolve 1.0 mm dots or better. |
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Term
| What slice is used to measure image intensity uniformity? |
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Definition
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Term
| How is the uniformity test performed? |
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Definition
| Place a 200 cm2 circular ROI over slice 7. Stay within this ROI. Set the window to its minimum. Adjust the level to find the area of lowest signal (last dark spot). Place a 1 cm2 ROI over the area and record the value. Repeat for the area of highest signal (last white spot). |
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Term
| What is the formula for calculating Percent Integral Uniformity? |
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Definition
| PIU = 100 * (1 - ((high - low) / (high + low)) |
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Term
| What is the criteria for PIU? |
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Definition
PIU > 87.5% for < 3 Tesla
PIU > 82.0% for 3 Tesla |
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Term
| Ghosting occurs in the _______ direction. |
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Definition
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Term
| Describe the setup for the Percent-Signal Ghosting measurement. |
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Definition
| Using slice 7, set a 200 cm2 circular ROI over the object. Place four 10 cm2 elliptical ROI's over the left, right, top, and bottom between the edge and the object. |
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Term
| What is the formula for ghosting ratio? |
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Definition
| ghosting ratio = ((top + btm) - (left + right)) / (2 x (large ROI)) |
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Term
| What is the criteria for the ghosting ratio? |
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Definition
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Term
| Describe the low contrast test tool. |
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Definition
| Low contrast disks are contained in slices 8 through 11. Each slice contains 10 spokes and each spoke has 3 disks. |
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Term
| What is the requirement for low contrast detectability? |
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Definition
9 spokes total for < 3 T
37 spokes total for 3 T |
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Term
| What is the minimum frequency of the technologist's QC testing? |
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Definition
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Term
| What are the technologist's QC tests? |
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Definition
| 1. Setup, 2. Table position accuracy, 3. Center frequency/transmitter gain, 4. geometirc accuracy, 5. high-constrast, 6. low contrast, 7. artifact evaluation, 8. Visual checklist |
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Term
| What is the action limit for the weekly center frequency test? |
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Definition
1 or 2 ppm week to week change
For a 3T scanner, that is 128 Hz to 256 Hz
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