8.4 Physicist Tests: Artifacts, Phantom Quality, Illuminance, Tech QC Program Review, Compression Application/Paddle Alignment, Workstation QC

The rest of the physicist visit

Section 8.3 covered whether the beam is collimated, detailed, automatic, and under the dose cap. This section is everything else ARRT lists for the medical physicist: artifact evaluation, phantom image quality evaluation, room illuminance, evaluation of the technologists' QC program, application of compression, compression paddle alignment, and acquisition and radiologist interpretation workstation QC. Your job on the exam is still purpose and frequency. Your job at work is to understand why the physicist is looking at your QC book and your monitors — and what happens when something fails.

Artifact evaluation

Purpose. Artifacts can mimic calcifications, hide masses, or make a detector look dirty enough that no one should interpret the study. The physicist looks for grid lines, ghosting from prior exposures, dead or noisy detector regions, filter scratches, paddle edges, and processing blotches — on flat fields and on the phantom, in 2D and in DBT slices when the unit has tomosynthesis.

Frequency. At MEE (a new or repaired detector is an artifact magnet) and as part of the annual survey, typically tied to phantom imaging and any manufacturer flat-field test. Technologists also hunt artifacts weekly on the phantom (8.1). The physicist evaluation is the independent, documented check that the weekly program is actually catching problems and that artifacts outside the wax insert — the rest of the large digital phantom — are not ignored.

Performance idea. ACR is explicit for the digital phantom: clinically significant artifacts fail, even if object counts pass. Do not argue that banding in the axillary tail of the phantom does not matter because fibers were visible in the center.

Physicist phantom image quality evaluation

Purpose. Same objects as your weekly test, scored by someone who does not have an incentive to pass a busy unit. The physicist also uses the phantom for CNR (8.3) and, on DBT, for z-resolution and volume coverage. ACR reviewers use the same 2018 scoring process the facility should use: two largest fibers, three largest speck groups, two largest masses, no clinically significant artifacts, chest wall flush, about 5 daN compression.

Frequency. MEE and at least annually. This does not replace weekly technologist phantoms. A unit that has only an annual physicist phantom and no weekly tech phantoms has a failed technologist QC program, which is itself a physicist finding (below).

Room illuminance

Purpose. Illuminance is light falling on a surface, measured in lux. The physicist measures ambient light at the interpretation display because glare and room light raise the black level the eye sees and destroy low-contrast masses. This is the metered version of the viewing-condition habits in 8.2.

Frequency. At MEE of a workstation (a new reading room is a new device) and with annual display evaluation. If the reading room is remodeled or lights are replaced, treat it as a reason to remeasure, not as interior decorating.

Performance idea. Quote the action limit on the QC form. Several manufacturer digital programs historically required reading-room illuminance not to exceed about 50 lux at the monitor, with a failed test blocking further interpretation on that display until lighting is fixed. ACR 2018 still expects the physicist to evaluate illuminance; do not memorize a lux number the stem did not give if the form in the question prints a different limit. Viewbox-era 3,000 nit luminance rules apply only if the site still uses a mammographic viewbox.

Evaluation of the technologists' QC program

Purpose. MQSA quality assurance is not complete because a physicist can pass a unit on a Tuesday. Someone has to verify that weekly phantoms, monthly checklists, monthly monitors, semiannual compression, and manufacturer calibrations actually happened at the required frequency, were scored against action limits, and were followed by corrective action. EQUIP later asks whether the LIP oversees that QC; the physicist's survey is the technical review of the same record.

Frequency. Each annual survey (and at MEE to the extent tests already exist). ACR states the physicist does not have to evaluate a minimum number of in-person QC days and may review the program remotely (records sent for review). A brand-new unit can only be judged on the tests performed since install.

What a fail looks like. Missing weeks of phantom images, checklists initialed in batches, no RWS QC, overdue compression-force tests, or failed tests with no documented repair. The physicist reports that to the LIP. Your defense is a complete, dated QC book — not a verbal claim that the phantom looked fine.

ACR 2018 splits unit QC review from display-device QC review because workstations are standalone. The physicist may pass the gantry and still fail the RWS program.

Application of compression

Purpose. Confirm that powered compression actually delivers a clinically useful, legally bounded force and that the device holds force without drifting off the breast. This is the physicist's version of the technologist's semiannual compression-force test, done with calibrated instrumentation at MEE/survey.

Frequency. MEE and as required on the annual survey / manufacturer list. After a compression-device repair, expect an MEE-level retest before patients.

Criteria to remember. Same MQSA window as 8.2: at least 25 pounds (111 N) available, initial power drive not above 45 pounds (200 N). The physicist is also watching for uneven application (one side of the paddle loads the scale, the other floats), which is a bridge to paddle alignment.

Compression paddle alignment

Purpose. If the paddle hangs out beyond the receptor at the chest wall, it can clip posterior tissue from the field or cast a shadow of the vertical paddle edge onto the image — a white or bright band that looks like a filter-edge artifact and can hide tissue. If the paddle sits too far in, the chest wall is not compressed.

MQSA (21 CFR 900.12). The chest-wall edge of the compression paddle shall not extend beyond the chest-wall edge of the image receptor by more than 1% of the SID when the paddle is at standard-breast thickness. The shadow of the vertical edge of the paddle shall not be visible on the image.

Worked example. SID 65 cm. One percent of SID is 0.65 cm. A paddle that overhangs the receptor by 1.0 cm at the chest wall fails. Two percent of SID (1.3 cm) is the x-ray field tolerance from 8.3, not the paddle tolerance. Mixing 1% and 2% is a designed trap.

Frequency. Measured with collimation geometry at MEE (and annually when collimation is annual, as with many DBT systems). After paddle replacement, do not assume alignment is still legal.

Acquisition and radiologist interpretation workstation QC (physicist)

Purpose. The physicist's workstation testing goes beyond the technologist's monthly TG18/SMPTE glance. It asks whether each AWS and each RWS, treated as a standalone device, meets luminance, uniformity, and pattern criteria in the QC program, and whether the technologist's display QC on that device is being done.

Frequency. MEE when a workstation is installed or newly associated with the mammography program, and at least annually thereafter. ACR 2018 forms track displays across rooms and even across facilities so a borrowed RWS does not vanish from the inventory.

Relationship to monthly tech QC. You still clean and pattern-check monthly (8.2). The physicist checks that those monthly tests exist and that the display itself still passes a deeper evaluation. A monthly tech pass does not waive the annual physics workstation test.

Corrective action without a homemade cheat sheet

When a test is a hard fail, the affected equipment stays out of clinical use until the problem is corrected and the test is repeated successfully. That is the concept to carry into the exam. Do not memorize an unofficial list of which digital tests are 30-day problems unless the question cites a form.

What you can cite:

• MEE fails: no clinical examinations until the unit (or repaired component) meets requirements.
• MQSA 21 CFR 900.12(e)(8): compare results with action limits (manufacturer limits for digital, or the alternative standard the facility adopted). If results are outside limits, identify the source and take corrective action before further examinations that would use a component that failed tests listed in (e)(1), (e)(2), specified (e)(4) items, (e)(5)(vi) dosimetry, (e)(6) other-modality/digital QC, or (e)(7) mobile checks. All other tests in (e) must be corrected within 30 days of the test date.
• ACR 2018 and manufacturer forms print a corrective-action column for each test. That column is the operational answer in the real department.

Phantom image quality, AGD over 3.0 mGy, a failed detector calibration, a failed primary RWS, and a failed MEE item are not situations where you keep scanning and file a note to fix it next month. Illuminance that exceeds the display's action limit means stop interpreting on that workstation until lighting or the display is fixed — manufacturer language has treated that as a before-further-review fail.

Purpose and frequency snapshot

How 8.1–8.4 fit on exam day

If the question names a technologist test, be ready for procedure and numbers: weekly phantom 2/3/2 at ~5 daN, monthly thickness ±5 mm, monthly visual and monitors, semiannual 25–45 lb force, manufacturer-timed detector calibration, optional repeats. If it names a physicist test, answer purpose and frequency: MEE before clinical use after install/move/major repair, annual survey at least, AGD ≤ 3.0 mGy, kVp ±5%, collimation 2% SID, paddle 1% SID. If it names manufacturer QC, do not paste ACR 2018 dates onto the stem. If it names a fail, stop using the failed component until the program's corrective action is done — and use MQSA (e)(8) plus the form in front of you, not a rumor about 30 days for every test.