I85PREP
⌘ K
ARRT Bone Densitometry study materials
ARRT Bone Densitometry study resources

ARRT Bone Densitometry practice questions

One question at a time. Build your ARRT Bone Densitometry understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Equipment and Instrumentation

Why do bone densitometers use two different photon energies when measuring bone mineral density?

  1. To shorten the scan by reducing the number of detector elements
  2. To remove the need for any calibration standard
  3. To increase the total radiation dose delivered to the patient
  4. To separate the attenuation of bone mineral from that of soft tissue Correct answer.

Explanation

Two energies exploit the fact that bone and soft tissue attenuate low- and high-energy photons differently. Solving the two attenuation equations together allows the soft tissue contribution to be subtracted and the bone mineral content isolated. A single energy cannot distinguish the two materials.

Question 2Equipment and Instrumentation

What is the approximate effective dose to the patient from a standard lumbar spine DXA examination?

  1. About 10 mSv
  2. Less than 10 µSv Correct answer.
  3. About 100 µSv
  4. About 1 mSv

Explanation

DXA is a very low-dose technique, with effective doses on the order of a few microsieverts, comparable to a few hours of natural background radiation. A chest radiograph is roughly 100 µSv, and CT examinations are in the millisievert range.

Question 3Equipment and Instrumentation

Which radionuclide served as the photon source in early generations of DXA scanners?

  1. Iodine-125
  2. Technetium-99m
  3. Gadolinium-153 Correct answer.
  4. Cobalt-57

Explanation

Gadolinium-153 emits gamma photons at two useful energies (approximately 44 and 100 keV), which allowed early densitometers to perform dual-energy subtraction. These isotope sources have largely been replaced by X-ray tubes.

Question 4Equipment and Instrumentation

Quantitative ultrasound of the calcaneus typically reports which two measurements?

  1. Areal BMD and T-score
  2. Trabecular thickness and cortical width
  3. Volumetric BMD and Z-score
  4. Speed of sound and broadband ultrasound attenuation Correct answer.

Explanation

Heel ultrasound devices transmit sound through the calcaneus and derive speed of sound (SOS) and broadband ultrasound attenuation (BUA). These parameters reflect bone density and structure but are not direct measurements of BMD in g/cm².

Question 5Equipment and Instrumentation

In a constant-potential DXA system, what is the purpose of the K-edge filter?

  1. It sweeps the beam across the patient during acquisition
  2. It produces two distinct photon energy peaks from a single beam Correct answer.
  3. It mechanically restricts the beam to the scan field
  4. It converts transmitted photons into visible light

Explanation

A rare-earth K-edge filter absorbs photons on either side of its absorption edge, leaving two peaks at different energies from one X-ray beam. This is how constant-potential systems generate the dual-energy spectrum needed for subtraction.

Question 6Equipment and Instrumentation

Which design feature is characteristic of a fan-beam DXA system?

  1. A multi-element detector array that images an entire line at once Correct answer.
  2. A rotating anode that spins throughout the scan
  3. An isotope source instead of an X-ray tube
  4. A single detector that moves in a rectilinear pattern

Explanation

Fan-beam systems collimate the beam into a fan that spans the scan width and capture it with a linear detector array, so the arm moves only in one direction. This shortens scan time relative to pencil-beam geometry.

Question 7Equipment and Instrumentation

The least significant change (LSC) at the 95% confidence level is obtained by multiplying the precision error by which factor?

  1. 3.50
  2. 2.77 Correct answer.
  3. 1.00
  4. 1.96

Explanation

The LSC is calculated as 2.77 times the precision error, which corresponds to 1.96 times the standard deviation of the difference between two measurements. A change smaller than the LSC cannot be distinguished from measurement noise with 95% confidence.

Question 8Equipment and Instrumentation

How is the in vivo precision of a DXA system properly determined?

  1. By comparing the system with a second densitometer on the same day
  2. By performing duplicate scans on 30 patients with repositioning between scans Correct answer.
  3. By scanning one volunteer 30 times over a full year
  4. By scanning the manufacturer's phantom 30 times in one day

Explanation

In vivo precision must include biological and repositioning variation, so it is derived from repeat scans of patients with the patient repositioned between acquisitions. Phantom scans reflect only instrument stability and exclude patient-related variability.

Question 9Equipment and Instrumentation

How does a pencil-beam DXA system acquire a scan?

  1. A detector ring rotates continuously around the patient
  2. A tightly collimated beam with a single detector moves in a rectilinear raster pattern Correct answer.
  3. Cross-sectional slices are reconstructed from multiple projections
  4. A fan of photons is detected simultaneously by a linear array

Explanation

Pencil-beam geometry uses a narrow collimated beam paired with a single detector that sweeps back and forth across the region of interest. The narrow beam keeps scatter and patient dose low, but scan times are longer than with fan-beam systems.

Question 10Equipment and Instrumentation

Which statement accurately describes spinal quantitative computed tomography (QCT) for bone densitometry?

  1. It measures the speed of sound through the vertebral body
  2. It derives density from two photon energies emitted by a gadolinium source
  3. It reports volumetric BMD in mg/cm³ and can isolate trabecular bone Correct answer.
  4. It reports areal BMD in g/cm² from a single projection

Explanation

QCT reconstructs cross-sectional images and expresses results as true volumetric density in mg/cm³. Because trabecular and cortical regions can be analyzed separately, it can sample the metabolically active trabecular compartment.

Question 11Equipment and Instrumentation

Why is a phantom scanned routinely as part of a DXA quality control program?

  1. To select the appropriate scan speed for each patient
  2. To measure the radiation dose received by each patient
  3. To recalibrate the reference database used for T-scores
  4. To detect drift in system calibration before it affects patient results Correct answer.

Explanation

A stable phantom provides a fixed attenuation reference, so repeated measurements reveal changes in system output rather than changes in anatomy. Serial phantom values plotted on a control chart detect drift or malfunction early.

Question 12Equipment and Instrumentation

Which equation describes the attenuation of a monoenergetic photon beam passing through tissue?

  1. I = I0 (1 − µx)
  2. I = I0 − µx
  3. I = I0 × e^(+µx)
  4. I = I0 × e^(−µx) Correct answer.

Explanation

Transmitted intensity falls exponentially with the linear attenuation coefficient and path length. DXA uses this relationship at two energies to solve for the amounts of bone mineral and soft tissue along the beam path.

Question 13Equipment and Instrumentation

What is the function of the collimator in a DXA scanner?

  1. It supplies a reference attenuation for internal calibration
  2. It measures the intensity of photons transmitted through the patient
  3. It shapes the beam to the scan region, reducing scatter and patient dose Correct answer.
  4. It holds the tube potential constant during the acquisition

Explanation

Collimators absorb off-axis photons so only the intended beam geometry reaches the patient. This improves image quality by limiting scatter and reduces the volume of tissue irradiated.

Question 14Equipment and Instrumentation

Increasing the scan speed of a DXA acquisition generally produces which effect?

  1. No change in dose but a longer scan time
  2. Lower patient dose and poorer precision Correct answer.
  3. Higher patient dose and better precision
  4. Lower patient dose and better precision

Explanation

Faster scanning shortens beam-on time and therefore reduces dose, but fewer counts are collected per pixel. The resulting noisier measurement lowers precision, so speed and precision must be balanced.

Question 15Equipment and Instrumentation

Two clinics use densitometers from different manufacturers and want to compare results in a shared patient population. What is the appropriate cross-calibration method?

  1. Use each system's own reference database without any adjustment
  2. Scan the same manufacturer phantom on both systems
  3. Apply a fixed 10% correction to one system's values
  4. Scan a group of patients on both systems and derive a regression equation Correct answer.

Explanation

Cross-calibration between different systems requires in vivo measurements, because phantoms do not reproduce patient anatomy or soft tissue effects. Regression of paired patient measurements allows values from one system to be translated to the other.

Question 16Equipment and Instrumentation

In many DXA systems, what is the role of a sodium iodide crystal?

  1. It absorbs transmitted photons and emits light in proportion to their energy Correct answer.
  2. It supports the patient in the correct scanning position
  3. It filters the beam to produce two energy peaks
  4. It collimates the beam into a narrow pencil shape

Explanation

Sodium iodide activated with thallium is a scintillator: incoming photons deposit energy that is re-emitted as visible light. A photomultiplier converts that light into an electrical signal proportional to the deposited energy.

Question 17Equipment and Instrumentation

Areal bone mineral density as reported by DXA is expressed in which units?

  1. dB/MHz
  2. g/cm² Correct answer.
  3. g/cm³
  4. mg/cm³

Explanation

DXA divides bone mineral content (grams) by the projected bone area (cm²), producing an areal density in g/cm². Because the result depends on bone thickness in the beam path, it is not a true volumetric density.

Question 18Equipment and Instrumentation

Peripheral DXA (pDXA) devices are most commonly used to measure which sites?

  1. Lumbar spine and proximal femur
  2. Whole body and total hip
  3. Mandible and tibial shaft
  4. Distal radius and calcaneus Correct answer.

Explanation

Peripheral systems are dedicated to appendicular sites, most often the distal forearm and the heel. These compact devices are portable and inexpensive but are used mainly for screening rather than for monitoring.

Question 19Equipment and Instrumentation

A weekly spine phantom scan falls outside the control limits and the technologist is unsure whether the change is real. What is the most appropriate next step?

  1. Repeat the phantom scan and check the setup before contacting service Correct answer.
  2. Replace the phantom because it has degraded
  3. Continue scanning patients and review the chart again next month
  4. Recalibrate the reference database that generates T-scores

Explanation

An isolated out-of-range value may result from a transient setup or analysis error, so the scan should be repeated with careful attention to phantom positioning and scan mode. If the repeat remains outside the limits, the system should be taken out of clinical use and service called.

Question 20Equipment and Instrumentation

A patient's spine BMD is 0.800 g/cm² at baseline and 0.818 g/cm² one year later. The facility's in vivo precision error for the spine is 1.0%. Is the change significant at the 95% confidence level?

  1. No, because changes must exceed 10% to be meaningful
  2. No, because the change is below the 2.8% LSC Correct answer.
  3. Yes, because a repeat scan would be more precise
  4. Yes, because any increase above 2% is real

Explanation

The change is about 2.25%, while the LSC at 95% confidence is 2.77 times the 1.0% precision error, or roughly 2.8%. Since the observed change is smaller than the LSC, it cannot be separated from measurement error.

Question 21Equipment and Instrumentation

Vertebral fracture assessment (VFA) performed on a DXA system requires which capability?

  1. High-resolution fan-beam imaging of the lateral spine Correct answer.
  2. A gadolinium-153 source and a water bath
  3. Volumetric acquisition with a rotating gantry
  4. Ultrasound transducers coupled to the back

Explanation

VFA uses the densitometer's fan-beam imaging mode to obtain a lateral view of the thoracolumbar spine at low dose. The images are then evaluated for vertebral deformity, allowing fracture detection at the time of the density study.

Question 22Equipment and Instrumentation

Why did manufacturers replace radionuclide sources with X-ray tubes in DXA systems?

  1. X-ray tubes eliminate the need for a detector
  2. X-ray tubes emit photons at exactly one energy
  3. X-ray tubes produce no scatter radiation
  4. X-ray tubes provide stable output and higher photon flux for faster scans Correct answer.

Explanation

An X-ray tube delivers a much higher and constant photon flux than a decaying isotope, which shortens acquisition time and keeps output reproducible over the life of the system. Dual energy is then achieved by filtration or by switching tube potential.

Question 23Equipment and Instrumentation

Which factor most often degrades in vivo precision of a lumbar spine DXA measurement?

  1. Fluctuation in detector crystal temperature only
  2. Variation in ambient humidity in the scan room
  3. Use of a fan-beam rather than a pencil-beam system
  4. Variability in patient positioning and in scan analysis between visits Correct answer.

Explanation

Reproducibility depends heavily on placing the patient in the same position and analyzing the scan with the same rules each time. Differences in vertebral level selection, rotation, or leg elevation introduce error that outweighs most instrument-level variation.

Question 24Equipment and Instrumentation

Phantom measurements on a DXA system drift steadily upward over several months and exceed the control limits month after month. What does this most likely indicate?

  1. An error in patient positioning on the scan table
  2. Progressive system calibration drift that requires service Correct answer.
  3. That the phantom's inserts have become denser over time
  4. Normal statistical variation that requires no action

Explanation

A sustained, directional shift in phantom values is the signature of drift in the system rather than random error. Trending rules on the control chart prompt corrective action, including service evaluation, before clinical results are affected.

Question 25Equipment and Instrumentation

Why is the precision error for femoral neck BMD typically larger than that for the lumbar spine?

  1. The hip reference database contains fewer subjects
  2. Fan-beam systems are unable to scan the proximal femur
  3. Hip positioning is less reproducible and the region is short and relatively low in density Correct answer.
  4. The femoral neck is scanned with a substantially higher radiation dose

Explanation

The femoral neck is a small, low-density region whose position depends on leg rotation and abduction, so repositioning error is greater. Smaller regions also contain fewer pixels, which reduces counting statistics and increases variability.

Question 26Equipment and Instrumentation

In a DXA system that uses k-edge filtration, how are the two photon energies produced?

  1. A radionuclide that emits two gamma energies
  2. Two separate X-ray tubes fired in alternation
  3. An X-ray beam filtered through a rare-earth metal Correct answer.
  4. A laser that excites bone to emit two wavelengths

Explanation

A k-edge filter made of a rare-earth element such as cerium or samarium absorbs photons just above its k-edge more strongly than higher-energy photons, splitting the bremsstrahlung spectrum into distinct low- and high-energy peaks that are measured at the same time. Only one X-ray tube is involved, and no radionuclide or laser is used.

Question 27Equipment and Instrumentation

Which technique produces dual-energy photons by rapidly alternating the tube voltage between approximately 70 and 140 kVp?

  1. Photon counting
  2. kVp switching Correct answer.
  3. K-edge filtration
  4. Beam hardening

Explanation

In kVp switching the generator alternates the tube potential between a low and a high value during the scan, so sequential measurements are made at two energies. K-edge filtration instead produces both energies simultaneously from a single tube potential.

Question 28Equipment and Instrumentation

Which value best approximates the effective dose delivered by a lumbar spine DXA scan?

  1. About 5 µSv Correct answer.
  2. About 0.5 mSv
  3. About 5 mSv
  4. About 50 mSv

Explanation

DXA uses a low-current, tightly collimated beam in a scanning geometry, so effective doses are on the order of a few microsieverts, well below a chest radiograph. Doses in the millisievert range belong to computed tomography, not densitometry.

Question 29Equipment and Instrumentation

To separate bone from soft tissue, the detector system of a DXA scanner sorts transmitted photons by their:

  1. polarization
  2. energy Correct answer.
  3. phase
  4. direction

Explanation

Because bone mineral and soft tissue attenuate low- and high-energy photons differently, the detector counts photons in two energy windows and the software solves for the bone and soft-tissue contributions. The sorting property is photon energy, not direction, phase, or polarization.

Question 30Equipment and Instrumentation

The two effective photon energies used by most DXA systems are approximately:

  1. 100 keV and 200 keV
  2. 40 keV and 70 keV Correct answer.
  3. 10 keV and 20 keV
  4. 500 keV and 1000 keV

Explanation

DXA operates with effective energies near 40 keV and 70 keV, a range where the attenuation difference between bone mineral and soft tissue is large enough to be exploited. Much lower energies are absorbed in soft tissue before reaching the detector, and much higher energies give poor contrast.

Question 31Equipment and Instrumentation

A patient is scanned on a fan-beam DXA system while lying on a thick foam pad that raises the body several centimeters above the table. What is the most likely effect on the measurement?

  1. BMD is overestimated because more bone is included
  2. BMD is underestimated because the bone is magnified Correct answer.
  3. Scan time increases because the beam travels farther
  4. No effect, because software corrects for table height

Explanation

In fan-beam geometry the beam diverges, so an object farther from the detector array is magnified more than the assumed geometry predicts; the larger projected area lowers the computed areal BMD. The analysis software assumes a fixed table-to-detector distance and does not compensate for added padding.

Question 32Equipment and Instrumentation

In lumbar spine DXA analysis, how is the soft-tissue attenuation beneath a vertebral body determined?

  1. By interpolating from soft tissue beside the vertebra Correct answer.
  2. From soft tissue measured over the femoral neck
  3. From a CT scan obtained the same day
  4. By assuming a fixed soft-tissue composition for all adults

Explanation

The analysis software estimates the soft-tissue contribution under each vertebra by interpolating from the soft-tissue regions immediately adjacent to it, where no bone is present. That interpolated baseline is subtracted so the remaining attenuation is attributed to bone mineral.

Question 33Equipment and Instrumentation

What distinguishes array (multi-detector) mode from single-detector mode in DXA?

  1. It uses a radionuclide source instead of a tube
  2. It acquires only lateral spine images
  3. It requires a longer scan time per centimeter
  4. It images a wider strip with each pass Correct answer.

Explanation

In array mode a linear array of detector elements samples many scan lines at once, covering a wider strip of the patient with each sweep and shortening the total acquisition. Single-detector systems must sweep one narrow line at a time.

Question 34Equipment and Instrumentation

What is the purpose of the internal reference wheel or step wedge in a DXA scanner?

  1. To measure the half-value layer of the beam
  2. To convert BMD into a T-score
  3. To correct for beam output and detector drift Correct answer.
  4. To align the patient's spine parallel to the table

Explanation

The reference material is measured during scanning so the system can compensate for fluctuations in tube output and detector response, keeping reported BMD stable over time. It does not perform unit conversion or database matching.

Question 35Equipment and Instrumentation

Which system characteristic most directly limits the spatial resolution of a DXA image?

  1. The length of the scan field
  2. The tube potential selected for the scan
  3. The size of the detector elements Correct answer.
  4. The reference database in use

Explanation

Resolution is governed by how finely the transmitted beam is sampled, which is set by detector element size and the reconstructed pixel size. Tube potential affects contrast and the reference database affects T-scores, but neither changes resolving power.

Question 36Equipment and Instrumentation

Obtaining a lateral lumbar spine DXA image requires the system to have which feature?

  1. A whole-body table with a step platform and handrails
  2. A pulsed generator with high frequency
  3. A second tube mounted at 90 degrees
  4. A rotating C-arm that moves the beam laterally Correct answer.

Explanation

Lateral spine imaging needs the source-detector geometry rotated 90 degrees, which is accomplished with a rotating C-arm on systems designed for lateral scanning. Table accessories and generator design do not provide a lateral projection.

Question 37Equipment and Instrumentation

Increasing the tube current (mA) during a DXA acquisition does which of the following?

  1. Shortens the total scan time considerably
  2. It raises photon counts and precision Correct answer.
  3. Improves the spatial resolution
  4. Reduces the dose to the patient

Explanation

Tube current determines how many photons are produced per unit time, so a higher mA improves counting statistics and reduces random error, thereby improving precision. Dose rises slightly, while resolution and scan time are governed by other parameters.

Question 38Equipment and Instrumentation

Many DXA systems automatically adjust technique according to body thickness. What is the main reason?

  1. To maintain adequate count rates in large patients Correct answer.
  2. To avoid measuring soft tissue in obese patients
  3. To keep the effective dose constant for all patients
  4. To shorten the scan for thin patients

Explanation

Attenuation rises with soft-tissue thickness, so technique is adjusted to ensure enough photons reach the detector for reliable separation of bone from soft tissue in larger patients. The goal is adequate counting statistics rather than a constant dose or a faster scan.

Question 39Equipment and Instrumentation

A facility installs a major software upgrade on its densitometer. What is the most appropriate action before resuming patient testing?

  1. Rescan the reference population for all patients
  2. Recalibrate the tube by measuring half-value layer
  3. Continue as before, because software does not affect BMD
  4. Scan the phantom and compare with the baseline Correct answer.

Explanation

Analysis software can change calibration factors or edge-detection algorithms, so the quality control phantom must be scanned and compared with the established baseline to confirm results are unchanged. If a shift is found, the manufacturer's guidance and cross-calibration are required.

Question 40Equipment and Instrumentation

A 70-year-old Hispanic woman is scanned, but the software compares her with a young white female reference population. What is the consequence?

  1. The scan must be repeated at a different kVp
  2. The measured area of the vertebra is altered
  3. The BMD is recalculated in different units
  4. The T-score is inaccurate; BMD is unchanged Correct answer.

Explanation

T-scores are derived from the selected reference database, so an unmatched population shifts the T-score while the measured BMD in g/cm² stays the same. The appropriate reference population for the patient should be selected in the software.

Question 41Equipment and Instrumentation

A technologist must stay in the scan room to assist an uncooperative patient. Which action most reduces occupational exposure from scattered radiation?

  1. Holding the patient's shoulders firmly
  2. Increasing tube current during the scan
  3. Standing as far from the scanner as possible Correct answer.
  4. Standing behind a lead apron draped over the table

Explanation

Scattered exposure falls off with the square of the distance, so moving as far from the scanner as the room allows gives the largest practical reduction. Time and shielding also matter, but distance offers the greatest benefit in this situation.

Question 42Equipment and Instrumentation

A DXA spine phantom used for quality control typically uses which material to simulate bone?

  1. Hydroxyapatite-equivalent plastic Correct answer.
  2. Steel cylinders suspended in air
  3. Calcium sulfate crystals in gelatin
  4. Cortical bone from animal femurs

Explanation

Phantoms contain inserts of a hydroxyapatite-equivalent plastic whose attenuation mimics bone mineral, embedded in a tissue-equivalent material. This provides a stable, known BMD reference that can be measured repeatedly over the life of the system.

Question 43Equipment and Instrumentation

In fan-beam DXA, what effect does a wider detector array have on the acquisition?

  1. It removes the need for baseline correction
  2. It covers more patient width per sweep Correct answer.
  3. It requires a higher tube potential
  4. It reduces the counts recorded per pixel

Explanation

A wider array captures more scan lines per sweep, so a larger fraction of the patient's width is covered in each pass and the total scan is shortened. Beam energy, count rate, and baseline correction are determined by other design features.

Question 44Equipment and Instrumentation

A whole-body DXA scan provides which of the following in addition to total BMD?

  1. Regional fat and lean mass Correct answer.
  2. Vertebral fracture grading
  3. Trabecular bone score
  4. Volumetric bone mineral density

Explanation

Whole-body acquisition covers the entire skeleton and simultaneously partitions soft-tissue attenuation into fat and lean compartments, permitting body composition analysis alongside total and regional BMD. It does not yield volumetric density or fracture assessment.

Question 45Equipment and Instrumentation

DXA images and results are sent to a PACS using which standard?

  1. DICOM Correct answer.
  2. IEEE 802.11
  3. TCP/IP
  4. HL7

Explanation

DICOM defines the image and result formats and the network services used to store and transfer them to a PACS. HL7 handles clinical messaging such as orders and reports, while the other choices are networking protocols.

Question 46Equipment and Instrumentation

Adding filtration to the beam before it reaches the patient has which combined effect?

  1. It converts fan geometry to pencil geometry
  2. It narrows the beam to the region of interest
  3. It removes low-energy photons and hardens the beam Correct answer.
  4. It increases low-energy photons at the skin surface

Explanation

Filtration preferentially absorbs the lowest-energy photons, which would otherwise be absorbed in skin without contributing to the image, so patient dose falls and the beam becomes more penetrating. Collimation, not filtration, defines the shape and width of the beam.

Question 47Equipment and Instrumentation

A vertebral body is deeper in the anteroposterior direction but has the same true volumetric density as a normal vertebra. What DXA result is expected?

  1. A higher areal BMD, since more bone is in the beam Correct answer.
  2. No change because DXA measures volume
  3. A lower areal BMD because the area is larger
  4. The same areal BMD, since density has not changed

Explanation

DXA reports areal BMD, the mineral content divided by the projected area. A thicker bone places more mineral in the beam path without enlarging the projected area, so areal BMD rises even though true volumetric density is unchanged.

Question 48Equipment and Instrumentation

Retained barium in the bowel overlies the lumbar spine during a DXA scan. What is the most likely effect on reported BMD?

  1. The BMD is always raised because barium mimics bone
  2. The scan aborts automatically
  3. No effect, because dense material is subtracted
  4. The BMD is altered by a miscalculated baseline Correct answer.

Explanation

Barium attenuates both photon energies far more than soft tissue, so the interpolated soft-tissue baseline beneath the vertebra is miscalculated and the reported BMD is erroneous. The system does not reject the image automatically, so the scan must be reviewed and repeated after the barium clears.

Question 49Equipment and Instrumentation

What should be done when a new densitometer is installed, before the first patient is scanned?

  1. Nothing, because factory calibration replaces testing
  2. Acceptance testing with phantom and dose checks Correct answer.
  3. Only a radiation survey of the room
  4. Only a repeat phantom scan at one year

Explanation

Acceptance testing confirms that the installed system meets specifications and regulatory requirements, including phantom BMD values, precision, and radiation measurements. Clinical use begins only after these checks are documented.

Question 50Equipment and Instrumentation

During a lumbar spine DXA scan, why are the patient's hips and knees flexed over a positioning block?

  1. To reduce the radiation dose to the pelvis
  2. To shorten the spine-to-detector distance
  3. To flatten lumbar lordosis and open disc spaces Correct answer.
  4. To allow both hips to be scanned in a single pass

Explanation

Flexing the hips and knees flattens the lumbar lordosis so the vertebral bodies face the beam squarely and the disc spaces are opened, which improves the reproducibility of vertebral analysis. It is a positioning aid and does not appreciably change dose or beam geometry.