Lecture 4: Magnification Mammography, Tomosynthesis, MQSA, and Artifacts

Magnification Mammography

  • Primary Purpose and Spatial Resolution:

    • Magnification mammography is performed specifically to improve spatial resolution and evaluate fine anatomical structures (such as microcalcifications and lesion margins).
    • Unlike general radiography, magnification levels in mammography cannot be chosen arbitrarily. The magnification level is fixed by the system design to maximize spatial resolution without degrading image quality.
    • An acrylic or Plexiglas magnification stand is placed on top of the detector. The breast rests directly on top of this stand, placing it at a precise, fixed distance between the X-ray tube focal spot (source) and the detector.
    • By federal law under the Mammography Quality Standards Act (MQSA), magnification mammography systems must provide a magnification factor ranging between a 40%40\% size increase (1.4×1.4\times) and a doubling (2.0×2.0\times) of the object size (1.4×−2.0×1.4\times - 2.0\times magnification).
    • Magnification is governed by the source-to-image distance (SID) divided by the source-to-object distance (SOD): Magnification Factor=SIDSOD\text{Magnification Factor} = \frac{\text{SID}}{\text{SOD}}

  • Focal Spot Size, Focal Spot Blur, and Tube Current Adjustments:

    • Geometrically magnifying an object also magnifies the focal spot, which increases geometric unsharpness (focal spot blur).
    • To counteract focal spot blur and maintain superior spatial resolution, the mammography system automatically switches to a much smaller focal spot size during magnification acquisitions.
    • Because a smaller focal spot concentrates thermal energy into a significantly smaller area on the target anode, it heats up extremely rapidly. To avoid thermal damage to the anode, the system cannot throw off as many X-ray photons per second.
    • Consequently, the tube current (mA\text{mA}) must be reduced dramatically, typically to between one-half (12\frac{1}{2}) and one-quarter (14\frac{1}{4}) of the mA\text{mA} used in contact (standard screening) mammography.

  • Exposure Time Adjustments:

    • Because the mA\text{mA} is reduced to prevent target overheating, the X-ray tube emits fewer photons per unit time.
    • To ensure the image receptor absorbs a sufficient total number of X-ray photons (mAs) to maintain an acceptable signal-to-noise ratio, the system must utilize a significantly longer exposure time.

  • Anti-Scatter Grid Removal and the Air Gap Effect:

    • The standard anti-scatter grid is completely removed during magnification mammography.
    • Removing the anti-scatter grid saves patient radiation dose, helping to offset the dose increase caused by placing the breast closer to the X-ray tube.
    • Without a grid, image contrast would normally degrade due to scatter radiation. However, magnification mammography inherently creates an Air Gap Effect:
      • The acrylic stand elevates the breast, creating a physical air gap between the underside of the breast and the detector surface.
      • X-ray photons that undergo Compton scatter within the breast diverge at wide angles.
      • Due to the physical distance provided by the air gap, a vast majority of these widely scattered photons completely miss the detector surface.
      • This natural loss of scattered radiation preserves image contrast without requiring an anti-scatter grid, thereby protecting the patient from unnecessary additional radiation exposure.

  • Radiation Dose Dynamics:

    • Positioning the breast closer to the X-ray source increases the entrance skin exposure due to the inverse-square law.
    • However, dose measurements in magnification mammography are complex: the field of view is smaller, the anti-scatter grid is removed, and mA\text{mA} is lowered.
    • While literature contains variable findings (some studies reporting slightly higher dose and others showing minimal change), for examination purposes, entrance radiation dose in magnification mammography is generally considered slightly higher than standard contact mammography due to proximity to the X-ray source.

Digital Breast Tomosynthesis (DBT)

  • System Overview and Principles:

    • Digital Breast Tomosynthesis (DBT) acquires multiple low-dose projection images as the X-ray tube pivots across an angular arc over the stationary or moving detector.
    • DBT functions as a limited-angle CT system. Compared to diagnostic CT (which acquires roughly 10001000 projection views across a full 360∘360^\circ rotation), DBT acquires a much smaller number of projections across a constrained angular range.
    • Acquisition tube voltage typically ranges between 25 kV25\,\text{kV} and 40 kV40\,\text{kV}.
    • Total sweep acquisition time varies by vendor, taking between 4 s4\,\text{s} and 25 s25\,\text{s}.
    • Grids: Depending on vendor design, some DBT systems utilize a high-transmission grid, while others remove the grid entirely during 3D acquisition.
    • X-ray Target/Filter: The market leader (Hologic/Solvologix) utilizes an aluminum (Al\text{Al}) filter during tomosynthesis mode, whereas traditional 2D systems or other DBT vendors may use rhodium (Rh\text{Rh}) or silver (Ag\text{Ag}) filters.

  • Mathematical Concept of Image Shift-and-Add Reconstruction:

    • Modern DBT systems use advanced reconstruction algorithms (such as filtered back-projection or iterative reconstruction), but the foundational mechanism is best understood through the historical "shift-and-add" method.
    • Consider a breast containing a superficial structure and a deep structure:
      • When the X-ray beam hits the breast at a negative angle (from the left), the superficial object projects onto the detector to the right of the deep object.
      • When the beam is perpendicular (0∘0^\circ angle), the two objects overlap entirely.
      • When the beam hits at a positive angle (from the right), the superficial object projects to the left of the deep object.
    • Reconstructing a Specific Depth Plane:
      • To reconstruct the superficial plane, the projection images are shifted relative to one another so that the superficial object aligns perfectly across all frames before summing the images. The signal of the superficial object reinforces itself, becoming sharp and high in contrast, while the deep object is smeared out across the background.
      • To reconstruct the deep plane, the projection images are shifted so that the deep object overlaps perfectly across all frames before summation. The deep object reinforces itself, becoming crisp, while the superficial object is smeared out.

  • Angular Arc and Through-Plane Resolution:

    • The total sweep angle varies by vendor, ranging from small arcs (15∘15^\circ) to wide arcs (50∘50^\circ).
    • Effect of Angular Arc Range:
      • A wider angular arc (e.g., 50∘50^\circ vs 15∘15^\circ or 2.5∘2.5^\circ) provides superior through-plane (z-axis) spatial resolution.
      • Wider arcs isolate individual focal slices more effectively and smear away out-of-plane overlapping tissue far more efficiently, eliminating pseudo-lesions caused by tissue superposition.
    • Effect of Number of Projections (Angular Sampling Density):
      • Vendors configure sampling differently (e.g., a 15∘15^\circ arc taking 1515 projections at 1∘1^\circ intervals, versus a 50∘50^\circ arc taking projections every 2∘2^\circ).
      • Denser angular sampling (more projections across the arc, e.g., every 0.5∘0.5^\circ versus every 4∘4^\circ) reduces angular sampling artifacts (such as out-of-slice streaking and replica artifacts).
      • Trade-off: Increasing the total number of projections requires splitting the total radiation dose across more exposures or increasing total patient dose, and can increase system mechanical complexity.

  • Clinical Advantages, Slice Thickness, and Synthetic 2D Mammograms:

    • Primary Clinical Advantage: DBT substantially reduces or eliminates tissue superposition (tissue overlap). It helps distinguish pseudomass architectural distortions (caused by overlapping normal tissue) from true malignant architectural distortions and unmasks true lesions hidden by dense fibroglandular tissue.
    • Slice Thickness: Reconstructed focal slices are typically 1 mm1\,\text{mm} thick. Reading dozens of slices per breast extends radiologist interpretation times.
    • Spatial Resolution: In-plane spatial resolution (x−yx-y axis) is comparable to standard 2D digital mammography, whereas through-plane resolution (zz axis) is lower but provides cross-sectional sectioning.
    • Synthetic 2D Mammograms (e.g., Solvologix C-View):
      • Synthetic 2D mammograms are generated mathematically by synthesizing data from the acquired 3D DBT projections, eliminating the need to perform an additional, separate 2D radiation exposure.
      • The combined radiation dose of a DBT sweep plus a synthesized 2D image is comparable to or only slightly higher than a standard 2D screening mammogram.
      • Regulatory image quality standards applying to traditional 2D mammograms apply equally to synthetic 2D mammograms.
      • Synthetic mammograms exhibit specific algorithmic differences compared to conventional 2D images, such as altered skin line appearance and specific dark halo artifacts surrounding high-density structures.

Mammography Quality Standards Act (MQSA) and Accreditation

  • Federal Regulatory Framework:

    • MQSA is a strict federal law passed by the U.S. Congress and enforced by the Food and Drug Administration (FDA).
    • The FDA grants authority to approved accrediting bodies (such as the American College of Radiology [ACR] or approved state-level accrediting bodies like Texas) to evaluate mammography facilities.
    • Facilities undergo mandatory comprehensive accreditation reviews at least once every 3 years3\,\text{years} (or sooner if triggered by patient or clinical complaints).
    • Non-compliance carries severe legal consequences: accrediting bodies can immediately pause or revoke a facility's legal authority to perform mammography and can levy administrative fines exceeding hundreds of thousands of dollars.
    • Exclusions: MQSA federal law specifically does not cover stereotactic breast biopsy procedures, interventional procedures, or Veterans Affairs (VA) facilities. (VA medical centers operate under internal federal guidelines that are substantially equivalent to MQSA).

  • MQSA and ACR Phantoms and Quality Control Standards:

    • Standard Phantom Specifications:
      • The standard MQSA testing phantom simulates a compressed breast thickness of 4.2 cm4.2\,\text{cm} with a composition of 50%50\% fibroglandular tissue and 50%50\% adipose tissue (50/50 glandularity50/50\,\text{glandularity}).
      • Phantom testing is performed weekly by quality control radiologic technologists.
    • Phantom Test Objects:
      1. Fibers (simulating tissue spicules and architectural distortion): Range in size from 1.6 mm1.6\,\text{mm} down to 0.75 mm0.75\,\text{mm} in diameter.
      2. Speck Groups (simulating microcalcifications): Punctate objects representing malignant microcalcification clusters down to ≈0.30 mm−0.32 mm\approx 0.30\,\text{mm} - 0.32\,\text{mm}.
      3. Masses (simulating soft tissue masses): Circular objects of 1 cm1\,\text{cm} diameter with varying thickness steps down to 0.75 mm0.75\,\text{mm} (representing a 90%90\% glandularity equivalent contrast on a 50%50\% background).
    • Passing Criteria:
      • Standard MQSA Phantom: Must successfully visualize at least 44 fibers, 33 speck groups, and 33 masses (4−3−34-3-3 score).
      • Newer ACR Digital Mammography Phantom: Must successfully visualize at least 22 fibers, 33 speck groups, and 22 masses (2−3−22-3-2 score).
      • In both phantom scoring systems, the minimum physical diameter required for a passing fiber score is 0.75 mm0.75\,\text{mm}.
      • The phantom background must be completely uniform; significant background artifacts cause phantom failure.

  • Quality Control Failures and Mandatory Corrective Action Timelines:

    • Category 1: Immediate Clinical Shutdown (Stop clinical imaging immediately until repaired and re-tested):
      1. Mean Glandular Dose (MGD) exceeds the legal limit.
      2. Phantom image quality failure.
      3. Film printer image quality failure (if hard-copy film printing is clinically utilized).
      4. Diagnostic viewstation display failure (unacceptable dead pixels, luminance issues, or inability to meet phantom display criteria).
      5. DBT volume reconstruction failure.
      6. Compression force failure (paddle fails to achieve or maintain required initial automated compression force).
    • Category 2: 30-Day Corrective Action Window:
      • Any other non-critical quality control failures allow the facility to continue patient imaging, but the failure must be formally corrected, re-tested, and documented within 30 calendar days30\,\text{calendar days}.
      • Facilities are legally mandated to document what was fixed, how it was fixed, and when it was fixed. Lack of documentation during regulatory audits results in citations and fines.

Image Storage, Display, and Computer-Aided Detection (CAD)

  • Data File Sizes and Data Compression Regulations:

    • A single 2D digital mammography view ranges from 12 MB12\,\text{MB} to 50 MB50\,\text{MB} in storage size.
    • A complete 3D Digital Breast Tomosynthesis study ranges from 400 MB400\,\text{MB} to 3 GB3\,\text{GB} (0.4 GB−3.0 GB0.4\,\text{GB} - 3.0\,\text{GB}).
    • Lossy Compression Prohibition: MQSA regulations strictly forbid the use of lossy image compression on diagnostic breast images. Lossy compression permanently discards fine data, which risks irreversibly altering microcalcifications or subtle tissue spicules. Facilities are legally restricted to using lossless compression algorithms only.

  • Legal Image Retention Mandates:

    • Active Patients: Facilities must retain mammographic images for a minimum of 5 years5\,\text{years} if the patient returns for subsequent examinations (ensuring at least 55 prior annual studies remain available for direct comparison).
    • Inactive Patients: If a patient does not return for further exams, the facility must retain the patient's final mammography study for at least 10 years10\,\text{years}.

  • Computer-Aided Detection (CADe) and Diagnosis (CADx):

    • Breast imaging was the clinical pioneer for computer-aided diagnosis and radiomics applications.
    • CADe (Computer-Aided Detection): Identifies and draws visual overlays (markers) around suspicious areas such as microcalcification clusters or spiculated masses.
    • CADx (Computer-Aided Diagnosis): Evaluates image features (e.g., degree of spiculation, border sharpness, density) and calculates a mathematical probability of malignancy (e.g., estimating a 70%70\% likelihood of cancer).
    • Sensitivity Adjustment: Most CAD software engines allow facilities to tune sensitivity thresholds; reducing sensitivity decreases the rate of annoying false-positive visual marks.
    • Legal Archival Requirement: Under MQSA regulations, facilities are strictly forbidden from saving only the CAD-marked images. Un-marked, pristine original images must always be preserved and available for diagnostic interpretation.

Radiation Dose Metrics in Breast Imaging

  • Mean Glandular Dose (MGD) / Average Glandular Dose (AGD):

    • MGD/AGD represents the calculated absorbed radiation dose specifically delivered to the radio-sensitive fibroglandular tissue of the breast. (Adipose tissue is insensitive to radiation-induced carcinogenesis and is excluded from risk models).
    • MQSA Legal Radiation Dose Limit: By federal law, the Mean Glandular Dose for a single projection view (specifically tested on the Cranial-Caudal [CC] view) must not exceed 3 mGy3\,\text{mGy} (0.3 rad0.3\,\text{rad} or 300 mrad300\,\text{mrad}) for a standard 4.2 cm4.2\,\text{cm} compressed breast phantom of 50/50 glandularity50/50\,\text{glandularity}.
    • Typical Clinical Dose: Modern digital mammography systems typically deliver between 1.2 mGy1.2\,\text{mGy} and 1.5 mGy1.5\,\text{mGy} per view for a 4.2 cm4.2\,\text{cm} breast—roughly half the federal maximum allowable limit.
    • Thickness Dependency: Absorbed dose increases non-linearly with increasing breast thickness. For a 7 cm7\,\text{cm} compressed breast, clinical MGD routinely exceeds 3 mGy3\,\text{mGy}. This is legally permissible because the 3 mGy3\,\text{mGy} regulatory limit applies strictly to the standardized 4.2 cm4.2\,\text{cm} phantom baseline.

  • Effective Dose:

    • The effective dose for a standard 4-view screening mammogram (bilateral CC and MLO views) in a typical patient is approximately 0.4 mSv0.4\,\text{mSv}.
    • Comparative Scale: A 0.4 mSv0.4\,\text{mSv} screening mammogram delivers a higher effective dose than a single PA chest radiograph, but a lower effective dose than an AP abdominal radiograph.

Mammography and Tomosynthesis Artifacts

  • Exogenous / Patient-Related Artifacts:

    • Deodorant / Antiperspirant Artifact: Appears as multiple hyperdense, focal punctate speckles residing in the axillary fold or superior tissue margin, closely mimicking clustered malignant microcalcifications. Most commercial antiperspirants contain aluminum-based metallic compounds (high atomic number, ZZ) that strongly attenuate X-rays.
    • Dry, Flaky Skin: Dense, calcified or desiccated epidermal flakes simulate microcalcifications.
    • Topical Lotions / Ointments: Produce broad hyperdense radiopaque streaks, smears, or variable density bands due to unequal attenuation of the X-ray beam across topical dermal residues.
    • Motion Artifact and Suboptimal Field of View: Patient motion causes blurring of fine structures (calcifications, vessel margins). Inadequate positioning truncates posterior breast tissue at the chest wall, creating a critical diagnostic failure under MQSA positioning guidelines.

  • Detector and Display Artifacts:

    • Dead Pixels / Dead Pixel Lines: Manifest as crisp white (hyperdense) or dark spots or linear streaks. Dead white pixels can easily mimic microcalcifications. Required fix: Service engineers must run software bad-pixel map algorithms and detector uniformity calibration.
    • Ghosting / Image Lag: Residual signal latent from a preceding high-exposure scan remains visible as a faint secondary shadow superimposed on the subsequent image acquisition.

  • Tomosynthesis-Specific Artifacts:

    • Peripheral Bright Area / Increased Edge Intensity: Prominent bright, hyperdense bands visible along the posterior chest wall or superficial borders, particularly on MLO views. Occurs because X-ray beams passing through tissue outside the defined 3D reconstructed volume are recorded by the detector; the reconstruction algorithm fails to allocate this extra attenuation correctly and aggregates it at the boundary edges of the reconstructed image volume.
    • Terracing / Staircase / Accordion Artifact: Step-like, terraced linear structures occurring along the chest wall or peripheral tissue boundaries. Caused by incomplete angular data sampling at peripheral boundaries where outer tissue is not covered across all acquisition arc angles. Clinical Warning: The absence of visible lesions in terraced marginal zones does not confirm healthy tissue, as reconstruction in this region is incomplete; suspicious findings seen within terraced zones require dedicated supplemental mammographic views.
    • Slinky / Off-Slice Replication Artifact: Highly dense, strongly attenuating objects (such as metallic surgical clips, biopsy markers, or localized coarse calcifications) appear to duplicate and spread out vertically across multiple adjacent non-focal reconstructed slices. The replication spreads perpendicular to the direction of X-ray tube travel across the arc. Denser angular sampling and iterative reconstruction algorithms mitigate this artifact.
    • Halo Artifact: A distinct dark, radiolucent (black) ring or outline surrounding a highly dense object (such as a metallic clip or dense calcification) on synthesized 2D or reconstructed DBT slices. Caused by local photon starvation and extreme X-ray beam hardening that break the linear attenuation assumptions of the 3D reconstruction algorithm.
    • False Skin Thickening: Out-of-focus blur from superficial skin structures projecting into adjacent focal plane slices, falsely simulating cutaneous edema or pathologic skin thickening on synthesized 2D or 3D slice displays.

Review Questions and Self-Assessment

  • Question 1: How long does digital breast tomosynthesis (DBT) typically take to acquire all projection images?

    • Options: (a) 1−3 s1 - 3\,\text{s}, (b) 4−25 s4 - 25\,\text{s}, (c) 30−60 s30 - 60\,\text{s}, (d) 1−2 min1 - 2\,\text{min}.
    • Correct Answer: (b) 4−25 s4 - 25\,\text{s}.
    • Explanation: Depending on the equipment manufacturer and angular sweep range, total acquisition time ranges between 4 s4\,\text{s} and 25 s25\,\text{s}.

  • Question 2: What parameter is typically increased in magnification mammography compared to contact (screening) mammography?

    • Options: (a) Focal spot size, (b) Exposure time, (c) Grid ratio, (d) Compression force.
    • Correct Answer: (b) Exposure time.
    • Explanation: Magnification mode forces the system to select a smaller focal spot to limit geometric blur. Because a smaller focal spot overheats rapidly, tube current (mA\text{mA}) must be reduced. To deliver sufficient photon fluence (mAs) to the receptor, exposure time must be increased.

  • Question 3: What fiber diameter represents the minimum passing threshold for standard phantom image quality?

    • Options: (a) 1.60 mm1.60\,\text{mm}, (b) 1.20 mm1.20\,\text{mm}, (c) 1.00 mm1.00\,\text{mm}, (d) 0.75 mm0.75\,\text{mm}.
    • Correct Answer: (d) 0.75 mm0.75\,\text{mm}.
    • Explanation: The smallest test fiber required to pass phantom quality evaluation in both MQSA and ACR digital phantoms is 0.75 mm0.75\,\text{mm} in diameter.

  • Question 4: Assuming identical glandularity percentage, image contrast is best for which compressed breast thickness?

    • Options: (a) Thinnest breast, (b) Average thickness breast, (c) Thickest breast, (d) Thickness has no impact on contrast.
    • Correct Answer: (a) Thinnest breast.
    • Explanation: A thinner compressed breast produces less Compton scatter radiation. Additionally, a thinner breast requires lower X-ray beam energy (kVp\text{kVp}), maximizing photoelectric interaction differences and enhancing soft-tissue subject contrast.

  • Question 5: Which of the following represents the largest single image series in terms of digital storage memory requirement?

    • Options: (a) Screening digital breast tomosynthesis study, (b) Screening 2D digital mammogram study, (c) Routine single-phase chest CT, (d) AP abdominal radiograph.
    • Correct Answer: (a) Screening digital breast tomosynthesis study.
    • Explanation: A 3D DBT study contains numerous reconstructed high-resolution slices, requiring 400 MB400\,\text{MB} to 3 GB3\,\text{GB} of storage memory, compared to 12−50 MB12-50\,\text{MB} for a 2D mammogram.

  • Question 6: Assuming a patient undergoes annual screening mammography at a facility, how many prior annual examinations must be retained by the facility under MQSA regulations?

    • Options: (a) 3 priors, (b) 5 priors, (c) 7 priors, (d) 10 priors.
    • Correct Answer: (b) 5 priors.
    • Explanation: MQSA requires facilities to retain images for at least 5 years5\,\text{years} if the patient has subsequent exams (maintaining 55 prior annual studies). If no subsequent exams are performed, the final exam must be retained for 10 years10\,\text{years}.

  • Question 7: What is the approximate effective dose delivered by a standard 4-view screening mammogram?

    • Options: (a) 0.4 mSv0.4\,\text{mSv}, (b) 1.5 mSv1.5\,\text{mSv}, (c) 3.0 mSv3.0\,\text{mSv}, (d) 5.0 mSv5.0\,\text{mSv}.
    • Correct Answer: (a) 0.4 mSv0.4\,\text{mSv}.
    • Explanation: A full 4-view screening mammogram delivers an effective radiation dose of approximately 0.4 mSv0.4\,\text{mSv}.

  • Question 8: What is the current MQSA federal regulatory limit for single-view Average Glandular Dose (AGD) evaluated on a standard baseline phantom?

    • Options: (a) 1.0 mGy1.0\,\text{mGy}, (b) 1.5 mGy1.5\,\text{mGy}, (c) 3.0 mGy3.0\,\text{mGy}, (d) 5.0 mGy5.0\,\text{mGy}.
    • Correct Answer: (c) 3.0 mGy3.0\,\text{mGy} (0.3 rad0.3\,\text{rad} or 300 mrad300\,\text{mrad}).
    • Explanation: Federal law under MQSA mandates that the mean glandular dose for a single projection view of a standard 4.2 cm4.2\,\text{cm} compressed breast phantom (50/50 glandularity50/50\,\text{glandularity}) shall not exceed 3.0 mGy3.0\,\text{mGy}.

  • Question 9: Automatic Exposure Control (AEC) systems in modern digital mammography can automatically adjust which acquisition parameters?

    • Options: (a) Source-to-Image Distance (SID), (b) Focal spot position, (c) kVp\text{kVp}, mA\text{mA}, and Beam Filtration, (d) Compression force.
    • Correct Answer: (c) kVp\text{kVp}, mA\text{mA}, and Beam Filtration.
    • Explanation: Mammography AEC assesses tissue attenuation during pre-exposure and optimizes tube voltage (kVp\text{kVp}), tube current-time product (mAs), beam filter material, and target selection. It cannot adjust mechanical SID.

  • Question 10: For a fixed breast thickness, how does increasing the X-ray beam's half-value layer (HVL) impact the Average Glandular Dose during screening mammography?

    • Options: (a) Average glandular dose decreases, (b) Average glandular dose increases, (c) Average glandular dose remains completely unchanged, (d) Average glandular dose drops instantly to zero.
    • Correct Answer: (a) Average glandular dose decreases.
    • Explanation: Increasing the half-value layer (HVL) means making the X-ray beam more penetrating (higher energy). A more penetrating beam passes through tissue more readily to reach the receptor, requiring less overall radiation exposure and thus decreasing absorbed glandular dose (at the expense of image contrast).

  • Question 11: What image quality aspect is improved most by Digital Breast Tomosynthesis (DBT) compared to standard 2D contact mammography?

    • Options: (a) Spatial resolution, (b) Geometric magnification, (c) Reduction of tissue overlap / tissue superposition, (d) Elimination of radiation dose.
    • Correct Answer: (c) Reduction of tissue overlap / tissue superposition.
    • Explanation: DBT's primary clinical advantage is depth sectioning, which eliminates out-of-plane tissue superposition, unmasking true cancers and eliminating false positives caused by overlapping normal tissue.