13.3 CT, BMD & Mammography Overview
Key Takeaways
- CT image formation depends on kV, mA (tube current), rotation/exposure time, pitch, and reconstructed slice thickness—entry-level RTRs should relate these to noise, dose, and coverage conceptually.
- Contrast-enhanced CT answers vascular and organ-enhancement questions; unenhanced CT remains appropriate for many stone, hemorrhage, and selected follow-up pathways—match protocol to the clinical question.
- CTDI and DLP are order-of-magnitude dose metrics used for comparison and documentation; they are not personal effective dose badges but signal that CT is a higher-dose modality than most plain radiography.
- DXA bone mineral densitometry reports T-scores and Z-scores; RTRs need awareness of what the exam is and score categories at overview depth (low blueprint weight).
- Screening mammography commonly uses CC and MLO projections with firm compression to reduce thickness, scatter, and motion and to improve contrast and dose efficiency—overview knowledge, not full mammography specialty certification.
13.3 CT, BMD & Mammography Overview
Quick Answer: CT (medium secondary weight) uses rotating x-ray acquisition with selectable kV, mA, pitch, and slice parameters; know contrast vs unenhanced indications at a protocol-awareness level and treat CTDI/DLP as dose metrics of interest. BMD (DXA) and mammography are low weight: know DXA T/Z-score meaning at a glance and mammography’s CC/MLO projections plus why compression matters. This is entry-level overview—not CT/MRI specialty or full mammo certification depth.
The CAMRT Radiological Technology blueprint weights CT as a medium secondary clinical procedure area, while BMD and mammography are low. Canadian RTRs increasingly rotate through CT or work in hybrid departments; even those who remain primarily in general radiography must triage, prepare, and educate patients, collaborate on protocols, and speak the dose language of modern imaging. Keep depth proportional to blueprint weight: stronger CT conceptual fluency, lighter BMD/mammo awareness.
CT Principles for the Entry-Level RTR
Computed tomography reconstructs cross-sectional images from many projections acquired as the x-ray tube rotates around the patient. Multi-detector systems acquire multiple slices per rotation, enabling helical (spiral) scanning of long body regions in seconds.
Core acquisition parameters
| Parameter | Concept | Typical trade-offs |
|---|---|---|
| kV (tube voltage) | Beam energy / penetrability | Higher kV → more penetration, often lower contrast, can reduce noise for large patients; dual-energy systems add advanced options |
| mA / mAs (tube current × time) | Photon quantity | Higher mA → lower noise, higher dose; automatic exposure control (AEC/ATCM) modulates mA along z-axis and angles |
| Pitch (helical) | Table movement per rotation relative to beam collimation | Higher pitch → faster coverage, generally lower dose, potentially more noise/artifacts if extreme |
| Detector configuration / collimation | Beam width along z | Wider coverage speeds exams; influences dose efficiency and overbeaming concepts at awareness level |
| Reconstructed slice thickness | Displayed/reconstructed section thickness | Thinner slices → better z-resolution, more noise unless dose/kernel adjusted |
| Kernel / algorithm | Reconstruction filter | Bone kernels sharpen edges (noisier); soft-tissue kernels smoother |
| Scan range / phases | Anatomic coverage and timing | More phases and longer ranges multiply dose—justify each phase |
Helical pitch (working definition)
Pitch ≈ (table travel per rotation) / (total collimated beam width). Pitch ≈ 1 means table travel roughly matches beam width per rotation. Pitch > 1 moves the table farther (faster scan, generally less overlap/dose). Pitch < 1 increases overlap (higher dose, sometimes used for fine detail strategies). Exact vendor definitions vary slightly; exam items test the direction of the trade-off, not calculator precision.
Unenhanced versus contrast-enhanced CT
| Study type | Common clinical aims (examples) | Notes for RTRs |
|---|---|---|
| Unenhanced (non-contrast) | Urinary stones, acute hemorrhage pathways, some follow-ups, patients who cannot receive IV contrast | Still a full CT dose study—not “dose free” because contrast is omitted |
| IV contrast-enhanced | Vascular opacification, organ lesions, infection/abscess characterization, staging pathways | Requires screening for contrast risk, IV access, timing (arterial/portal/delayed phases per protocol) |
| Oral/rectal contrast (when used) | Selected bowel/leak protocols | Prep instructions and timing matter; many trauma protocols omit oral contrast |
Match the clinical question to the protocol. Giving IV contrast “because CT always uses contrast” is incorrect; omitting contrast when the question is vascular/lesion characterization may make the study non-diagnostic. Screening for allergy, renal risk, metformin policies, pregnancy, and breastfeeding guidance follows site and radiologist protocols (see contrast chapters).
Dose awareness: CTDI and DLP
CT doses are higher than most plain radiographic exams for the same body region. Two metrics appear on modern consoles and dose reports:
- CTDIvol (CT dose index volume) — standardized measure related to output for the selected technique and scan mode, expressed in mGy. Useful for comparing protocols and scanners, not a direct organ dose to your specific patient without further calculation.
- DLP (dose–length product) — roughly CTDIvol × scan length (mGy·cm). Reflects total output for the scan range; longer ranges raise DLP even if CTDIvol is constant.
Order-of-magnitude mindset: a multiphase abdominal CT delivers substantially more dose than a two-view chest radiograph. Pediatric protocols must use child-sized techniques (image gently principles). Your role includes verifying correct protocol selection, avoiding unnecessary multiphase repeats, centering the patient for AEC accuracy, and removing metallic artifacts from the range when possible.
Practical CT safety and quality behaviours
- Identity, pregnancy status, and order appropriateness before scanning.
- Positioning and centering within the gantry—off-center patients degrade AEC and image quality.
- Arms up for chest/abdomen when feasible to reduce artifact and dose.
- Breath-hold coaching for motion control on thoracic/abdominal exams.
- Shielding policies follow current evidence-based site rules (some historical shield practices have changed—follow department protocol).
- Critical results pathways for unexpected findings per site policy.
BMD Overview (DXA) — Low Weight
Bone mineral densitometry most commonly uses dual-energy x-ray absorptiometry (DXA/DEXA). Two x-ray energies estimate bone mineral density, typically at the lumbar spine and hip (and sometimes forearm).
T-score and Z-score awareness
| Score | Comparison group | Clinical framing (awareness level) |
|---|---|---|
| T-score | Young adult reference mean | Used in postmenopausal women and men of specified ages for osteopenia/osteoporosis classification thresholds in clinical guidelines |
| Z-score | Age-matched (and often sex/ethnicity-matched) peers | Emphasized in younger patients; very low Z-scores prompt search for secondary causes |
WHO-style categories (awareness, not endocrinology mastery) often taught as: T-score ≥ −1.0 normal; between −1.0 and −2.5 low bone mass (osteopenia); ≤ −2.5 osteoporosis—always interpreted by qualified clinicians with clinical context. RTRs performing DXA focus on correct positioning, artifact exclusion (buttons, prior surgery), serial scan consistency, and QC phantoms per site—not diagnosing patients from the console alone.
DXA effective doses are typically very low compared with CT. Blueprint weight is low: know what DXA is, what T/Z scores represent, and that serial comparisons need consistent technique.
Mammography Overview — Low Weight
Screening and diagnostic mammography are specialized; many Canadian RTRs hold additional mammography credentials for independent practice. Entry-level CAMRT RT content expects overview knowledge:
Standard screening projections
| Projection | Name | Purpose (concept) |
|---|---|---|
| CC | Craniocaudal | Superior–inferior compression; shows medial/lateral extent |
| MLO | Mediolateral oblique | Oblique compression including pectoral muscle; visualizes axillary tail better than CC alone |
Diagnostic workups may add true lateral, spot compression, magnification, and tomosynthesis views—recognize that they exist.
Why compression matters
Firm, even compression:
- Decreases breast thickness → less scatter, better contrast, lower dose for adequate image quality
- Spreads tissues → reduces superposition of structures
- Immobilizes → reduces motion blur
- Equalizes thickness → more uniform exposure
Patients need clear explanation: compression is brief and purposeful, not punitive. Pain varies; communicate, position carefully, and never use compression as a substitute for correct positioning geometry.
Awareness boundaries
- Screening intervals and eligibility are program/province-specific—do not invent national rules on the exam unless given in the stem.
- Breast implants, male patients, and post-surgical breasts require modified protocols—flag for specialized pathways.
- Digital breast tomosynthesis (DBT) is widely used; conceptual value is reduced tissue overlap versus 2D alone.
Integrating the Three Modalities in Patient Pathways
General radiography often feeds CT (e.g., abnormal chest radiograph → CT chest). Trauma pathways may skip plain film for CT. Osteoporosis risk may lead from plain films showing compression fractures to DXA. Breast symptoms may go directly to mammography/ultrasound rather than chest x-ray. Your clinical expert role includes knowing the next best imaging conversation, preparing patients accurately, and not overselling modalities outside your authorization.
Exam Focus
CT items: relate higher mA to lower noise/higher dose; higher pitch to faster/lower-dose trends; thinner slices to more noise; contrast vs non-contrast indications; CTDI/DLP as dose report metrics. BMD items: T-score vs Z-score reference groups. Mammography items: CC and MLO as standard screening pair; compression rationale. Avoid over-detailed specialty protocol numbers unless taught as concepts. Depth should match medium for CT, low for BMD/mammo—exactly the CAMRT secondary procedure weighting.
In helical CT, increasing pitch while holding other factors constant most typically results in which combination?
Which clinical question is most appropriately answered with unenhanced CT rather than routine IV contrast-enhanced CT?
What do CTDIvol and DLP primarily provide for CT practice?
Why is firm compression used in mammography?