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Radiology Study Notes: Essential Rad Tech Guide
2026-05-29·Education·~1,500 words
Whether you're studying for the ARRT registry, the Philippine RTLE, or just want to consolidate your rad tech knowledge, this guide brings together the essential facts, formulas, and concepts in one place — a structured study reference you can return to throughout training.
1. The Five Radiographic Densities
Every structure in the body appears in one of five shades on an X-ray, determined by its ability to attenuate (absorb) X-rays:
1. Air / Gas (blackest) — Lungs, bowel gas, trachea
2. Fat (dark gray) — Subcutaneous tissue, retroperitoneal fat
3. Soft tissue / Water (medium gray) — Muscle, liver, kidneys, blood
4. Bone / Calcium (light gray to white) — Skeleton, calcifications
5. Metal (whitest) — Implants, wires, contrast media, foreign bodies
2. Key Physics Formulas (ARRT High-Yield)
Inverse Square Law: I₁ / I₂ = (D₂ / D₁)²
Intensity decreases with the square of distance. Double SID = ¼ intensity.
Direct Square Law (mAs Compensation): mAs₁ / mAs₂ = (SID₁ / SID₂)²
Used to calculate mAs when changing SID.
mAs Reciprocity Law: mA × Time = mAs
100 mA × 0.1 s = 10 mAs = 200 mA × 0.05 s = 10 mAs. Same density.
15% Rule: kVp × 1.15 ≈ 2× density (can halve mAs)
The most tested clinical relationship. 15% kVp boost = double exposure.
Grid Conversion (Bucky Factor): New mAs = Old mAs × Grid Factor
No grid = 1×, 5:1 = 2×, 8:1 = 3-4×, 12:1 = 4-5×, 16:1 = 5-6×
Magnification Factor: Image Size / Object Size = SID / SOD
Shorter SID or longer OID = more magnification.
3. Anatomic Landmarks for Positioning
Knowing these reference points allows you to center any exam correctly without palpation errors:
| Landmark | Vertebral Level | Relevant Exam |
|---|
| Vertebra prominens (C7) | C7 | Cervical spine, thoracic inlet |
| Jugular notch | T2–T3 | Trachea, upper chest |
| Sternal angle (Angle of Louis) | T4–T5 | Carina of trachea, aortic arch, chest PA |
| Xiphoid process | T9–T10 | Heart, diaphragm, upper abdomen |
| Lower costal margin | L2–L3 | Kidneys, adrenal glands |
| Iliac crest | L4–L5 | Lumbar spine, abdomen, IVP |
| Anterior superior iliac spine (ASIS) | S1–S2 | Pelvis, hip, sacrum |
| Pubic symphysis | Same level as greater trochanters | Pelvis, hip, femur |
4. Radiation Safety and ALARA
Annual Dose Limits (NRC)
| Group | Effective Dose Limit | Lens of Eye | Skin / Extremity |
|---|
| Occupational (annual) | 50 mSv | 150 mSv | 500 mSv |
| Occupational (5-year avg) | 20 mSv/yr | — | — |
| General public (annual) | 1 mSv | 15 mSv | 50 mSv |
| Declared pregnant worker | 0.5 mSv/month
(2 mSv/gestation) | — | — |
The Three Pillars of ALARA
- Time — Minimize exposure duration. Use shortest exposure times possible
- Distance — Maximize distance. At 2 m from the source, intensity drops to ¼ of 1 m (inverse square law)
- Shielding — Use lead apron (0.25-0.5 mm Pb equivalent), thyroid shield, lead glasses, and mobile lead screens
5. Contrast Media Quick Reference
| Type | Examples | Route | Key Facts |
|---|
| Barium sulfate | Baritop, Liquid E-Z | Oral / Enema | Never if perforation suspected. Never before CT (causes streak artifacts). Contraindicated if large bowel obstruction. |
| Iodinated (water-soluble) | Iohexol, Iopamidol (non-ionic), Diatrizoate (ionic) | IV, Intra-arterial, Intrathecal | Non-ionic = lower adverse reaction rate. Check creatinine before IV. Risk of CIN (contrast-induced nephropathy). |
| Gadolinium (MRI) | Gadobutrol, Gadoterate | IV | NSF risk with linear agents (poor renal function). Check GFR. More paramagnetic = shorter T1 = brighter on T1-weighted images. |
| Negative contrast | Air, CO₂, O₂ | Enema, Joint injection | CO₂ is used for venography in patients with iodine allergy. Air is used in double-contrast barium studies. |
6. Common Projections and When to Use Them
PA vs AP Chest
PA (posteroanterior): Patient faces the detector. Minimizes cardiac magnification, better visualization of lungs. Preferred for routine chest X-ray.
AP (anteroposterior): Patient faces the tube. Used for portable (bedside) exams. Heart appears larger, clavicles are more horizontal, scapulae may overlap lungs.
Common Special Views
- AP Axial (Towne) — Skull, occipital bone, foramen magnum
- AP Axial (Ferguson) — Atlanto-occipital joint, odontoid process
- Lateral (Swimmer's) — C7-T1 when obscured by shoulders on lateral cervical
- AP (Grashey) — Glenohumeral joint (shoulder) without superimposition
- AP Axial Outlet (Taylor) — Pelvic ring, pubic rami
- AP (Inlet) — Pelvic ring, sacroiliac joints
- Sunrise / Skyline — Patellofemoral joint
- Tunnel (Intercondylar / Holmblad) — Intercondylar notch of knee
- AP (Mortise) — Ankle joint, distal tibiofibular joint
- PA (Carpal Bridge) — Dorsal aspect of carpal bones
7. kVp vs mAs — The Cheat Sheet
| Problem | Primary Adjust | Secondary Adjust |
|---|
| Image too light (underexposed) | ↑ mAs (double it) | ↑ kVp 15% |
| Image too dark (overexposed) | ↓ mAs (halve it) | ↓ kVp 15% |
| Too gray (low contrast) | ↓ kVp 10-15% | May need to ↑ mAs to compensate |
| Too chalky (high contrast) | ↑ kVp 10-15% | May need to ↓ mAs to compensate |
| Noisy / grainy | ↑ mAs | Consider faster imaging plate (DR) |
| Patient motion | ↑ mA (shorter time, same mAs) | Use reciprocity law |
8. Patient Care Essentials
- Informed consent — Required for contrast administration, invasive procedures, and research. Must include: nature of procedure, risks, benefits, alternatives
- Hematomas after injection — Apply firm pressure for 5-10 minutes. Check for swelling, discoloration, pulse distal to site
- Contrast reaction management — Know where the crash cart is. Recognize urticaria (hives), bronchospasm, laryngeal edema, anaphylactic shock. Epinephrine (1:1000, 0.3-0.5 mL IM) for severe reactions
- Fall prevention — Assist patients on and off tables. Lock wheels before transfers. Side rails for vulnerable patients
- Infection control — Hand hygiene before and after every patient. Clean transducer/table between patients. Sterile technique for invasive procedures
9. Exposure Index and Deviation Index (Digital Radiography)
In digital radiography, the Exposure Index (EI) indicates how much radiation reached the detector. The Deviation Index (DI) shows how far the actual EI is from the target EI:
- DI = 0 = Perfect exposure
- DI = +1 = 1.3× overexposed (acceptable)
- DI = +3 = 2× overexposed (unnecessary dose)
- DI = -1 = 0.7× underexposed (acceptable)
- DI = -3 = 0.5× underexposed (excessive noise)
- Target range: DI ±1 is acceptable, ±2 is borderline, ±3 requires repeat evaluation
10. Helpful Study Resources on This Site
About the author: This study reference was compiled by the Radiography 101 Education Team, drawing from ARRT content specifications, the Philippine PART exam syllabus, and standard radiography textbooks including Clark's Pocket Handbook and Radiography PREP.