What are the most important physics formulas for the ARRT exam?

Commonly taught relationships within the ARRT radiography content domains include the inverse square law (I1/I2 = (D2/D1)²), mAs (mA × time), the approximate 15% kVp rule, grid conversion, distance compensation, and magnification (image size/object size = SID/SOD). Equal mAs under otherwise unchanged conditions produces approximately equal detector exposure—not necessarily equal displayed brightness in digital imaging. ARRT publishes content categories rather than promising a fixed number of questions on any one formula.

What are the essential anatomic landmarks for radiography positioning?

Common surface landmarks include the vertebra prominens (usually C7), jugular notch (about T2–T3), sternal angle (about T4–T5), xiphoid tip (about T9–T10), lower costal margin (about L2–L3), iliac crest (about L4–L5), ASIS (about S1–S2), pubic symphysis, greater trochanter, and acromion. Vertebral correlations are approximate and positioning/centering must follow the requested projection and local protocol.

What are the different types of contrast media in radiography?

Positive contrast agents include barium sulfate for selected gastrointestinal studies and iodinated agents for routes such as enteric, intravenous, intra-arterial, or intrathecal administration when the specific product is approved/appropriate. Gas such as air or carbon dioxide can provide negative contrast in selected procedures. When gastrointestinal perforation is suspected, water-soluble contrast is generally preferred initially; dilute barium is nevertheless commonly used for selected CT examinations. Iodinated contrast can cause allergic-like or physiologic reactions. Kidney events should be described as contrast-associated AKI (correlative) or contrast-induced AKI (causal), and renal-function testing is risk-based rather than universal.

What are the ALARA principles of radiation safety?

ALARA stands for As Low As Reasonably Achievable. Practical controls include minimizing time near a radiation field, maximizing distance when feasible, and using appropriate structural or personal shielding under the facility radiation-safety program. Under U.S. NRC rules, the adult occupational annual total effective dose equivalent limit is 50 mSv and the public annual total effective dose equivalent limit from licensed operations is 1 mSv (with stated exclusions). These are regulatory ceilings, not dose targets.

What are the five basic radiographic densities?

A common teaching convention groups radiographic appearances from darkest to lightest as air/gas, fat, soft tissue/water, bone/calcium, and metal/high-density contrast. Actual image values form a continuum and displayed brightness also depends on acquisition and image processing, especially in digital radiography.

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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

A common teaching convention groups radiographic appearances into five broad categories. Actual attenuation and pixel values are continuous, and digital display processing also affects brightness:

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

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. With other factors unchanged, detector exposure is approximately the same; digital display brightness is processing-dependent.

Approximate 15% Rule: kVp × 1.15 ≈ 2× receptor exposure (so mAs may be halved to maintain receptor exposure)
This empirical teaching rule is approximate and varies with anatomy, beam filtration, detector response, and technique range.

Grid Conversion (Bucky Factor): New mAs = Old mAs × (New Grid Factor / Old Grid Factor)
Traditional teaching factors are approximate (for example, no grid 1×, 5:1 about 2×, 8:1 about 4×, 12:1 about 5×, 16:1 about 6×); use the grid/system manufacturer's or department's validated factor.

Magnification Factor: Image Size / Object Size = SID / SOD
At a fixed object size, shorter SOD (or greater OID at fixed SID) produces more magnification.

3. Anatomic Landmarks for Positioning

These surface-to-vertebral correlations are approximate and vary with habitus and position. Use them with the requested projection and the department's validated positioning protocol—not as a substitute for it:

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)	About T4–T5	Upper-thoracic landmark; the carina level varies with respiration and position
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	No fixed vertebral level	Pelvis, hip, femur

4. Radiation Safety and ALARA

U.S. NRC Dose Limits

Category	Limit	Important qualification
Adult occupational, annual	50 mSv total effective dose equivalent (TEDE)	Separate annual limits: lens dose equivalent 150 mSv; shallow dose to skin or an extremity 500 mSv
Individual member of the public, annual	1 mSv TEDE	From licensed operations, with exclusions stated in 10 CFR 20.1301; also 0.02 mSv in any one hour from external sources in an unrestricted area
Embryo/fetus of a declared pregnant worker	5 mSv for the entire pregnancy	The licensee must seek a substantially uniform monthly exposure rate; this is not a 0.5 mSv-per-month statutory limit

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 structural barriers and task-appropriate personal protective equipment specified by the facility's radiation-safety program; required lead equivalence depends on the procedure and applicable regulation

5. Contrast Media Quick Reference

Type	Examples	Route	Key Facts
Barium sulfate	Various suspensions	Oral / rectal	Water-soluble contrast is generally preferred first when GI perforation is suspected. Dilute barium is commonly used for selected CT exams, so it is not categorically prohibited before CT.
Iodinated (water-soluble)	Iohexol, iopamidol (nonionic); diatrizoate (ionic)	Enteric, IV, intra-arterial, or intrathecal, depending on the product	Renal-function testing before intravascular use is risk-based, not universal. Distinguish contrast-associated AKI (correlation) from contrast-induced AKI (causation).
Gadolinium-based (MRI)	Gadobutrol, gadoterate	IV	NSF risk differs by ACR agent group. Group II agents are strongly preferred in at-risk patients; ACR states their NSF risk at standard dose is very low or possibly nonexistent and renal testing is optional before IV administration.
Negative contrast	Air; carbon dioxide in selected angiographic procedures	Procedure-specific	Air is used in some double-contrast GI studies. CO₂ angiography has important anatomic and procedural restrictions and is not simply interchangeable with iodinated contrast.

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 sacroiliac joints (Ferguson method) — Bilateral SI joints; tube angle and naming can vary by positioning reference/protocol
Lateral (Swimmer's) — C7-T1 when obscured by shoulders on lateral cervical
AP oblique (Grashey method) — Glenohumeral joint space in profile
AP Axial Outlet (Taylor) — Pelvic ring, pubic rami
AP axial inlet — Pelvic ring and pelvic inlet; angle is protocol- and patient-dependent
Sunrise / Skyline — Patellofemoral joint
Tunnel (Intercondylar / Holmblad) — Intercondylar notch of knee
AP oblique mortise — Ankle mortise and distal tibiofibular articulation
Tangential carpal bridge — Dorsal aspect of the carpal bones with the wrist hyperflexed; use only when clinically safe

7. Digital Technique Troubleshooting

Problem	Primary Adjust	Secondary Adjust
Low detector exposure / quantum mottle	Check positioning, collimation, EI/DI validity, and technique before changing exposure	If underexposure is confirmed, adjust the validated technique chart rather than using brightness alone
High detector exposure	Check EI/DI and technique against the exam-specific target	Reduce technique according to the validated chart when image quality remains adequate
Displayed image too light/dark	Do not diagnose exposure from digital brightness alone	Review processing, histogram/field recognition, EI/DI, and raw-data quality
Inadequate subject contrast	Confirm positioning, collimation, scatter control, and protocol	kVp changes affect penetration, receptor exposure, and patient dose; compensate only under a validated technique chart
Patient motion	Use the shortest practical exposure time	Increase mA to preserve prescribed mAs only within tube/loading limits and the validated technique

8. Patient Care Essentials

Informed consent — Requirements depend on the procedure, state law, and institutional policy. ACR notes that routine IV contrast injection may be exempted because adverse events are uncommon; invasive procedures and research follow their applicable consent rules.
Injection-site bleeding or hematoma — Stop the injection, apply pressure until bleeding is controlled, and assess the site and distal neurovascular status according to local policy; escalate persistent bleeding, expanding swelling, severe pain, or neurovascular change.
Contrast reaction management — Recognize allergic-like and physiologic reactions, stop contrast when appropriate, assess airway/breathing/circulation, call the facility response team, and follow the current reaction protocol within scope. The ACR adult algorithm lists epinephrine 0.3 mg IM (0.3 mL of 1 mg/mL [1:1,000]) for indicated bronchospasm/laryngeal edema scenarios, repeatable every 5–15 minutes; treatment varies with presentation.
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 standardized Exposure Index (EI) estimates detector exposure for the image and is not a direct patient-dose measurement. The Deviation Index (DI) is logarithmic: DI = 10 log₁₀(EI/EI_T), where EI_T is the exam-specific target:

DI = 0 — EI equals the target EI; this does not by itself prove correct positioning, image quality, or patient dose
DI = +1 — EI is about 1.26× target
DI = +3 — EI is about 2× target
DI = -1 — EI is about 0.79× target
DI = -3 — EI is about 0.50× target and quantum mottle may increase
Action ranges: Set locally by examination and validated protocol; DI alone is not a universal accept/repeat rule and should never trigger an automatic repeat without reviewing diagnostic adequacy and repeat policy

10. Helpful Study Resources on This Site

ARRT Exam Prep: How to Pass the Registry — Study strategies and test-day tips
X-Ray Physics Made Simple — kVp, mAs, density, and contrast
Radiographic Density & Contrast: The Complete Guide — Deep dive into image quality
Radiation Safety for Rad Techs — ALARA, shielding, dose limits
Philippines PART/RTLE Board Exam Guide — PRC exam coverage, study plan
RTLE Practice Quiz — Interactive 100-question quiz for Philippine exam
Radiography Calculators — mAs, grid, magnification, and inverse square law calculators

Accuracy sources

Key claims were checked against the ARRT Examination Content Specifications, 10 CFR Part 20 (NRC regulations), the ACR Manual on Contrast Media, and AAPM Report 116. Projection names and angles can vary among positioning references and institutional protocols.

About this guide: Radiography 101 prepared this educational study reference for radiologic technology students. Use the linked current standards and your program's validated procedures; positioning details and regulatory requirements can vary by jurisdiction and protocol.