Study guide
11.1 Forearm Anatomy, Landmarks, and Adjacent Structures
ARRT Bone Densitometry Exam
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1 Chapter 1: Exam Orientation & the ARRT Blueprint
1.1 About the ARRT Bone Densitometry Examination 1.2 Eligibility, Structured Education, and Clinical Experience
2 Chapter 2: Osteoporosis, Bone Biology & Risk Factors
2.1 Osteoporosis: Definitions, Types, and WHO Diagnostic Criteria 2.2 Bone Functions, Structural Anatomy, and Bone Types 2.3 Bone Cells, the Remodeling Cycle, and Calcium Homeostasis 2.4 Risk Factors, Secondary Causes, and Bone-Compromising Medications
3 Chapter 3: Indications, Prevention & Treatment
3.1 The Bone Mass Measurement Act: Covered Indications and Coding 3.2 Prevention: Nutrition, Exercise, Smoking Cessation, and Fall Prevention 3.3 Osteoporosis Drug Therapy: Antiresorptive, Anabolic, and Hormonal Agents
4 Chapter 4: Patient Preparation, Support & Radiation Safety
4.1 Patient Support, Disabilities, Ergonomics, and Transfer Techniques 4.2 Patient Instructions: Explaining the Exam, Dose, Duration, and Motion Control 4.3 Patient History, Contraindications, and Pregnancy Screening 4.4 Scan Preparation: Patient Data Entry, Artifact Removal, and Infection Control 4.5 Radiation Safety in DXA: ALARA, Distance, and Dose Levels
5 Chapter 5: DXA Physics & Instrumentation
5.1 Physical Principles of Dual-Energy X-ray Absorptiometry 5.2 Beam Geometries: Pencil Beam and Fan Beam Systems 5.3 Fan-Beam Geometry: Source–Object–Detector Distances, Magnification, and Centering 5.4 The Scan Analysis Algorithm: Soft Tissue, Bone Edge Detection, BMC, Area, and BMD
6 Chapter 6: Measuring & Reporting Bone Mineral Density
6.1 Statistics for Densitometry: Mean, Standard Deviation, and Coefficient of Variation 6.2 T-Scores, Z-Scores, and Reference Database Selection 6.3 Clinical Indications for BMD Testing and the FRAX Tool 6.4 Vertebral Fracture Assessment and Trabecular Bone Score 6.5 Whole Body DXA, Body Composition, and Pediatric Scanning
7 Chapter 7: Quality Control, Precision & Equipment Management
7.1 Accuracy, Precision, and the Sources of Precision Error 7.2 Daily Quality Control, Phantoms, and Control Charts 7.3 In Vivo Precision Studies and RMS-SD Calculation 7.4 Determining and Applying the Least Significant Change 7.5 Maintenance, Repair, Relocation, Cross-Calibration, and Software Upgrades
8 Chapter 8: DXA Informatics & Data Management
8.1 DXA Informatics, DICOM, PACS, and Data Management
9 Chapter 9: Lumbar Spine DXA
9.1 Lumbar Spine Anatomy, Landmarks, and Regions of Interest 9.2 Lumbar Spine Scan Acquisition and ROI Edge Placement 9.3 Lumbar Spine: Compensating for Body Habitus, Variant Anatomy, Pathology, and Low Bone Density 9.4 Lumbar Spine Positioning: The Leg Block, Centering, and Common Challenges 9.5 Lumbar Spine Problems: Poor Bone Edge Detection and Obscured Intervertebral Spaces 9.6 Lumbar Spine: Variant Anatomy, Fractures, Degenerative Pathology, Calcification, and Surgery 9.7 Lumbar Spine Analysis: Vertebral Exclusion and Reporting Rules 9.8 Lumbar Spine Follow-Up Scans and Rate of Change
10 Chapter 10: Proximal Femur DXA
10.1 Proximal Femur Anatomy, Landmarks, and Regions of Interest 10.2 Femur Selection: Right, Left, and Dual Femur Scanning 10.3 Proximal Femur Scan Parameters and Compensation 10.4 Proximal Femur Positioning: Neck Rotation, Shaft Placement, and Positioning Aids 10.5 Proximal Femur Problems: Ischium, Short Neck, Arthritis, Fracture, and Hardware 10.6 Femoral ROI Analysis: Femoral Neck, Total Hip, and Ward's Area 10.7 Proximal Femur Follow-Up Scans and Rate of Change
11 Chapter 11: Forearm DXA
11.1 Forearm Anatomy, Landmarks, and Adjacent Structures 11.2 Forearm Indications and Side Selection 11.3 Forearm Length Measurement, Scan Parameters, and Positioning 11.4 Forearm Regions of Interest: One-Third, Mid, Ultradistal, and Total 11.5 Forearm Problems: Edge Detection, Artifacts, Low Bone Density, and Joint Disease 11.6 Scan Analysis: Region Placement, Window Level, and Reported Values 11.7 Forearm Follow-Up Scans and Rate of Change
12 Chapter 12: Cross-Site Artifacts & Clinical Integration
12.1 Recognition, Impact, and Management of DXA Artifacts 12.2 Clinical Interpretation, Serial Comparison, and Reporting
11.1 Forearm Anatomy, Landmarks, and Adjacent Structures
Key Takeaways Explain This Topic AI
The radius is the lateral forearm bone on the thumb side and the ulna is the medial bone on the little-finger side.
The ulnar styloid process distally and the olecranon process proximally are the landmarks used to measure forearm length and locate regions of interest.
The distal radius is metaphyseal and trabecular-rich, while the shaft at the one-third site is predominantly cortical.
The radius and ulna must be separated in the image, because the software locates regions of interest using the gap between them.
The carpal bones are the principal adjacent structures, and including them in a region of interest invalidates the measurement.
Last updated: September 2026
11.1 Forearm Anatomy, Landmarks, and Adjacent Structures
Quick Answer: The forearm holds two bones: the radius laterally (thumb side) and the ulna medially (little-finger side). The ulnar styloid distally and the olecranon proximally are the landmarks that define forearm length , which the software uses to place regions of interest. The distal radius is trabecular-rich metaphysis ; the shaft at the one-third site is predominantly cortical . The carpal bones and the radioulnar gap are the adjacent structures that determine analyzability.
Why the Forearm Has Its Own Anatomy Section
The forearm is not simply a smaller spine. Its regions of interest are located by measured distance along a bone , not by recognizing a vertebral body or a femoral neck. That makes landmark identification and length measurement into primary determinants of the result, in a way that has no equivalent at the other two sites.
- Osteology
- The Radius
Lateral bone of the forearm, on the thumb side.
Small proximally (the radial head at the elbow) and large distally at the wrist — the opposite of the ulna.
The distal radius carries most of the load transmitted from the hand through the wrist, which is why a fall on an outstretched hand fractures the distal radius (Colles fracture) rather than the ulna.
Radial styloid process : the distal lateral projection, palpable at the base of the thumb.
The distal metaphysis is trabecular-rich ; the mid and proximal shaft are cortical .
The Ulna
Medial bone, on the little-finger side.
Large proximally (the olecranon at the elbow) and small distally .
Olecranon process : the prominent point of the elbow — the proximal landmark for forearm length measurement.
Ulnar styloid process : the small distal projection, palpable on the little-finger side of the wrist — the distal landmark.
The Relationship Between Them
The radius and ulna lie side by side, separated by the interosseous space occupied by the interosseous membrane. Distally they converge; proximally they diverge. That geometry matters because:
The gap between the two bones widens proximally , and analysis software uses the point at which the radius and ulna are separated by a defined distance as a reference for locating the ultradistal region on some systems.
Pronation and supination rotate the radius around the ulna. In full supination the bones lie parallel; in pronation the radius crosses over the ulna. A pronated, rotated forearm projects the two bones overlapping, which defeats the software's ability to locate regions.
Landmarks Used in DXA
Landmark Location Use in DXA
Ulnar styloid process Distal ulna, little-finger side of wrist Distal reference for forearm length; often the zero point for ROI placement
- Olecranon process Point of the elbow Proximal reference for forearm length measurement
- Radial styloid process Distal lateral radius, thumb side Orientation and alignment check
- Radioulnar gap Interosseous space Used by some systems to locate the ultradistal region
- Distal radioulnar joint Where radius and ulna articulate at the wrist Marks the distal limit of usable bone
Forearm length — measured from the ulnar styloid to the olecranon on most systems — is entered into the scanner so the software can compute where along the bone to place the percentage-defined regions. A length entered incorrectly places the ROI at the wrong point along a bone whose cortical thickness changes continuously with position, so the reported BMD is a measurement of a different piece of bone than intended.
Radiographic Appearance on DXA
A correctly acquired forearm image shows:
The radius and ulna running parallel along the long axis of the image, without overlap.
The distal ends — radial and ulnar styloids — at one end of the field, with the carpal bones just beyond.
Clear separation between the two bones through the region of interest zone.
Soft tissue on both sides of the forearm for the baseline estimate.
The cortical shell of each bone visible as a dense rim, thick in the shaft and thin distally.
Density changes visibly along the bone: the distal metaphyseal region appears less dense (trabecular) and the shaft appears denser (cortical). That gradient is precisely why ROI position matters.
Significant Adjacent Structures
Structure Significance
Carpal bones Dense, immediately distal to the radius. Including them in a region of interest falsely elevates the value. The scan field must be set so the ROI does not extend into the carpus
Soft tissue Needed on both sides for the baseline estimate; a forearm pressed against a rail or another body part contaminates it
- Distal radioulnar joint Marks the distal limit; degenerative change here alters local density
- Humerus / elbow Proximal limit; must not enter the analysis field
The contralateral limb, torso, or table rail Any structure entering the scan field corrupts the baseline. Position the arm clear of the body
Skeletal Composition and Site Choice
Region Composition Behavior
Ultradistal radius Predominantly trabecular More responsive to disease and therapy than the shaft, but higher precision error
Mid-radius Mixed Intermediate
One-third (33%) radius Predominantly cortical Stable, reproducible; the ISCD diagnostic forearm site; preferentially lost in hyperparathyroidism
Total radius Combined Reported by most systems; not the diagnostic site
The cortical composition of the one-third radius explains both of its uses. It is reproducible , which makes it the diagnostic forearm site. And it is cortical , which makes it the preferred site in primary hyperparathyroidism — a disease in which chronically elevated parathyroid hormone preferentially resorbs cortical bone, so the one-third radius shows the disease while the trabecular-rich spine may look deceptively normal.