Skeleton, Spine, Skull, and Joints, with the Brain, Nervous, and Endocrine Systems
Unit 1: Anatomy, Positioning, and Medical Language. Lesson 3 of 60 in CRES Academy.
Most plain-film volume is bones, and bone is what drives technique: it is dense, it varies hugely in thickness from a finger to a hip, and it is what the radiographer blames when an image looks flat.
Axial and appendicular
The skeletal system divides in two. The axial skeleton is the center of the body: skull, vertebral column, ribs, and sternum. The appendicular skeleton is the limbs and the girdles that hang them on: shoulders, arms, hands, pelvis, legs, feet. There are five primary bone types: long bones such as the femur, short bones such as the carpals of the wrist, flat bones such as the ribs, sesamoid bones such as the patella, and irregular bones such as the vertebrae. Marrow inside bone makes blood cells, which is also why bone marrow is one of the most radiosensitive tissues in the body.
A requisition names the bone and not the region, so learn the limb list as it runs outward from the trunk. At the shoulder, the girdle is the clavicle in front and the scapula behind; the upper arm includes the humerus; the forearm carries two bones, the radius under the thumb and the ulna under the little finger; and the hand is carpals at the wrist, metacarpals across the palm, and phalanges in the fingers. The lower limb mirrors it: the pelvis with its hip socket, the femur down to the knee with the patella in front of it, then the weight-bearing tibia beside the slimmer fibula, and the foot as tarsals at the ankle, metatarsals through the arch, and phalanges again in the toes. Every step outward is thinner than the one before it, which is why no single extremity technique serves a shoulder and a finger.
The vertebral column
Cervical 7 neck; the atlas and axis at the top carry the skull Thoracic 12 each pair of ribs attaches here Lumbar 5 the heavy load-bearing bodies, the thickest part to penetrate Sacral 5 fused into the sacrum Coccyx 4 fused bones at the tail Total 33 bones, so a spine series covers more thickness change than any other exam
Skull and joints
The cranium surrounds the brain: the frontal bone at the forehead, parietal bones at the sides, temporal bones housing the ear structures, the occipital bone at the back, plus the ethmoid and sphenoid inside. The face carries the mandible, maxilla, zygomatic, nasal, and lacrimal bones. Joints are classified by structure as fibrous, cartilaginous, or synovial, and by function as immovable, slightly movable, or freely movable. The synovial joints, with a capsule and fluid, are the freely movable ones imaging is mostly ordered for.
Brain, nerves, and the endocrine glands
The brain and spinal cord are the central nervous system; the nerves branching from them are the peripheral nervous system. Bone hides soft neural tissue, so plain films show the skull and spine while computed tomography and magnetic resonance show the brain, cord, and discs themselves, which is why a stroke or a disc complaint moves to those rooms. The endocrine glands, at recognition level, are the pituitary, thyroid, parathyroid, adrenals, pancreas, and gonads, and the thyroid is the one imaging and nuclear medicine work with most.
What bone does to technique
- Density and thickness drive kilovoltage: a hand needs far less than a hip, and a lumbar spine needs a grid where a finger does not.
- Musculoskeletal pathologies imaging is ordered for are fractures and dislocations, arthritis and joint space loss, bone infection, and tumors.
- Thin extremities need a small focal spot and short exposures, so detail complaints on extremity work point at focal spot, distance, and motion rather than at dose.
Skeleton, Spine, Skull, and Joints, with the Brain, Nervous, and Endocrine Systems, explained slowly
Bone is a technique problem before it is an anatomy problem
A hand and a hip are both bone, and the exposure one needs is nothing like the other. Everything the skeleton does to your equipment follows from two variables: how thick the part is and how dense it is. Thick and dense means more kilovoltage, more exposure, and a grid; thin means a small focal spot, a short exposure, and no grid. That is why a room's technique chart is organized by body part, and why "the extremity images are soft" is a focal spot, distance, and motion question while "the lumbar images are flat" is a grid and kilovoltage question.
The two halves of the skeleton
Axial skeleton skull, vertebral column, ribs, sternum
Appendicular skeleton shoulders and arms, hands, pelvis, legs, feet
Upper limb, by name clavicle and scapula at the girdle, then humerus, then
radius and ulna, then carpals, metacarpals, phalanges
Lower limb, by name pelvis at the girdle, femur with the patella at the knee,
tibia and fibula below it, then tarsals, metatarsals,
and phalanges
Five bone types long (femur), short (carpals), flat (ribs),
sesamoid (patella), irregular (vertebrae)
Marrow inside bone, makes blood cells, and is one of the most
radiosensitive tissues there isThat last line is why shielding and collimation matter even on a limb: a field edge that spills onto a pelvis is spilling onto marrow.
The vertebral column, counted and weighted
Region Count What it means on the console Cervical 7 thin neck, low technique, but the shoulders block a lateral Thoracic 12 ribs and lung behind it, so density changes down the series Lumbar 5 the thickest part of the spine; grid, higher kilovoltage Sacral 5 fused heavy, overlapped by bowel gas Coccyx 4 fused small and low Total 33 no other single exam crosses this much thickness
A spine series is the best test of whether a room's automatic exposure control and technique chart are honest, because the same patient presents four different thicknesses in one visit.
Skull and joints
The cranium is the frontal bone at the forehead, parietal bones at the sides, temporal bones around the ear structures, the occipital bone at the back, and the ethmoid and sphenoid inside; the face adds mandible, maxilla, zygomatic, nasal, and lacrimal bones. Joints are classified by structure as fibrous (immovable, like skull sutures), cartilaginous (slightly movable, like the discs between vertebral bodies), or synovial (freely movable, with a capsule and fluid: knee, shoulder, hip). Synovial joints are what most joint imaging is ordered for, and their service consequence is motion: a freely movable joint has to be supported and imaged quickly.
Soft neural tissue needs a different room
The brain and spinal cord are the central nervous system; the nerves branching out are the peripheral nervous system. Plain films show you the bone around them, so a skull or spine radiograph answers questions about fracture and alignment and nothing about the tissue inside. Computed tomography and magnetic resonance show the brain, cord, and discs directly, which is why stroke, bleed, and disc complaints move to those rooms, and why their table motion, alignment, and reconstruction computers are the service items that matter to neurologic work. The endocrine glands, at recognition level, are the pituitary, thyroid, parathyroid, adrenals, pancreas, and gonads, with the thyroid the one imaging and nuclear medicine work with most.
Musculoskeletal pathology, at recognition level
Fractures and dislocations, degenerative joint disease with narrowed joint spaces, bone infection, and tumors. You do not read them, but you should know that all four are judged on fine detail at the cortex and the joint space, which tells you which image quality factors are clinically load-bearing: focal spot size, object-to-image distance, and motion.
What to memorize
- Axial is skull, column, ribs, sternum; appendicular is limbs and girdles, named clavicle, scapula, humerus, radius, ulna, carpals, metacarpals, and phalanges in the arm, and pelvis, femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges in the leg.
- Five bone types: long, short, flat, sesamoid, irregular. Marrow is highly radiosensitive.
- Column 7, 12, 5, 5, and a coccyx, 33 bones in all.
- Joints fibrous, cartilaginous, synovial. Thickness and density decide kilovoltage and whether a grid is used.