Anatomy

Anatomy is the study of structures that can be observed both grossly (without magnification) and microscopically (with magnification). The term 'anatomy' itself is derived from the Greek word 'temnein,' meaning 'to cut,' highlighting its historical link to dissection.

Gross anatomy, or macroscopic anatomy, focuses on structures visible to the naked eye. Microscopic anatomy, also known as histology, involves the study of cells and tissues using a microscope.

Anatomy is foundational to the practice of medicine, aiding physicians in understanding patient diseases through physical examinations and advanced imaging. It is also crucial for dentists, chiropractors, physical therapists, and other healthcare professionals involved in patient treatment.

The primary techniques for learning anatomy are observation and visualization. While anatomical terminology is important, a deeper understanding requires visualizing the spatial relationships of structures, going beyond simple memorization.

Gross anatomy can be studied through dissection of cadavers, viewing previously dissected material, using plastic models, or employing computer teaching modules and virtual/augmented reality experiences.

In the regional approach, each body region is studied comprehensively, examining all its structures (vasculature, nerves, bones, muscles, organs) simultaneously before moving to the next region.

The systemic approach involves studying each body system (e.g., cardiovascular, nervous, skeletal) in its entirety throughout the whole body before moving to the next system.

The regional approach is effective for cadaver dissection but may limit understanding of system continuity. The systemic approach enhances understanding of entire systems but can be difficult to coordinate with dissection and may lack detail.

The anatomical position is the standard reference point for describing body structures. It involves standing upright, feet together, arms at the sides, palms facing forward, and face looking forward.

Coronal planes are vertical planes that divide the body into anterior (front) and posterior (back) parts.

Sagittal planes are vertical planes, perpendicular to coronal planes, that divide the body into right and left parts. The median sagittal plane divides the body into equal right and left halves.

Transverse, horizontal, or axial planes divide the body into superior (upper) and inferior (lower) parts.

Anterior (ventral) and posterior (dorsal) describe front and back positions. Medial and lateral describe positions relative to the median sagittal plane and the sides of the body, respectively.

Superior and inferior refer to positions along the vertical axis (head to toe). Proximal and distal are used primarily for limbs, indicating closeness to or farness from the origin of a structure.

Cranial and caudal are sometimes used instead of superior and inferior, respectively. Rostral is used, especially in the head, to indicate a position towards the nose.

Superficial and deep describe the relative positions of structures with respect to the body's surface. Superficial structures are closer to the surface, while deep structures are farther away.

Recognizing that not all individuals fit a sex-binary model, anatomical terminology is evolving to be more inclusive. Terms like 'cis,' 'trans,' and 'non-binary' are used, and preferred patient terminology should be prioritized.

Plain radiography uses X-rays, a form of electromagnetic radiation, to create images. Different tissues attenuate X-rays to varying degrees: air attenuates the least, fat more than air, and bone the most, resulting in different shades of gray on the image.

Fluoroscopy uses a continuous flow of X-rays to produce real-time, dynamic images, allowing visualization of movement, such as in joints or during angiography.

Contrast agents, like barium sulfate for the GI tract or iodine-based compounds for vascular imaging, are used to enhance the visibility of specific structures by attenuating X-rays more than surrounding tissues.

Subtraction angiography uses digital imaging to remove overlying bony and soft tissue structures, allowing for clearer visualization of contrast-filled blood vessels.

Ultrasound uses high-frequency sound waves, not electromagnetic radiation, to create real-time images of internal organs. It is widely used for abdominal, fetal, and soft tissue imaging.

Doppler ultrasound is a specialized form of ultrasound that measures the flow, direction, and velocity of blood within vessels, aiding in the detection of blockages.

CT scanners acquire multiple X-ray images in the axial plane, which are then processed by a computer to create cross-sectional 'slices' of the body. Contrast agents are often used to improve visualization.

MRI utilizes strong magnetic fields and radio waves to align hydrogen protons in water molecules. As these protons return to their aligned state, they emit signals that a computer interprets to create detailed images. Different pulse sequences (T1-weighted, T2-weighted) highlight different tissue characteristics.

DWI, a type of MRI, provides information about the movement of water molecules within tissues. It is particularly useful for identifying abnormal tissues like tumors or infarcted areas where diffusion is restricted.

Nuclear medicine uses gamma rays emitted from radioactive tracers administered to the patient. A gamma camera detects these rays to create images reflecting physiological function. Technetium-99m (99mTc) is a commonly used radionuclide.

PET imaging detects positron-emitting radionuclides. It is valuable for identifying metabolically active tissues, such as tumors, and for assessing brain function. Fluorodeoxyglucose (FDG) labeled with fluorine-18 is a common PET tracer.

SPECT is a nuclear medicine technique that uses a rotating gamma camera to create 3D images of radionuclide distribution, aiding in the diagnosis of conditions like coronary artery disease and bone fractures.

Interpreting medical images requires understanding the imaging technique, normal anatomical variations, and how images are obtained. Without a solid anatomical foundation, it is impossible to accurately identify abnormalities.

When viewing radiographs, the right side of the patient is typically displayed on the observer's left, as if viewing the patient in the anatomical position. For AP radiographs, the anterior part is closer to the tube, and the posterior part is closer to the detector.

A quality PA chest radiograph should demonstrate the lungs, heart and mediastinum, diaphragm, ribs, and soft tissues. Markers indicating the patient's right or left side are crucial for correct interpretation.

Plain abdominal radiographs are usually obtained in the AP supine position. Erect views may be used to assess for small bowel obstruction. Contrast studies, like barium enemas, are used to visualize the gastrointestinal tract.

CT images are typically acquired in the axial plane and viewed from below, looking upwards towards the head. This means the patient's right side appears on the left of the image, and the top of the image is anterior.

MRI images are generally viewed using the same principles as CT scans, with the patient's right side on the left of the image and the top of the image representing the anterior aspect.

The skeletal system is composed of cartilage and bone. It is divided into the axial skeleton (skull, vertebral column, ribs, sternum) and the appendicular skeleton (limbs and girdles).

There are three types of cartilage: hyaline (found on articular surfaces), elastic (found in the external ear), and fibrocartilage (found in intervertebral discs). Cartilage is avascular and nourished by diffusion.

Cartilage supports soft tissues, provides smooth gliding surfaces at joints, and enables the growth of long bones.

Bone tissue is classified as either compact (dense, outer shell) or spongy (trabecular, with internal cavities containing marrow).

Bones provide support, protect vital organs, act as reservoirs for calcium and phosphorus, serve as levers for muscle action, and house blood-producing marrow.

Bones are classified by shape: long (e.g., femur), short (e.g., carpals), flat (e.g., skull bones), irregular (e.g., vertebrae), and sesamoid (e.g., patella).

Accessory bones are extra bones not typically part of the normal skeleton, while sesamoid bones are embedded within tendons. These can sometimes cause pain or be mistaken for fractures on imaging.

Bones are vascular and innervated. Nutrient arteries supply the marrow and inner bone layers, while the periosteum, a membrane covering the bone, is richly supplied with blood vessels and sensory nerves.

Bones develop through two primary processes: intramembranous ossification (direct ossification of mesenchymal models) and endochondral ossification (ossification of cartilaginous models).

Radiation exposure from X-ray and nuclear medicine imaging should be minimized ('as low as reasonably achievable'). Ultrasound and MRI are preferred when radiation risk is a concern, especially in pregnant patients.

The effective dose of radiation from various imaging procedures can be compared to the equivalent duration of natural background radiation exposure. For example, a chest radiograph is equivalent to about 3 days of background radiation.

Medical imaging investigations should be ordered judiciously, based on a sound clinical history and examination. The benefits of the imaging must significantly outweigh the risks, particularly concerning radiation exposure.