Sectional anatomy for imaging professionals, 4th edition

Chapter 1. Introduction to Sectional

Anatomy

Sectional anatomy has had a long history. Beginning as early as the 16th century, the great anatomist and artist Leonardo da Vinci was among the first to represent the body in anatomic sections. In the following centuries, numerous anatomists continued to provide illustrations of various body structures in sectional planes to gain greater understanding of the topographical relationships of the organs. The ability to see inside the body for medical purposes has been around since 1895, when Wilhelm Conrad Roentgen discovered x-rays. Since that time, medical imaging has evolved from the two-dimensional (2D) image of the first x-ray to the 2D cross-sectional images of computed tomography (CT) and magnetic resonance imaging (MRI), then to the three-dimensional (3D) imaging techniques used today. These changes warrant the need for medical professionals to understand and identify human anatomy in both 2D and 3D images.

Sectional anatomy emphasizes the physical relationship between internal structures. Prior knowledge of anatomy from drawings or radiographs may assist in understanding the location of specific structures on a sectional image. For example, it may be difficult to recognize all the internal anatomy of the pelvis in cross-section, but by identifying the femoral head on the image, it will be easier to recognize soft tissue structures adjacent to the hip (Fig. 1.1).

OBJECTIVES

 Define the four anatomic planes.

 Describe the relative position of specific structures within the body using directional and regional terminology.

 Identify commonly used external landmarks.

 Identify the location of commonly used internal landmarks.

 Describe the dorsal and ventral cavities of the body.

 List the structures located within the four abdominal quadrants.

 List the nine regions of the abdomen.

 Describe the gray scale used in CT and MR imaging.

 Describe MPR, CPR, SSD, MIP, and VR.

 Differentiate between 2D and 3D images.

ANATOMIC POSITIONS AND PLANES

For our purposes, sectional anatomy encompasses all the variations of viewing anatomy taken from an arbitrary angle through the body while in anatomic position.

In anatomic position, the body is standing erect, with the face and toes pointing forward and the arms at the side, with the palms facing forward. Sectional images are commonly acquired and displayed according to one of the four fundamental anatomic planes that pass through the body (Fig. 1.2). The four anatomic planes are defined as follows:

1. Sagittal plane: a vertical plane that passes through the body, dividing it into right and left portions

2. Coronal plane: a vertical plane that passes through the body, dividing it into anterior (ventral) and posterior (dorsal) portions

3. Axial (transverse) plane: a horizontal plane that passes through the body, dividing it into superior and inferior portions

4. Oblique plane: a plane that passes diagonally between the axes of two other planes

Medical images of sectional anatomy are, by convention, displayed in a specific orientation. Images are viewed with the right side of the image corresponding to the viewer’s left side (Fig. 1.3).

TERMINOLOGY AND LANDMARKS

Directional and regional terminology is used to help describe the relative positions of specific structures within the body. Directional terms are defined in Table 1.1, and regional terms are defined in Table 1.2 and demonstrated in Fig. 1.4.

TABLE 1.1 Directional Terminology

Direction

Definition

Superior

Above; at a higher level

Inferior

Below; at a lower level

Anterior/ventral

Toward the front or anterior surface of the body

Posterior/dorsal

Toward the back or posterior surface of the body

Medial

Toward the midsagittal plane

Lateral

Away from the midsagittal plane

Proximal

Toward a reference point or source within the body

Distal

Away from a reference point or source within the body

Superficial

Near the body surface

Deep

Farther into the body and away from the body surface

Cranial/cephalic

Toward the head

Caudal

Toward the feet

Rostral

Toward the nose

Ipsilateral

On the same side

Contralateral

On the opposite side

Thenar

The fleshy part of the hand at the base of the thumb

Volar

Pertaining to the palm of the hand or flexor surface of the wrist or the sole of the foot

Palmar

The front or palm of the hand

Plantar

The sole of the foot

TABLE 1.2 Regional Terminology

Direction

Definition

Abdominal

Abdomen

Antebrachial

Forearm

Antecubital

Front of elbow

Axillary

Armpit

Brachial

Upper arm

Buccal

Cheek

Carpal

Wrist

Cephalic

Head

Cervical

Neck

Costal

Ribs

Crural

Leg

Cubital

Posterior surface of elbow area of the arm

Cutaneous

Skin

Femoral

Thigh, upper portion of leg

Flank

Side of trunk adjoining the lumbar region

Frontal

Forehead

Gluteal

Buttock

Inguinal

Groin

Lumbar

Lower back between the ribs and hips

Occipital

Back of the head

Ophthalmic

Eye

Otic

Ear

Pectoral/mammary

Upper chest or breast

Pedal

Foot

Pelvic

Pelvis

Perineal

Perineum

Plantar

Sole of foot

Popliteal

Back of knee

Sacral

Sacrum

Sternal

Sternum

Sural

Calf

Tarsal

Ankle

Thoracic

Chest

Umbilical

Navel

Vertebral

Spine

External Landmarks

External landmarks of the body are helpful in identifying the location of many internal structures. The commonly used external landmarks are shown in Figs. 1.5 and 1.6.

Internal Landmarks

Internal structures, in particular, vascular structures, can be located by referencing them to other identifiable regions or locations, such as organs or the skeleton (Table 1.3).

BODY CAVITIES

The body consists of two main cavities: the dorsal and ventral cavities. The dorsal cavity is located posteriorly and includes the cranial and spinal cavities. The ventral cavity, the largest body cavity, is subdivided into the thoracic and abdominopelvic cavities. The thoracic cavity is further subdivided into two lateral pleural cavities and a single, centrally located cavity called the mediastinum. The abdominopelvic cavity can be subdivided into the abdominal and pelvic cavities (Fig. 1.7). The structures located in each cavity are listed in Table 1.4.

ABDOMINAL AND PELVIC DIVISIONS

The abdomen is bordered superiorly by the diaphragm and inferiorly by the pelvic inlet. The abdomen can be divided into four quadrants or nine regions. These divisions are useful in identifying the general location of internal organs and provide descriptive terms for the location of pain or injury in a patient’s history.

TABLE 1.3 Internal Landmarks

Landmark

Location

Aortic arch

2.5 cm below jugular notch

Aortic bifurcation

L4-L5

Carina

T4-T5, sternal angle

Carotid bifurcation

Upper border of thyroid cartilage

Celiac trunk

4 cm above transpyloric plane

Circle of Willis

Suprasellar cistern

Common iliac vein bifurcation

Upper margin of sacroiliac joint

Conus medullaris

T12-L1, L2

Heart—apex

5th intercostal space, left midclavicular line

Heart—base

Level of 2nd and 3rd costal cartilages behind sternum

Inferior mesenteric artery

4 cm above bifurcation of abdominal aorta

Inferior vena cava

L5

Portal vein

Posterior to pancreatic neck

Renal arteries

Anterior to L1, inferior to superior mesenteric artery

Superior mesenteric artery

2 cm above transpyloric plane

Thyroid gland

Thyroid cartilage

Vocal cords

Midway between superior and inferior border of thyroid cartilage

TABLE 1.4 Body Cavities

Main Body Cavities

Contents

Dorsal

 

Cranial

• Brain

Spinal

• Spinal cord and vertebra

Ventral

Thoracic

• Mediastinum

• Thymus, heart, great vessels, trachea, esophagus, and pericardium

• Pleural

• Lungs, pleural membranes

Abdominopelvic

• Abdominal

• Peritoneum, liver, gallbladder, pancreas, spleen, stomach, intestines, kidneys, ureters, and blood vessels

• Pelvic

• Rectum, urinary bladder, male

 

and female reproductive system

Quadrants

The midsagittal and transverse planes intersect at the umbilicus to divide the abdomen into four quadrants (Fig. 1.8A):

Right upper quadrant (RUQ)

Right lower quadrant (RLQ)

Left upper quadrant (LUQ)

Left lower quadrant (LLQ)

For a description of the structures located within each quadrant, see Table 1.5.

Regions

The abdomen can be further divided by four planes into nine regions. The two transverse planes are the transpyloric and transtubercular planes. The transpyloric plane is found midway between the xiphisternal joint and the umbilicus, passing through the inferior border of the L1 vertebra. The transtubercular plane passes through the tubercles on the iliac crests, at the level of the L5 vertebral body. The two sagittal planes are the midclavicular lines. Each line runs inferiorly from the midpoint of the clavicle to the midinguinal point (Fig. 1.8B). The nine regions can be organized into three groups:

Superior

 Right hypochondrium

 Epigastrium

 Left hypochondrium Middle

 Right lateral

 Umbilical

 Left lateral Inferior

 Right inguinal

 Hypogastrium

 Left inguinal

TABLE 1.5 Organs Found Within Abdominopelvic Quadrants

Quadrant

Organs

Right upper quadrant (RUQ)

Right lobe of liver, gallbladder, right kidney, portions of stomach, small and large intestines

Left upper quadrant (LUQ)

Left lobe of liver, stomach, tail of the pancreas, left kidney, spleen, portions of large intestines

Right lower quadrant (RLQ)

Cecum, appendix, portions of small intestine, right ureter, right ovary, right spermatic cord

Left lower quadrant (LLQ)

Most of small intestine, portions of large intestine, left ureter, left ovary, left spermatic cord

IMAGE ACQUISITION

The images displayed in this text are acquired from MRI and CT scanners. MRI uses a strong magnetic field in conjunction with nonionizing radiofrequency (RF) energy to acquire images. CT uses ionizing radiation to acquire images. Both modalities are capable of creating 2D and 3D images.

IMAGE DISPLAY

Each digital image can be divided into individual regions called pixels or voxels that are then assigned a numerical value corresponding to a specific tissue property of the structure being imaged (Fig. 1.9). The numerical value of each voxel is assigned a shade of gray for image display. In CT, the numerical value (CT number) is referenced to a Hounsfield unit (HU), which represents the attenuating properties or density of each tissue. Water is used as the reference tissue and is given a value of zero. A CT number greater than zero will represent tissue that is denser than water and will appear in progressively lighter shades of gray to white. Tissues with a negative CT number will appear in progressively darker shades of gray to black (Fig. 1.10). In magnetic resonance (MR), the gray scale represents the specific tissue relaxation properties of T1, T2, and proton density. The gray scale in MR images can vary greatly because of inherent tissue properties and can appear different with each patient and across a series of images (Fig. 1.11).

The appearance of digital images can be altered to include more or fewer shades of gray by adjusting the gray scale, a process called windowing. Windowing is used to optimize visualization of specific tissues or lesions. Window width (WW) is a parameter that allows for the adjustment of the gray scale (number of shades of gray), and window level (WL) basically sets the density of the image or the center of the gray scale (Fig. 1.10).

Fig. 1.10 CT numbers and windowing on axial CT of chest.

MULTIPLANAR REFORMATION AND 3D IMAGING

Several postprocessing techniques can be applied to the original 2D digital data to provide additional 3D information. All current postprocessing techniques depend on creating a digital data stack from the original 2D images, thereby generating a cube of digital information (Fig. 1.12).

Multiplanar Reformation (Reformat) (MPR)

Images reconstructed from data obtained along any projection through the cube result in a sagittal, coronal, axial, or oblique image (see Figs. 1.13 and 1.14).

Curved Planar Reformation (Reformat) (CPR)

Images are reconstructed from data obtained along an arbitrary curved projection through the cube (Fig. 1.15).

3D Imaging

All 3D algorithms use the principle of ray tracing in which imaginary rays are sent out from a camera viewpoint. The data are then rotated on an arbitrary axis, and the imaginary ray is passed through the data in specific increments. Depending on the method of reconstruction, unique information is projected onto the viewing plane (Fig. 1.16).

Shaded Surface Display (SSD).

A ray from the camera’s viewpoint is directed to stop at a particular user-defined threshold value. With this method, every voxel with a value greater than the selected threshold is rendered opaque, creating a surface. That value is then projected onto the viewing screen (Fig. 1.17).

Maximum Intensity Projection (MIP). A ray from the camera’s viewpoint is directed to stop at the voxel with the maximum signal intensity. With this method, only the brightest voxels will be mapped into the final image (Fig. 1.18).

Volume Rendering (VR). The contributions of each voxel are summed along the course of the ray from the camera’s viewpoint. The process is repeated numerous times to determine each pixel value that will be displayed in the final image (Fig. 1.19).

REFERENCES

Curry, R. A., & Tempkin, B. B. (2010). Sonography: Introduction to normal structure and functional anatomy (3rd ed.). St. Louis: Saunders.

Frank, E., & Long, B. (2011). Merrill’s atlas of positioning and radiographic procedures (12th ed.). St. Louis: Mosby.

Seeram, E. (2008). Computed tomography; physical principles, clinical applications, and quality control (3rd ed.). Philadelphia: Saunders.


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