Introduction

Anatomic ImagingAnatomic imaging began at the turn of the

century with Roentgen's discovery of x-rays in1896. Since that time, medical imaging hasevolved into computerized systems that allow sci-entist and physicians to look inside the body withamazing accuracy and without the trauma and riskof explanatory surgery. Many of the advances inmedical diagnosis and treatment are only possiblebecause of anatomic imaging. A list of modernimaging techniques includes traditional x-rays,computer tomography (CT), magnetic resonanceimaging (MRI), dynamic spatial reconstruction(DSR), digital subtraction angiography (DSA),positron emission tomography (PET), opticalcoherent tomography (OCT), echo-planar MRI(EPI), and ultrasound.

Typically, the views of the body produced bythese technologies are presented as transverse,sagittal, or frontal sections. Sagittal and frontalsections are usually less difficult to understandbecause most anatomy textbooks primarily usesagittal and frontal views to illustrate anatomy.However, most CT and MRI images are present-ed as transverse sections, which is why under-

standing cross-sectional anatomy is important.This miniatlas is designed to help students acquirea basic knowledge of cross-sectional anatomythat will not only be useful, but will also maketheir careers more interesting and meaningful.However, before explaining how to use the mini-atlas, some background information on CT andMRI is appropriate.

Computerized TomographyComputerized tomography (CT) l i t e r a l l y

means to cut or section (Gr. tomos) the body anduse the computer to write (Gr. grapho) a pictureof the section. In computerized tomography, thepatient is placed into a scanner and is exposed toa narrowly focused x-ray beam (Figure 1).Radiation that is not absorbed by the patient'sbody is called remnant radiation. The remnantradiation passes through the patient's body and ismeasured by a series of detectors that send thedata to a computer. The data is converted intopicture elements called pixels (picture x ele-ments), which are the tiny boxes of varying colorsthat make up a digital picture. In a process calledimage reconstruction, the computer combines

FIGURE 1 CT SCANNER The patient lies on the cot, which automatically moves through the gantry opening.The circular gantry can produce x-rays that pass through the patient's body from every angle to be detected bysensors within the gantry.

the pixels to form an image. Each image is like asingle slice from a loaf of bread. After eachimage is made, the patient is moved slightly andanother image is taken, producing additionalslices through the body.

On the average, it takes over 262,000 pixels toform a single CT image, which is like a complexmosaic formed by arranging and connectingtogether small pieces of glass or stone. The com-puter can arrange the pixels to form transverse,sagittal, or frontal images of the body.

The amount of radiation absorbed by differenttissues determines the appearance of the tissues inCT images. The denser the tissue, the greater isthe amount of radiation absorbed by the tissue.C o n s e q u e n t l y, less remnant radiation passesthrough denser tissues than through less dense tis-sues. The computer uses the amount of remnantradiation that is detected by the sensors as a basisfor coloring the tissues. Dense tissues, such as

bone, appear white. Air, which is not dense com-pared to tissues, is easily penetrated by x-rays andappears black. Thus, air-filled structures, such asthe lungs, appear black. Fat, muscle, and organsthat fall between the densities of bone and airappear as varying shades of gray (Table 1).

In order to better see certain hollow organs,such as blood vessels, a contrast medium can beplaced in the organ. A contrast medium is adense liquid that prevents the passage of remnantradiation and appears white in a CT image (seeTable 1). The size and shape of hollow organs canbe determined in CT images because a liquid con-trast medium assumes the shape of the organs it iscontained within. For example, iodinated con-trast medium is injected into blood vessels, theurinary bladder, or ureters, and barium sulfate isgiven orally or rectally to enhance the gastroin-testinal tract.

The newest generation of CT scanners are

Tissue CT Image T1 MRI Image T2 MRI Images

Bone White Black Black

Muscle Gray Gray Dark gray

BrainWhite matter Dark gray Gray Dark grayGray matter Gray Dark gray Gray

Fat Dark gray White Grayto black

Cerebrospinal fluid (CSF) Black Black White

Blood

Flowing White* Black Black

Nonflowing White* Gray White

Contrast media White White Not used

Air-filled structures such as Black Black Blackthe lungs, stomach, and colon

*Only if iodinated contrast media is present; otherwise appears gray.

TABLE 1 APPEARANCE OF DIFFERENT TISSUES IN CT AND MRI IMAGES

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able to produce an image so fast that physiologi-cal motions, such as respiratory or cardiac move-ments, do not blur the image. For this reason, CTis used for studies of the chest, abdomen, pelvis,and soft tissues of the neck.

Magnetic Resonance ImagingTo make a magnetic resonance image

(MRI), the patient is placed into a scanner thatproduces a very powerful magnetic field, whichaffects the nuclei of the atoms in the patient'sbody. Many atomic nuclei, including the protonsthat form the nuclei of hydrogen atoms, rotate orspin about, much like tiny tops or gyroscopes.Because a moving charge creates a magnetic fieldaround itself, the hydrogen protons are like littlebar magnets. Normally, the "axes" of the hydro-gen proton "magnets" are oriented in many differ-ent directions. The magnetic field produced bythe scanner causes the hydrogen protons in thepatient's body to align in the same direction, inmuch the same way that the needle of a compassaligns with the magnetic north pole.

After the protons are aligned, a radiofre-quency (RF) pulse is used to establish a secondmagnetic field that is typically at a ninety degreeangle to the first magnetic field. The second mag-netic field causes the hydrogen protons to "tipover" in relation to the first magnetic field. Theenergy in the RF pulse also raises the energy levelof the hydrogen protons. When the RF signal isturned off, the hydrogen protons are said to relax,which means they realign with the first magneticfield. As they relax, the hydrogen protons releasethe energy acquired from the RF pulse and returnto their previous lower energy level by giving offenergy that can be detected by sensors in the scan-ner. As with CT technology, the data collected bythe sensors is used to form pixels, and throughimage reconstruction, the computer can formtransverse, sagittal, or frontal sections of thepatient's body.

The process just described is called magneticresonance imaging because it requires magneticfields and the resonance of the hydrogen protons.To use an analogy, each hydrogen proton is like atuning fork. When energy is applied to a tuning

fork by striking the tuning fork, it releases theenergy by producing sound waves. If many tun-ing forks that can produce the same sound wavesare struck at the same time, then they all vibratetogether at the same sound frequency and are saidto be in resonance. When the RF pulse is appliedto the hydrogen protons, they are all excited to thesame higher energy level and are in resonance.

When the RF pulse ends, the hydrogen pro-tons release an energy signal as they realign withthe first magnetic field. The time required for theexcited hydrogen protons to realign with the firstmagnetic field is called the relaxation time. Thereturn to the unexcited state exhibits two relax-ation times called T1 and T2. The energy signal

released from the hydrogen protons during thefirst second is T1, and the signal after one second

is T2. The signals received by the computer for

T1 and T2 are used to create different shades of

gray for different types of tissues (see Table 1).Magnetic resonance imaging detects the pres-

ence of hydrogen protons, and not other atoms,because the RF pulse used causes the resonanceof only hydrogen protons. The nuclei of otheratoms have different resonance frequencies, muchlike different tuning forks produced diff e r e n tsounds. The use of hydrogen protons in MRI isnot by accident. Over 75 percent of the humanbody is water and each water molecule containstwo hydrogen protons. In addition, many organicmolecules such as carbohydrates, lipids, and pro-teins contain abundant hydrogen. The number ofhydrogen protons in a tissue, and the relationshipof the hydrogen protons to surrounding atoms,determines the different energy signals recordedfor different tissues when the hydrogen protonsrelax.

General Features of the MiniatlasThe miniatlas consists of 25 photographs of

transverse sections through a cadaver. The num-ber of sections has been kept to the minimum nec-essary to show major anatomical features. Thelevel of each section is indicated in a level orien-tation figure, and a numbered key is used to iden-tify structures on each photograph. A systemic

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description of the features of each cadaver sectionis also provided. In addition, there are CT andMRI pictures that match as closely as possible thecadaver sections. Any structures seen in a CT orMRI, but not seen in a cadaver section, are brieflydescribed. At the end of the atlas there are CT andMRI pictures that illustrate the use of these tech-nologies for detecting pathologies.

How to Use The MiniatlasIn typical undergraduate anatomy courses, the

body is studied from a systemic approach. A sys-tem is a group of structures that have one or morecommon functions. Examples are the digestive,circulatory, nervous, respiratory, and skeletal sys-tems.

Most cross-sectional anatomy atlases assumethat the reader already knows the parts of eachsystem, and understands the three-dimensionalrelationship of the parts within a system to eachother. Therefore, when a section is made throughthe body, the emphasis is on the relationshipbetween the parts of different systems within thatparticular section. However, without an under-standing of the systems, this approach is quiteconfusing to beginning students.

This miniatlas of human cross-sectionalanatomy is designed to be used at the same timethat the systems are being learned. The parts ofeach system are described by system in a SystemicDescriptions section. Thus, it is possible to con-centrate on one system at a time. Furthermore,the parts of a system seen at one level are relatedto the parts seen in sections at other levels. Forexample, starting with a section through the oralcavity, the systemic description section describesthe parts of the digestive system seen at this level,such as the tongue and the parotid salivary glands.In succeeding sections, the systemic descriptionfor the digestive system describes the new struc-tures encountered and their relationship to othersections. Thus, the oral cavity joins the pharynx(throat), which is connected by the esophagus tothe stomach.

In order to effectively present a systemicapproach, more than a verbal description is need-ed. High quality photographs and illustrations of

each system from a sagittal or frontal view arealso required, because they show the superior toinferior relationships between the parts of a sys-tem. To assist with understanding head and neckanatomy, a sagittal and frontal view of the headand neck are presented on pages 6 and 7 of thisintroduction. Additional illustrations and pho-tographs are provided in the third edition ofAnatomy and Physiology by Seeley, Stephens,and Tate (SST).

The recommended procedure for using theminiatlas is to read and understand the anatomy ofa system using an introductory anatomy and phys-iology text. In the systemic descriptions in theminiatlas, the parts of a system are followed bynumbers, which indicate the location of the partson the cadaver section. In addition, it may be use-ful to refer to an introductory anatomy and phys-iology text to see the structures from a frontal orsagittal view. For example, Figure 2 refers to asagittal view of the brain in A n a t o m y a n dPhysiology by Seeley, Stephens, and Tate.

Next, relate what is seen in the frontal orsagittal view to what is seen in the transversecadaver section. The approximate location of thecut on the frontal or sagittal section can be deter-mined by using the level orientation diagram pro-vided for each section and by noting what partsare cut in the cadaver section. For example, theorientation diagram for Section 1 indicates a cutthrough the superior part of the braincase, and thesystemic description for the nervous systemdescribes a cut through the cerebrum, corpus cal-losum, and lateral ventricles. The location of thecut is shown as a black line in Figure 2.

A ruler was used to draw the location of thecut on the illustration. You may want to straight-en out a paper clip and lay it on the illustration atthe location of the cut. Thoughtful considerationof the cross-sections in the miniatlas and the illus-trations in the textbook will result in a three-dimensional understanding of each system.

Although this atlas can be used to study onlyone system at a time, as you study a particularsystem you probably will note some features ofthe other systems. For example, you probablywill already have studied the skeletal system

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before you study the nervous system. Examinethe cross-sections and identify the bones and partsof bones because they are easily seen. Then, asyou study additional systems, use the bones as"landmarks" to guide you through the sections. Inaddition, begin to relate the parts of one system toanother. For example, while studying the diges-tive system you might note that the esophagus isposterior to the trachea and anterior to thedescending aorta.

After you understand a system on the cadaversections, take some time to examine the CT andMRI images. These technologies produce amaz-ingly good pictures of the inside of the body, andwith your new knowledge of cross-sectionalanatomy, you will be able to identify the majorparts of the body. Also examine the CT and MRIsections at the end of the atlas to discover howthey can reveal pathologies within the humanbody.

FIGURE 2 SAGITTAL VIEW OF THE BRAIN Taken frm Anatomy and Physiology by Seeley, Stephens, andTate. The black line indicates the level of Cadaver Section 1.

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1. Atlas2. Axis3. Base of occipital bone4. Body of sphenoid bone5. Cerebellum6. Left cerebral hemisphere7. Cervical vertebrae8. Corpus callosum

9. Posterior ridge of the sellaturcica

10. Falx cerebri11. Foramen magnum12. Fourth ventricle13. Pituitary gland14. Sella turcica15. Hypothalamus

16. Medulla oblongata17. Midbrain18. Nose19. Oral cavity20. Oropharynx21. Pons22. Spinal cord23. Thalamus

FIGURE 3 SAGITTAL SECTION OF THE HEAD Reprinted with permission from England, M.A. and Wakely, J. 1991. Color Atlas of the Brain and Spinal Cord.

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1. Atlas2. Axis3. Caudate nucleus4. Cerebellum5. Dura mater6. Ear7. Falx cerebri8. First cervical nerve root9. Glossopharyngeal and vagus

nerve roots10. Hippocampus (a cerebral

gyrus)11. Internal capsule

12. Internal jugular vein13. Lateral geniculate body14. Lateral ventricle15. Medial geniculate body16. MIddle cerebellar peduncle17. Posterior neck muscles18. Parietal lobe19. Petrous part of temporal bone20. Pons21. Posterior cranial fossa22. Sigmoid sinus23. Spinal cord24. Accessory nerve root

25. Neck muscle26. Superior sagittal sinus27. Midbrain28. Pons29. Temporal lobe30. Tentorium cerebelli31. Thalamus32. Third cervical vertebrae33. Third ventricle34. Vertebral artery35. Vestibulocochlear nerve

FIGURE 4 FRONTAL SECTION OF THE HEAD AND NECK Posterior view showing the cerebrum and cere-bellum surrounding the brainstem (midbrain, pons, and medulla oblongata). Reprinted with permission fromEngland, M.A. and Wakely, J. 1991. Color Atlas of the Brain and Spinal Cord.

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