2 Clinical Imaging Basics Introduction

Imaging is the primary diagnostic method used by physicians in nearly all medical specialties. All physicians should have a basic understanding of radiology concepts and how imaging can be optimally utilized in the care of their patients. The clinical imag-ing basics covered in this text are meant to be a brief introduction to the extraordinary diagnostic and therapeutic capabilities of radiology. Although imaging can be quite daunting to the first year medical student who has yet to master anatomy, you are encour-aged to become familiar with the basics outlined here and in other introductory works. It is upon these basic fundamentals that you will build your foundation to better understand anatomy and physiology, and also to achieve the ultimate goal of using imaging to take the best care of your patients, regardless of which career path you choose.

The four most commonly used imaging modalities are

1.Radiographs (X-rays)

2.CT (computed tomography)

3.MRI (magnetic resonance imaging)

4.Ultrasound

X-Rays

—X-rays are a form of electromagnetic energy and are ionizing radiation. An x-ray tube generates energy in the form of photons, which are directed toward the patient. An electronic detector is positioned behind the patient, which detects the photons (x-ray energy) that pass through the patient to produce an image (Fig. 2.1). The degree to which that energy is able to pass through different tissues of the body produces the light, dark, and intermediate grays that are seen on the image. Tissues are rendered as

•white when the energy is blocked (metal)

•black when the energy is not blocked (air)

•gray when the energy is partially blocked (soft tissues)

—The amount of x-ray energy passing through the body depends on the type of tissue encountered by the x-ray beam and the thickness of the tissues. Based on these principles, five densities can be identified on a radiograph

(Table 2.1).

Fig. 2.1 Example of a simplified schematic of x-ray machine.

(From Gunderman R. Essential Radiology, 3rd ed. New York: Thieme; 2014.)

Table 2.1 Radiographic Densities

—Additional descriptive terms are “opacity,” “density,” and “shadow,” which indicate a whiter area on the x-ray; “lucency” indicates a blacker area.

—The image created is a summation of shadows based on the type and thickness of tissues the x-ray photons encounter (Fig. 2.2). This summation of shadows creates a two-dimensional representation of the threedimensional structure of the body. Because it is frequently difficult to determine the depth of a structure in a two-dimensional image, an additional orthogonal (right angle) projection may be required to mentally “construct” a three-dimensional visualization (Fig. 2.3).

—The position of the patient and direction of the x-ray beam are manipulated to optimize image quality and produce different physical and physiologic effects such as fluid moving to the most inferior (dependent) part of the patient due to gravity. The resulting image is described in terms of both patient position and x-ray beam direction, for example, upright frontal view

(orthogonal to the frontal view) could confirm this. Using the frontal and lateral view can help triangulate the position of the objects and visualize the abdomen in three dimensions. (From Gunderman R. Essential Radiology, 3rd ed. New York: Thieme; 2014.)

•Prone

•Decubitus (lying on the side)

•Oblique

Standard direction of x-ray beams include

•Frontal (either PA or AP.) Note that the distinction between Posterior Anterior (PA) or Anterior Posterior (AP) x-ray beams is beyond the scope of this text, and we will refer to them collectively as a frontal view.

•Lateral

—Radiographs are oriented in a standard fashion for viewing.

•Frontal images are viewed with the patient in the anatomic position (always facing the observer). The patient’s right side is seen on the observer’s left and vice versa, and each side is labeled as R or L.

•The lateral projection can be viewed with the patient facing either right or left, but you should be consistent with your viewing patterns. The lateral view is best used in conjunction with the frontal view to convey the 3-dimensional nature of the structure. The lateral view is also helpful in seeing “hidden” areas on the frontal view such as behind the sternum and heart.

—When evaluating radiographs, a systematic approach is important and should include a checklist of major anatomic structures, which are discussed in the appropriate units.

CT (Computed Tomography)

—CT scans are created by rotating an x-ray beam around the patient. The computer reconstructs these data into image sets of consecutive sections or “slices” (think of a loaf of bread cut into slices). (Fig. 2.5; see also Fig. 2.7). Because the CT scan is made up of a large number of individual x-rays, the radiation dose is considerably higher. For this reason CT scanning is used judiciously.

—The images produced are based on the computer calculation of the x-ray attenuation of each pixel on each slice and expressed in Hounsfield units (Hounsfield was one of the discoverers of CT). Water is arbitrarily set to 0 Hounsfield units; more dense structures are more white (bone), and less dense structures are darker (air). However, CT can discrimi-nate more subtle shades of gray, black, and white than radiographs, which improves soft

tissue detail. For example, CT can distinguish between fluid and organs and between blood and other types of fluids. When IV contrast is used, soft tissue detail/contrast is improved significantly.

—Individual CT slices are oriented in the transverse (axial) plane and, by convention, are viewed from the inferior perspective (looking from the feet toward the head) (Fig. 2.6). However, CT scanners can also generate a volume of data that can be viewed in any plane, including three dimensions (3D reconstruction). Additionally, individual images can be “windowed,” which involves changing the brightness or the contrast, to optimize the appearance of structures with specific densities, such as a bone or soft tissue “window.”

Fig. 2.4 Positioning of patient and direction of x-ray beam..

(From Gilroy AM, MacPherson BR, Wikenheiser JC. Atlas of Anatomy. Illustrations by Voll M and Wesker K. 2nd Edition. New York: Thieme Publishers; 2012.)

Fig. 2.5 How computed tomography (CT) scanners work..

The x-ray tube rotates continuously around the patient, as the patient, lying on the table, slides through the machine. The curved x-ray detector is opposite to the x-ray tube and registers the amount of x-ray energy that passes through the body. Taking into account the tube position and patient position at each time point of measurement, the computer constructs a data matrix and ultimately a set of images. (From Eastman G, et al. Getting Started in Radiology. Stuttgart: Thieme; 2005.)

MRI (Magnetic Resonance Imaging)

—A magnetic resonance image is produced by the interactions of a strong magnetic field on the protons within cells and a radiofrequency energy pulse that disturbs the protons. These spinning protons produce a “signal” that is picked up by a receiver and converted via computerized mathematical manipulation into images.

—MRIs are especially useful because they lack ionizing radiation, produce superb soft tissue contrast, and can be oriented in any plane (Fig. 2.7). The soft tissue contrast of MRI is far superior to other imaging modalities and is a key feature that makes MRI so powerful. Disadvantages include the high cost, long study acquisition time, and small confined space that patients must tolerate. MRI machines look similar to CT machines, but the tube that the patient slides into is smaller in diameter and longer.

—MRI images are traditionally viewed in the axial, sagittal, and coronal planes, but these can be skewed to optimize “out of plane” structures such as the heart. As in CT, axial images are viewed as if the patient is supine and the observer is looking superiorly from the feet (i.e., inferior view).

—An MRI exam consists of multiple sequences with each sequence highlighting different types of tissue. The same tissues can look different on different sequences. The two basic MRI sequences are T1 and T2 (T represents time constant).

•T1 sequence: fluid appears dark (black).

•T2 sequence: fluid appears bright (white).

•Cortical bone usually appears black on all sequences.

Fig. 2.6 Example of single axial slice from a normal abdominal CT..

This image is shown in soft tissue windows to highlight the solid organs and blood vessels. (From Moeller TB, Reif E. Pocket Atlas of Sectional Anatomy, Vol 2, 3rd ed. New York: Thieme; 2007.)

Ultrasound

—Ultrasound images are created by using a transducer, which emits highfrequency sound waves that penetrate the body. It then “listens” for the return echo, similar to the way sonar operates in a submarine (Fig. 2.8). Because tissues of different densities attenuate the sound wave to different degrees, the returning echo produces an image of varying shades of gray

(Fig. 2.9).

—Ultrasound travels most easily through water, which appears black on the image, but it travels poorly through air and bone, which block the sound energy and appear white on the image. Common terms used to describe ultrasound images include hypoechoic, meaning blacker, and hyperechoic or echogenic, meaning gray or white.

—Ultrasound is relatively inexpensive and is radiation free, so it is used whenever possible and is the primary imaging modality in pediatrics, obstetrics, and when imaging both male and female gonads.

—Ultrasound images are displayed as a single slice, usually as a longitudinal or transverse section through the target organ, but also in any plane necessary to identify certain features or pathology.

—Note that the images are oriented to the organ itself, not the whole body.

—Ultrasound can also make use of the Doppler effect to identify and

characterize motion. In medical imaging, Doppler is primarily used to characterize flow in blood vessels and to assess heart physiology. Color Doppler encodes direction and velocity into colors. By convention, blood flowing toward the ultrasound transducer is red and blood flowing away from the transducer is blue (regardless of vessel type). Spectral Doppler gives a graphical representation of the speed of flow with respect to time and thus can demonstrate characteristics of arterial vs. venous flow and identify abnormal flow patterns.

Fig. 2.7 Example of the improved soft tissue contrast and sensitivity of magnetic resonance imaging (MRI)..

(From Gunderman R. Essential Radiology, 2nd ed. New York: Thieme; 2000)

The probe was positioned on the abdomen such that the kidney could be seen oriented longitudinally. The image is through the mid-kidney. Ultrasound differentiates the renal pyramids, which are blacker because of higher relative water content. The renal sinus fat is shown to be whiter. Note the liver tissue anterior and superior to the kidney. (From Gunderman R. Essential Radiology, 3rd ed. New York: Thieme; 2014.)

Unit I Review Questions: Introduction

1.Which of the following is associated with membranous bone formation?

A.Primary ossification center

B.Epiphyses

C.Direct ossification of mesenchymal templates

D.Long bones of the limbs

E.Diaphysis

2.A portal system is associated with

A.venous shunts that divert blood toward the heart

B.the pulmonary circulation

C.arterial-venous anastomoses

D.capillary beds in the liver

E.lymphatic capillaries

3.In the anatomic position the

A.eyes are directed forward

B.palms are directed forward

C.body is erect with arms at the sides

D.feet are directed forward

E.All of the above

4.Which of the following is true of splanchnic nerves?

A.They synapse in ganglia near their target organ.

B.They innervate viscera of the abdomen.

C.They may carry sympathetic fibers.

D.They may carry parasympathetic fibers.

E.All of the above

5.A 43-year-old high school teacher complained to her physician of

abdominal bloating and pelvic pain. Radio-graphic studies showed a large tumor involving her right ovary. Although the patient was scheduled for surgery to remove the tumor, the physician was concerned about the spread of the cancer along lymphatic channels. What is the lymphatic drainage pattern of pelvic viscera?

A.Ipsilateral drainage to the right and left lymphatic ducts

B.Contralateral drainage to the right and left lymphatic ducts

C.Bilateral drainage to the right and left lymphatic ducts

D.All pelvic viscera drain to the right lymphatic duct.

E.All pelvic viscera drain to the left lymphatic duct.

6.As an inexperienced medical student, you volunteer at an outpatient

clinic where you’re asked to assist in adminis-tering vaccines to its large homeless population. As you deliver these percutaneous (through the skin) injections, you recall your knowledge of subcutaneous connective tissue and understand that deep to the skin the needle will first pass through

A.a fatty layer of regular connective tissue

B.a fatty layer of areolar and adipose connective tissue

C.a membranous layer of dense connective tissue

D.two layers of dense connective tissue

E.a layer of fascia that is devoid of all superficial vessels and nerves

7.As a junior orthopedic resident, you are tasked with delivering a lecture

to first-year medical students on the anatomy of joints. Which of the following statements might you include as a take-away point on your final summary slide?

A.Most synovial joints are stabilized by intrinsic ligaments and intraarticular structures such as menisci.

B.Synchondroses are defined as temporary cartilaginous joints that subsequently form synostoses, sites of bony fusion.

C.Synovial joints typically contain a joint space lined by a synovial membrane.

D.The complementary articulating surfaces of bones within a joint

provide the greatest stability.

E.Syndesmoses are fibrous joint

8.Which of the following pertains to visceral muscles?

A.The most prevalent type of muscle in the body

B.Tendons connect them to bony attachments

C.Tendon sheaths facilitate movement across joints

D.They include a striated type of muscle found in the heart

E.Aponeuroses attach them to other muscles or organs

9.After experiencing sudden unexplained weight loss and severe

abdominal pain, your uncle sought advice from his primary physician. A CT scan revealed a 3-cm pancreatic tumor in the center of his abdomen. The physician explained that the pain resulted from pressure of the tumor on nearby nerves and ganglia associated with his sympathetic nervous system. The nerves transmitting the pain contain which type of fibers?

A.Special visceral sensory

B.Special somatic sensory

C.Visceral sensory

D.Somatic sensory

E.Special visceral motor

10.Which of the following imaging methods do not use ionizing radiation as its energy source?

A.Ultrasound

B.Radiographs

C.CT scan

D.MRI

11.Which of the following structures would be the darkest as seen on an x-ray?

A.Liver

B.Spleen

C.Heart

D.Gas-filled bowel loop

E.Fluid-filled bowel loop

Answers and Explanations

1.C. In the process of membranous ossification, embryonic mesenchymal

templates are replaced by bone (Section 1.6).

A.A primary ossification center, usually in the shaft of the long bones, is the site at which endochondral ossifica-tion begins.

B.Epiphyses, located on either end of the long bones, are the secondary ossification centers for endochondral ossification.

D.Long bones of the limbs undergo endochondral bone formation, in which a cartilaginous template forms from the embryonic mesenchyme before being replaced by bone.

E.A diaphysis is the shaft of a long bone, which undergoes endochondral ossification.

2.D. The body’s largest portal system diverts blood from capillary beds in

the gastrointestinal tract to secondary capillary beds in the liver before returning it to systemic veins (Section 1.8).

A.A portal system diverts venous blood from one capillary network to another, instead of allowing it to flow directly into systemic veins toward the heart.

B.A portal system is a venous system within the systemic (general body) circulation, but not within the pulmonary (lung) circulation.

C.A portal system diverts blood from one capillary network to another, unlike arterial-venous anastomoses, which divert blood away from capillary beds.

E.Lymphatic capillaries are restricted to the lymphatic system and are not involved with portal venous systems.

3.E. The anatomic position is the standard position of the body used in

medical references. The body is upright, facing the observer, with arms at the side and the head, eyes, palms, and feet directed forward (Section 1.1).

A.Eyes are directed forward, and other positions are correct as well (E).

B.Palms are directed forward, and other positions are correct as well

(E).

C.Body is erect with arms at side, and other positions are correct as well (E).

D.Feet are directed forward, and other positions are correct as well (E).

4.E. All of the above (Section 1.10).

A.Splanchnic nerves synapse near their target organ either in prevertebral ganglia or in small ganglia on the viscera. B through D are correct also (E).

B.Splanchnic nerves are autonomic nerves that inner-vate viscera in the thorax, abdomen, and pelvis. A, C, and D are correct also (E).

C.Sympathetic fibers arise from the T1–L2 spinal cord to form thoracic, lumbar, and sacral splanchnic nerves. A, B, and D are correct also (E).

D.Parasympathetic fibers arise from the S2–S4 spinal cord and form pelvic splanchnic nerves. A through C are correct also (E).

5.E. The left lymphatic duct receives lymph from the entire body below

the diaphragm, as well as from the left side of the thorax, head, and neck and the left upper limb. The right lymphatic duct receives lymph only from the right side of the thorax, head, and neck and the right upper limb (Section 1.9).

A.Viscera from the right and left sides of the pelvis drain to the left lymphatic duct.

B.All pelvic viscera drain to the left lymphatic duct.

C.All pelvic viscera drain to the left lymphatic duct.

D.Only the right upper quadrant of the body drains to the right lymphatic duct.

6.B. The needle would first pass through the most superficial layer of the

subcutaneous connective tissue, which is composed of areolar and adipose tissues (Section 1.4).

A. Both areolar and adipose connective tissues, which make up the fatty layer, are irregular types of connective tissue.

C. The needle would pierce the membranous layer after first passing

through the more superficial fatty layer.

D.Subcutaneous connective tissue is composed of a superficial fatty layer and deeper membranous layer.

E.The fatty layer is traversed by the superficial vessels and nerves.

7.C. Synovial joints typically have a joint space enclosed by a fibrous

capsule lined by a synovial membrane (Section 1.6).

A.Most synovial joints are stabilized by extrinsic ligaments, although some also have intrinsic ligaments. Intra-articular structures are found in only certain joints, like the knee or shoulder.

B.While some synchondroses subsequently become fully fused (synostoses) many remain as cartilaginous joints.

D.Extrinsic ligaments are the primary stabilizers of most joints.

E.Syndesmoses are also found in the adult skull as well in the interosseous membranes that join bones of the forearm and leg.

8.D. The heart is made of cardiac muscle, one of two types of visceral

muscles (Section 1.8).

A.Somatic, or skeletal muscle, is the most prevalent type of muscle.

B.Visceral muscles do not have tendons and are not attached to bones.

C.Tendon sheaths surround the tendons of somatic muscles to facilitate movement as they cross joints.

E.Aponeuroses are tendons that form flat sheets and are associated with somatic muscles.

9.C. Visceral sensory fibers transmit sensation from internal organs

(Section 1.10).

A.Special visceral sensory fibers transmit only taste from the tongue and smell from the olfactory mucosa.

B.Special somatic sensory fibers transmit information only from the retina of the eye and auditory and vestibular apparatus of the ear.

D.Somatic sensory fibers conduct information from somatic structures, the skin, and skeletal muscles.

E.Special visceral motor fibers innervate only specific muscles that are

derived from the branchial arches.

10.A and D. Ultrasound utilizes acoustic energy (high-frequency sound

waves), and MRI uses radiofrequency energy within a high-strength magnetic field (Chapter 2).

B.Radiographs (or x-rays) utilize ionizing radiation in the form of highenergy electromagnetic waves.

C.CT scans utilize the same type of electromagnetic energy as x-rays.

11.D. The gas (air) in a gas-filled bowel loop does not attenuate the X-ray

energy as much as soft tissues, so therefore will be rendered as darker gray (Chapter 2).

A, B, C, and E. The liver, spleen, heart, and fluid-filled bowel would all be similar densities on an x-ray. These are all “soft tissue densities,” which cannot be reliably differentiated by conventional radiography. Remember that CT has much higher soft tissue contrast than regular x-rays.

Unit II Back

3Back

3.1The Vertebral Column

3.1Developmental Correlation: Spinal Development

3.2Clinical Correlation: Abnormal Curvatures of the Vertebral Column: Kyphosis, Lordosis, and Scoliosis

3.3Clinical Correlation: Osteoporosis

3.4Clinical Correlation: Spondylolysis and Spondylolisthesis

3.5Clinical Correlation: Injuries of the Cervical Spine

3.6Clinical Correlation: Herniation of Intervertebral Disks

3.7Clinical Correlation: Age-Related Changes in Vertebrae

3.8Clinical Correlation: Metastasis and the Vertebral Venous Plexus

3.1Joints of the Vertebral Column

3.2The Spinal Cord

3.9Clinical Correlation: Lumbar Puncture, Spinal Anesthesia, and Epidural Anesthesia

3.10Developmental Correlation: Developmental Changes of the Spinal Cord, Dural Sac, and Vertebral Column

3.3Spinal Nerves

3.11Clinical Correlation: Referred Pain

3.2Effects of the Sympathetic and Parasympathetic Nervous Systems

3.4Muscles of the Back and Suboccipital Region

3.3Muscles of the Back and Suboccipital Region

4Clinical Imaging Basics of the Spine

4.1Suitability of Imaging Modalities for the Back and Spine

Review Questions: Back