Ординатура / Офтальмология / Английские материалы / Clinical Medicine in Optometric Practice_Muchnick_2007

58 DIAGNOSTIC PROCEDURES

used to communicate with the patient. In addition, music can help reduce patient anxiety.

Positron Emission Tomography

In positron emission tomography (PET) analysis, radioactive isotopes are used that emit gamma rays from within a designated area of the body. Once the radioactive isotopes have migrated to the tissue in question, gamma rays are emitted from this tissue and are received by a set of counters set up in a ring around the patient.

The radioactive isotope that is administered to the patient has a short half-life and is bound to an appropriate chemical. Isotopes chosen for tagging are those that remain in the organ to be studied long enough to produce a usable image, but with relatively short half-lives to minimize radiation to the patient’s tissues. The chemical is injected and migrates to the tissue to be studied. This process occurs because the selected chemical substance to which the isotope has been attached is involved in the physiologic metabolism of that organ. It should remain there long enough to be imaged.

An image is obtained because the radioactive isotope emits gamma rays for a brief period of time. The patient is passed through a ring of gamma-ray counters that record the gamma emissions. A few hours or days later, the isotope stops emitting detectable rays when it returns to its stable state. The return to stability is measured in terms of its half-life, that is, the period until it is seen to be emitting half as much radiation as it did initially.

PET yields an image of organ function. Such biochemical activity in vivo includes glucose metabolism, regional blood flow evaluation, heart studies, and dopamine receptor sites in the brain. The results of PET are displayed as real-time color-coded moving images. Computers can be used to process PET images to display three-dimensional images.

Isotope Scan

Technetium has proved to be the most useful radioactive tracer, and it is linked to various physiologic substances that seek different organs. This substance is deposited temporarily in bone and is called a “bone-seeker.” The image obtained shows areas of more or less intensity of radiation related to the portion of the bone having increased turnover. “Hot spots” showing markedly increased activity of bone are thus seen as dense black areas on a gamma camera or rectilinear scan of the whole skeleton (Figures 5-9 and 5-10).

Unfortunately, this scan is very nonspecific and does not tell us the cause of the increased bone turn-

FIGURE 5-9 ■ Technetium bone scan. This is a radioactive tracer that can show “hot spots,” which are usually indicators of metastases. The bone scan here is normal.

FIGURE 5-10 ■ Technetium bone scan. Dark spots on the scan show areas of uptake subsequently diagnosed as metastases.

over. If located in symmetric joint areas, for example, the increased bone turnover may be caused by acute inflammation secondary to arthritis. If located eccentrically, these spots may indicate the location of the bone metastases from the patient’s known or suspected cancer. This scan is not definitive, therefore, but can be helpful with physical findings to aid in diagnosis.

Technetium may also be linked to a sulfur colloid that is normally picked up by the liver and remains there long enough to be imaged as a densely homogeneous, liver-shaped area of activity on the isotope scan of the abdomen. With any such radioactive pharmaceutical used, areas on the scan may exist where “cold spots” indicate physiologic uptake. These areas also are nonspecific in that they indicate only an area of less metabolic turnover. A large solitary cold spot in an otherwise homogeneously imaged liver thus might indicate the location of a large tumor metastasis or a benign cyst. An ordinary radiograph of a patient’s abdomen might show his liver to be enlarged but would not differentiate the tumor-mass areas from the normal, metabolically active liver tissue around them. Isotope scans, like all other radiological images, must always be interpreted in tandem with clinical information about the patient.

In the typical isotope scan, the image obtained is produced by gamma radiation from the entire thickness of the organ, not from the single slice as in CT, MRI, and sonography. Like fluoroscopy, plain radiography consists of continuous or intermittent observation of tissues penetrated by x-rays. This produces dynamic radiographic information. The motion of the fetal heart is routinely monitored by “realtime” sonography and is evidence that a quiet fetus is in fact alive. Dynamic studies using rapidly sequenced CT scans during the intravenous injection of contrast material produce time-lapse information about the vascularity of a liver mass. Similarly, sequential isotope scans are used to document flow patterns such as blood flow through the heart chambers in a patient suspected of having a congenital heart anomaly.

Ultrasonography

In sonography sound waves are sent into the body and the received echo waves are processed into images. By directing these narrow beams of high-energy sound waves and then recording the manner in which the sound is reflected from organs and internal structures, ultrasonography yields an image of a slice of the body. Ultrasound does not produce an image that is as sharp and clear as CT, but it has four singular advantages: it does not produce ionizing radiation and thus pro-

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duces no biological injury; it can be used at any orientation required by the anatomic region being investigated; it is far less expensive than CT or MRI; and, it can be performed at the bedside of very sick patients.

The ultrasound probe both transmits and receives echoes. The sound produced is above human hearing (20-20,000 Hz). Typical frequencies produced for diagnostic medical purposes are between 2 and 10 MHz (million hertz). Higher frequencies will produce better detail resolution but less depth penetration. For ophthalmic scanning, as in the technique of biomicroscopic ultrasonography, only a few centimeters of penetration are required; therefore, higher frequencies (7 to 10 MHz) can be used to provide optimal structural detail.

Various types of diagnostic ultrasonography are available, including A-mode, B-mode, duplex Doppler, and color Doppler imaging (CDI).

A-mode, or amplitude mode, is a single point of sound that has only one dimension. The echoes are received and interpreted as spikes on the imaging screen. Each spike, displayed on an oscilloscope screen, represents a different tissue and the distance between spikes yields significant data concerning the separation in depth of internal tissues. The use of A-scan has been largely subjugated to eye-care, where it is used primarily to measure the orbital structures and the axial length.

B-mode, or brightness mode, is the most common form of diagnostic ultrasonography in use today. This two-dimensional form of image production is used for evaluating anatomic structures within the body (Figure 5-11). A computer allows the formation of two-dimensional gray-scale images depending on signal strength. Strong reflections are displayed as white (on a black background), and progressively weaker appear as gradually darker shades of gray (Figure 5-12). Clear fluid, such as the vitreous body and aqueous, contains no reflective particles, and therefore is displayed as black. Rapid updating of a succession of two-dimensional images results in dynamic real-time moving images of structures in motion. In this way, B-mode can be used to visualize the fetal heart motion.

Duplex Doppler imaging combines two-dimensional imaging with Doppler ultrasonography for the evaluation of blood flow. The Doppler effect causes frequency changes between various moving structures. Using the two-dimensional image to identify the location of a vessel, a cursor called a range gate is positioned to provide blood flow information from a selected vessel segment. In a duplex scanner, a transducer probe contains both a transmitter and a receiver. The probe is directed at an artery and a measurement of the blood flow velocity is obtained. Doppler information can be in the form of an audible signal or can be visually displayed

60 DIAGNOSTIC PROCEDURES

FIGURE 5-11 ■ Conventional long-axis B-mode ultrasound image of multiple gallstones (ARROWHEADS) within the bilefilled gallbladder (GB). Note the acoustic shadow cast by the sound-attenuating stones (ARROWS).

on the monitor through a special trace, allowing quantification of flow. In part because of the small size of the blood vessels supplying the eye, duplex Doppler ultrasonography has limited applications in ophthalmic scanning.

In CDI, nonmobile structures are depicted in gray scale, and moving structures, such as blood flow, are imaged in color. The operator selects colors to depict movement toward the transducer (usually red) and away from the transducer (usually blue). CDI allows rapid assessment of the presence and direction of flowing blood. The most common applications for CDI technology includes echocardiography (ultrasonography of the heart), peripheral vascular (blood flow of the limbs), and cerebrovascular (blood flow to the brain). CDI can also demonstrate blood flow in the small vessel supplying the eye, including the central retinal artery and vein located within the optic nerve (Figure 5-13). Suspected occlusion of these vessels can thus be confirmed with CDI. In addition, the technique is useful in differentiating a retinal detachment from an intraocular tumor, especially if intraocular bleeding obscures the view (Figure 5-14).

FIGURE 5-13 ■ Transverse color Doppler image demonstrating the normal central retinal (CR) artery and vein located within the hypoechoic optic nerves (ARROWS). Blood flow is also visualized at the back of the eye corresponding to flow in retinal and choroidal vessels. Red and blue indicate flow toward and away from the transducer, respectively.

ORIENTING THE IMAGE

A light box, or view box, is used to backlight an x-ray image. Images should always be oriented so that it appears that the patient is facing the viewer. “Position” is used to specify the part of the body closest to the film.

Three anatomic planes are imaged in plain films, ultrasound, CT, and MRI: the axial, sagittal, and coronal planes (Figure 5-15). The axial projection is exposed with the top of the patient’s head closest to the film cassette, and the beam emitted from the throat to the crest of the skull. The lower border of the image thus is occipital (dorsal) and the upper border is nasal (ventral). The left margin visualizes the right side of the body, and the right margin visualizes the left side of the body. The coronal projection, also known as posterior-anterior (PA or AP) projection, is exposed with either the chest (PA projection; Figures 5-16 and 5-17) or the back (AP projection; Figures 5-18 and 5-19) closest to the film cassette, and the beam emitted from opposite the film plane. A PA image of the skull is taken with the nose closest to the film cassette, and an AP image of the skull is taken with the back of the head closest to the film cassette. The resultant image is positioned so that the upper border is cranial and the lower border is caudal. Laterally, the image is positioned as if the patient is facing the examiner.

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FIGURE 5-14 ■ Transverse color Doppler image of the eye in a patient with choroidal malignant melanoma (ARROW). Blood flow is detected from vessels in the tumor.

FIGURE 5-15 ■ Planes of the body.

FIGURE 5-16 ■ Schematic of PA chest, showing the physical set-up with the x-ray tube behind the patient and the film plate in front of the patient.

The sagittal projection is exposed by positioning one side of the patient’s body (Figures 5-20 and 5-21) or head closest to the film cassette and the beam emitted from opposite the film plane. This technique produces a cross-section of the eye from the cornea to the optic nerve with the resultant image positioned so that the upper border is cranial, the lower border is caudal, the right border is dorsal, and the left border is ventral.

Skull Films

Bony structures are best visualized when they are closest to the film cassette. The best image of the orbits is therefore achieved with a PA, or coronal, view (with the face of the patient nearest the film) (Figure 5-22), and the sharpest image of the occipital bone is achieved

FIGURE 5-18 ■ Schematic of the anteroposterior (AP) chest radiograph showing the physical set-up with the x-ray tube in front of the patient and the film behind the patient.

with an AP view (with the back of the patient’s head nearest the film).

Skull films invariable demonstrate dark gray suture lines. Skull and orbital fractures appear darker than sutures.

Computed Tomography Scan

The CT scan is normally viewed as though one were looking up at it from the patient’s feet. The CT scan should be held so that the patient’s left side is presented as if the patient is facing the examiner. After the scan is performed, permanent images are produced by photographing the monitor screen with a camera. The CT scan slices in sequence so that one slice can be linked to another. The slices above and

FIGURE 5-19 ■ AP chest radiograph. Note that the spinal column actually restricts the normal view of the chest. The AP view is usually used when the patient is not ambulatory and the portable x-ray machine is needed for the bedside.

FIGURE 5-20 ■ Lateral view of the chest.

FIGURE 5-21 ■ Lateral view of the chest, cross-section showing the anterior level of the chest with a metallic foreign body.

FIGURE 5-22 ■ Coronal (Caldwell’s) view of both orbits showing the sphenoid wings and ridge, and part of the orbital floor. Note the presence of a metallic foreign body in the left orbit which is easily visualized on plain films.

below give additional information about the structure of an organ (Figure 5-23).

The usual CT series of scans consists of contiguous 10-mm-thick slices through the region requested (Figure 5-24), but slices as thin as 1.5 mm can be obtained when finer detail is needed for diagnosis. In ordering orbital scans, the slices are usually 2-mm-thick cuts through the tissue in question (Figure 5-25). The x-ray dose per slice varies from 1 to 4 rad (but only to the slice being imaged) and is comparable to the exposure for conventional radiographic studies of the area. (Rads are units of radiation.)

The image is degraded by the presence of highdensity metallic materials used for joint prosthesis, and by motion initiated by the patient or by internal organs. CT scanners require only 1 to 10 seconds to complete a slice, and a patient must hold his or her breath to prevent motion artifacts on the image.

the clinician should write the patient’s name, age, and diagnosis clearly. A tentative diagnosis is adequate, and the type of scan desired should be indicated. If any questions arise, the radiologist will contact the ordering physician.

RADIOLOGY OF THE EYE AND ORBIT

Four basic methods exist to image the eye and orbital structures: plain films, computed tomography, MRI, and orbital ultrasonography. An orbital scan can be ordered through the local hospital radiology department. On a blank prescription pad or radiology form available from the local hospital radiology department,

Plain Films of the Eye and Orbit

A radiological work-up for a patient with orbital disease or ocular disease involves specific helpful views. These views include Caldwell’s position, Waters’ positions, lateral view, basilar view, and optic canals view.

Caldwell’s position is a coronal section that allows one to view both orbits, the sphenoid wings and ridge, and part of the orbital floor (Figure 5-26).

FIGURE 5-24 ■ Level four CT of orbits. This schematic shows approximately where the cuts are made. The cuts do not overlap.

FIGURE 5-25. ■ Level five CT of the orbits. This schematic shows approximately where the cuts are made. The cuts are overlapping.

Waters’ position is a transverse section that allows a view of the paranasal sinuses and floor and roof of the orbits and the maxillary antrum (Figure 5-27). The Waters’ position scan is used frequently by otolaryngologists and allergists. The ophthalmic application is

FIGURE 5-26 ■ Caldwell’s position, which is the same as a coronal, or PA, view.

FIGURE 5-27 ■ Waters’ view, a transverse scan. This scan allows the paranasal sinuses and the roof and floor of the orbits to be viewed.

limited to cases of suspected orbital floor fractures in trauma.

The lateral view is a cross-sectional view taken from the side (Figure 5-28). This view should enhance subtle pathologic changes so that the depth of level of the lesion is indicated. For example, the lateral view of the sella turcica, as seen in Figure 5-29, shows the sella and anterior clinoids so the examiner can detect problems like empty sella syndrome and pituitary disease.

Scans of the optic canals are magnified views that can demonstrate the foramen in the skull (Figure 5-30).

The basilar view is a transverse scan that allows the posterior wall of the orbit, the maxillary sinus and the optic canals, to be visualized (Figure 5-31).

FIGURE 5-28 ■ Lateral view, which is the same as a sagittal view.

Visualization of vascular structures of the eye may still require intravenous contrast media. Arteriography is used to selectively show the vascular tree within an organ (Figure 5-32). The radiopaque dye is injected into an artery, and subsequent arteries are viewed according to the study in question. This method is particularly helpful in the study of malignancies, because they are usually highly vascular in nature. Arteriography is also helpful in diagnosing aneurysms. This method does, however, have a higher morbidity rate than CT, therefore it is used with caution and only when necessary, not as a routine test.

FIGURE 5-30 ■ Optic canal scan. This scan gives an enlarged view of a very specific area within the orbit. This view can detail the foramen in the skull.

Computed Tomography of the Eye and Orbit

Axial images of the eye and orbit parallel to orbital axis, the optic canal, and the chiasm visualize the optic nerve and the horizontal recti muscles along their entire path. The axial image is best used to visualize the entire anterior visual pathway. Data taken in the axial plane may be reformatted to yield images in the coronal, sagittal, and oblique parasagittal planes. This

reformation minimizes radiation exposure to the patient, because these additional planes of section are generated by the computer information after the patient has left the scanner. If the orbit is angled 20 degrees downward so the plane of the image is parallel to the orbital floor, better images of the oblique muscles and inferior rectus muscle are obtained.

Orbital CT scans rarely require contrast enhancement because a wide variation in tissue densities exists

FIGURE 5-31 ■ Basilar view, a transverse scan allowing visualization of the posterior wall of the orbit, the maxillary sinus, and the optic canals.

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within the major orbital structures. High-contrast differentiation therefore occurs in CT images despite the lack of contrast media involvement. Contrast material may be necessary in cases of visual loss, possible infection, and intracranial involvement.

CT can define fine anatomic detail and thus can well visualize and characterize the morphology and the site of globe, extraocular muscle, optic nerve, and orbital tissue pathology. The CT findings should be correlated with the patient’s history, symptoms, clinical signs, laboratory testing, and other imaging data.

Direct coronal views are best used to evaluate the orbital roof, floor, and medial walls for signs of erosion, blowout fracture, neoplasm, and chronic infection. Axial views are typically used for almost all other evaluations of ocular conditions. One significant exception is extraocular muscle involvement in Graves’ disease, which is best visualized by ordering a paraxial CT reformation perpendicular to the course of the muscles to best judge true muscle thickness.

Ocular Pathology on Computed Tomography Scans

CT can define fine anatomic detail and so can well visualize and characterize the morphology and site of globe, muscle, optic nerve, and orbital tissue pathology. The CT findings should be correlated with the patient’s history, symptoms, clinical signs, laboratory testing, and other imaging data.

Axial sections are typically 1.5 mm thick to maximize detail. The most desirable plane to use is parallel to the optic nerve. Data taken in the axial plane may be reformatted to yield images in the coronal, sagittal, and oblique parasagittal planes. This reformation minimizes radiation exposure to the patient,