Ординатура / Офтальмология / Английские материалы / Imaging of Orbital and Visual Pathway Pathology_Muller-Forell_2005

CONTENTS

1.1Color Doppler Ultrasonography

of the Eye and Orbit 3

Wolfgang E. Lieb

1.1.1Introduction 3

1.1.2 Ophthalmic Examination Technique 4

1.1.3Safety Considerations 5

1.1.4Vascular Topography of the Normal Eye and Orbit 5

1.1.5Retinal and Retinal Vascular Disease

of the Eye 7

1.1.6Intraocular Tumors 8

1.1.7Orbital Disorders 9 References 10

1.2Computed Tomography 15

W. Müller-Forell

1.2.1Technical Principles 15

1.2.2Radiation Burden 15

1.2.3Contrast Medium 16

1.2.4Imaging Protocol 16

W. Wichmann

1.3.1Basic Physical and Technical Principles of MRI 18

1.3.2General Considerations of

MRI Imaging Protocols 20

References 23

W. Lieb, MD, PhD

Professor, Eye Clinic, University of Würzburg, Julius Maximilians University, Josef-Schneider-Strasse 11, 97080 Würzburg, Germany

PD W. S. Müller-Forell, MD

Institute of Neuroradiology, Medical School, University of Mainz, Langenbeckstrasse 1, 55101 Mainz, Germany

W. Wichmann, MD, PhD

Professor, Institute of Neuroradiology and Radiology, Klinik im Park AG, Seestrasse 220, 8027 Zurich, Switzerland

1.1

Color Doppler Ultrasonography of the Eye and Orbit

Wolfgang E. Lieb

Color Doppler imaging is the significant development of the last decade in ultrasonography that allows for simultaneous two-dimensional structural imaging in the Doppler evaluation of blood flow.With this technique, it has become possible for the first time to display indirectly the fine orbital vessels such as the ophthalmic artery and its branches, the central retinal artery, the posterior ciliary artery, and the lacrimal artery. On the other hand, also the display of venous structures such as the superior ophthalmic vein, the vortex veins, and the central retinal vein is possible. In addition to this qualitative display, it also enables quantitative assessment of the hemodynamics in those vessels by looking at the Doppler spectrum and determining flow velocities during various periods of the cardiac cycle.

This technique is now being used in ophthalmology to evaluate orbital tumors and vascular lesions, intraocular tumors, carotid cavernous sinus fistulas, and hemodynamic changes in patients with retinal vascular disease such as central retinal artery occlusion, central retinal vein occlusion, and diabetic retinopathy. Several studies have even been made to study drug effects on the hemodynamics.

1.1.1 Introduction

Real time A-mode and B-mode ultrasonography has been used for the diagnostic evaluation of ophthalmic disorders since the early 1960s. Modern digital high-resolution equipment has improved diagnostic imaging and made it an essential part of certain ophthalmologic evaluations. Doppler ultrasound detects

changes in the frequency of sound reflected from flowing blood, allowing estimation of the flow velocity. Doppler ultrasonography of the carotid arteries and the periorbital vessels is frequently employed in patients with ischemic ocular disease. The technology of Duplex scanning allows for simultaneous B-mode imaging and Doppler spectral analysis. Since the diameters of the vessels in the eye and orbit are too small to be imaged with conventional Duplex scans, Doppler spectra are obtained without precise localization and without knowledge of the Doppler angle. The latest technological change in the character of diagnostic ultrasound is color Doppler imaging. To facilitate localization of vascular structures, the two-dimensional flow information in color Doppler imaging (CDI) is encoded in color and superimposed on the gray scale structural image. Since the sensitivity of detecting Doppler shifts is not limited by the resolution of the gray scale image, Doppler shifts in very small vessels can be detected, depicting the course of the vessels (Grant et al. 1989, 1992;

Merritt 1987; Mitchell 1990; Mitchell et al. 1988, 1989; Powis 1988; Pozniak et al. 1992; Ranke et al. 1992).

The introduction of the so called“power Doppler”, which represents the summation of the square from the spectral amplitudes of the Doppler signal, was an important new step in CDI technology. Hereby the coding for direction is neglected in order to improve sensitivity for very small vessels or vessels with low blood flow (Adler et al. 1995; Allard et al. 1999; Babcock et al. 1996; Bascom and Cobbold 1996; Griewing et al. 1996a,b; Hamper et al. 1997;

Martinoli et al. 1998; Murphy and Rubin 1997;

Pugh et al. 1996; Rubin et al. 1994; Winsberg 1995; Wu et al. 1998). Modern broad band transducers with frequencies up to 15 MHz have greatly improved orbital gray-scale and color Doppler imaging.

The three major areas of application of CDI and its new modifications of signal processing such as power Doppler, tissue harmonic imaging, 3-dimen- sional CDI, as well as the use of contrast agents can be described as primarily:

1.Vascular evaluation

Application of CDI includes the detection and measurement of arterial stenosis and flowrestricting or flow-disturbing abnormalities. This can be performed for the large abdominal vessels, the aorta, iliac, femoral, and popliteal artery, and other peripheral arteries, and of course, for large vessels of the head and neck, the carotid bifurcation, and the vertebral arteries (Allard and

Cloutier 1999; Bazzocchi et al. 1998; Bendick

et al. 1998; Carroll 1996; Erickson et al. 1989b,

Erickson et al. 1989c; Ferrara and DeAngelis

1997; Foley et al. 1989; Grant et al. 1990, 1992;

Landwehr and Lackner 1990; Landwehr et al. 1989, 1990, 1991; Merritt 1987, 1989; Whelan et al. 1992)

2.Organ perfusion

CDI can be utilized to visualize the perfusion of the liver, kidneys, spleen, placenta, and brain. It can be used as a guide to obtain selective Doppler information which allows better assessment of the hemodynamics in those organs. The major indications of this category is the assessment of perfusion in kidney transplants (Bazzocchi et al. 1998; Becker and Cooperberg 1988; Deane et al. 1992; Fleischer and Kepple 1992; Lerner et al. 1990; Levine et al. 1997; Lewis and James 1989; Middleton et al. 1989; Rifkin et al. 1993;

Seibert et al. 1998; Wilson and Thurston 1992; Winkler 1998; Winters 1996).

3.Tumor neovascularity

CDI adds a new dimension to the ultrasound evaluation of mass lesions (Maresca et al. 1991; Orr and Taylor 1990; Taylor et al. 1991). This technique is already being successfully used to differentiate some benign from malignant tumors of the liver (Goldberg et al. 1990), tumors of the female breast, the testicles, as well as tumors of the eye and orbit (Falco et al. 1992; Guthoff et al. 1989, 1991a; Jain et al. 1992; Lieb 1998; Lieb et al. 1990b; Wolff-Kormann et al. 1992a,b).

1.1.2

Ophthalmic Examination Technique

The ultrasound transducer is applied to the closed eyelids using sterile ophthalmic methylcellulose as a coupling gel . During the examination, the patient lies in a supine position, and care is taken not to apply pressure to the eye to avoid artifacts. Horizontal and vertical scans through the eye and orbit are performed. Depending on the direction of flow with respect to the transducer, the blood flow data displayed are either in red or blue. The colors can be arbitrarily assigned, but in this study flow toward the transducer is depicted as red and away from the transducer as blue. The color saturation in the image represents the average frequency (first moment average) from a spectral analysis performed at each sample site. These frequencies can be turned into velocities by solving the Doppler equation for velocity.

When examining the eye and orbit through the eyelids, the ultrasound beam is essentially parallel to the orbital and ocular vessels, and thus most arterial flow is depicted in red. Arteries can usually be distinguished from veins by noting the pulsatility of the former. Pulsed Doppler spectral analysis also helps to distinguish between pulsatile arterial and the usually more continuous or minimally pulsatile venous flow, and allows for the quantification of data. When the ultrasound beam is at an angle of 90 deg to a vascular structure or if a vessel contains only stagnant blood, no Doppler flow information is obtained, and the structure is shown in gray scale display only.

All examinations are performed in a “low” or “medium” flow setting to allow for optimal detection of low to medium Doppler frequency shifts of the slow-flowing blood in the small orbital vessels.For the ophthalmic artery, medium to high flow settings are applied,since flow in this vessel is faster.Color threshold levels are adjusted to minimize artifacts by lid and involuntary eye movements. To obtain Doppler spectra, a sample volume of approximately 0.2×0.2 mm is chosen within the vessel . The proximal and distal portions of the vessel are imaged to facilitate determination of the Doppler flow angle for estimation of velocity. The scan images can be recorded on videotape and later reviewed with the benefit of cine-loop and frame by frame analysis of selected segments. Images can be photographed during cine-loop replay by using a 35 mm camera that photographs directly from an isolated on-board color monitor. The duration of the examination ranges from 20 to 30 min for both (Belden et al. 1995; Guthoff 1988; Guthoff et al. 1991a; Lieb 1993, 1998; Lieb et al. 1991; Williamson and Harris 1996).

sound with intensities of 1000 mW/cm2 (Lizzi and Mortimer 1988; Lizzi et al. 1981).

Currently, in the USA, the Food and Drug Administration (FDA) has established guidelines of ultrasound intensity limits for various clinical applications. The basis for these limits was linked to the measured output level of machines sold for ophthalmic and other particular applications prior to the enactment of the 1976 Medical Device Amendment. These parameters have not been based upon the assessment of risks published today.

For the instrument used in our studies (QAD 1 and QAD 2000, Siemens-Quantum, Issaquah, Wash., USA), the estimated in situ peak temporal average intensity (SPTA) in the color imaging mode is 2–3 mW/cm2 for the 7.5 MHz transducer. During spectral analysis, the in situ peak temporal average intensity is approximately 50–100 mW/cm2, exceeding the currently approved FDA upper guidelines of 17 mW/cm2. The Spatial Peak Average Intensity (SPAI) is the highest intensity within the ultrasound field average over an entire scan frame period.

Depending on the device settings for commercially available color Doppler units, the acoustic intensity values given in the AIUM bioeffect statements and in the FDA guidelines may be exceeded, especially during pulsed Doppler spectrum analysis. However, the SPTA intensities of 100 mW/cm2 should not be treated as a magic number. According to studies by Lizzi et al. (1981; Lizzi and Mortimer 1988) and Coleman et al. (1986), who did intensive experiments on ultrasound bioeffects to ocular tissues, the intensities used for diagnostic imaging are significantly lower than those which would be expected to cause unwanted ocular side-effects, especially cho- rio-retinal lesions and cataract formation.

1.1.3

Safety Considerations

As in any diagnostic test, there may be some risk in diagnostic ultrasound. Therefore, the sonographer and physician need to know something about this risk.

The American Institute of Ultrasound in Medicine (AIUM) has reviewed reports of bioeffects in ultrasound and has issued a statement in respect of ultrasound bioeffects on in vivo mammalian tissue.

According to the AIUM, there is in the low MHz frequency (0.5–10.0 MHz) no independently confirmed significant biological effect in mammalian tissue exposed in vivo to unfocussed ultrasound with intensities below 100 mW/cm2 or focussed ultra-

1.1.4

Vascular Topography of the Normal Eye and Orbit

Horizontal and vertical sections of the globe and normal orbit at the level of the optic nerve display Doppler signals along the course of the central retinal artery (CRA) and the central retinal vein (CRV) (Fig. 1.1a).

The CRA and the accompanying vein can be depicted within the anterior 2 mm of the optic nerve shadow. In some instances, they can be traced up to the point of entering into the optic nerve. The CRV usually runs next to the CRA and can be differenti-

a

b

c

ated from it by the color coding and also by its Doppler characteristics and its continuous flow in systole and diastole. The spectrum of the CRA shows usually a venous overlap from the CRV (Fig. 1.1b). On either side of the optic nerve, slightly posterior to the CRA, the short and long posterior ciliary arteries can be identified. Several groups have published their expe-

Fig. 1.1.a Horizontal section through the globe at the level of the optic nerve. Displayed are the central retinal artery (CRA), the central retinal vein (CRV), and the temporal short posterior ciliary artery (PCA). b Analysis of the Doppler spectrum. A cursor is placed on the vessel, and the angle is corrected according to the vessel’s course. The spectrum of the CRA shows usually venous overlap caused by the CRV running close to the CRA. c Doppler spectrum and course of the ophthalmic artery

riences and normal values (Table 1.1) with good overall reproducibility (Aburn and Sergott 1993a,b; Belden et al. 1995; Erickson et al. 1989a; Gillies et al. 1999; Giovagnorio et al. 1993; Greenfield et al. 1995; Guthoff et al. 1991b; Lieb et al. 1991; Niwa et al. 1998; Senn et al. 1996; Williamson et al. 1995). The Doppler spectrum of the posterior ciliary arte-

Ophthalmologic Imaging Methods

Table 1.1. Blood flow velocities in orbital vessels: data from 222 normal eyes (Lieb and Schenk 1998)

ries shows velocity time spectra which are similar to those of the CRA. The end-diastolic flow in the posterior ciliary arteries (PCA) is higher, however, reflecting the low resistance vascular channels of the choroid (Lieb et al. 1992a,b). Further posterior in the posterior orbit, segments of the main ophthalmic artery can be seen. The ophthalmic artery can be traced temporally to the optic nerve to the point where it usually crosses over the optic nerve towards the medial orbit.In Hayreh’s series,the crossing of the ophthalmic artery over the optic nerve in the midorbit was a common finding in 80% of cases studied (Hayreh 1963a,b; Lang and Kageyama 1990).

The flow velocity wave form of the ophthalmic artery (OA) is similar to that of the internal carotid artery, showing a high maximum peak systolic flow and low diastolic flow velocity. Sometimes the superior orbital artery and the lacrimal artery can be identified (Fig. 1.1c). Of the venous structures, flow in the vortex veins can be demonstrated in all four quadrants, the superior ophthalmic vein can be identified at the posterior aspect of the globe and in the superior nasal orbit (Fig. 1.2). The superior ophthalmic vein (SOV) can be traced posteriorly until it crosses over

7

the optic nerve (Berges 1992; Erickson et al. 1989; Lieb et al. 1991; Williamson and Harris 1996). The spectrum with the continuous, nonpulsatile flow pattern together with the color coding in blue is characteristic of venous flow.

1.1.5

Retinal and Retinal Vascular Disease of the Eye

Few reports have dealt with CDI in the evaluation of retinal disorders. Wells et al. and others (Verbruggen et al. 1999; Wells et al. 1991) have used CDI to depict a patent hyaloid artery in a case of persistent hyperplastic primary vitreous (PHPV). Wong et al. (1991) used the vascularity in the retinal vessels of retinal detachments as an additional criterion to distinguish a detached retina from dense vitreous strands (Fig. 1.3). In a study comparing the flow velocities of the CRA and OA in patients with arterial

Fig. 1.2. Section of the superior hemisphere of the globe displaying the nasal and temporal vortex veins

Fig. 1.3. Total funnel-shaped retinal detachment inserting at the optic nerve (N) with a membrane behind the lens . There is blood flow visible in the detached retina (R), the central retinal (cra), and the posterior ciliary arteries (pca)

hypertension and carotid artery disease, Cesarone et al. found significantly reduced systolic and end-dia- stolic flow velocities compared with a normal control group (Cesarone et al. 1992). We found a group of patients with proliferative diabetic retinopathy who showed a significant decrease of the peak systolic and end-diastolic flow velocity of the CRA. The blood flow velocities in the PCA and OA were unchanged com-

pared to age-matched controls (Göbel et al. 1993; Goebel et al. 1995). The pharmacological influence on flow parameters has been investigated by Belfort (1992) and Baxter et al. (1992) using CDI. In the first study,Belfort was able to detect a significant reduction in pulsatility and Pourcelot’s index of the CRA and PCA in a group of pre-eclamptic women who were treated with magnesium sulfate. The study by Baxter evaluated the effect of posture and topical ß-block- ers on the hemodynamics of the orbital vessels. They found no effect of posture but a fall in Pourcelot’s index. Since the identification of the orbital vessels is difficult to reproduce in their study, caution should be used when interpreting this information as an indication that CDI is capable of demonstrating subtle pharmacologic effects on orbital hemodynamics. Ho et al. (1992) published the first study to use CDI in the diagnosis and investigation of ocular ischemic syndrome (OIS). It demonstrated reduced ocular blood flow in one of the ophthalmic artery, PCA, or CRA in eyes with OIS.Furthermore, in some eyes with OIS, CDI demonstrated nondetectable or reversal of blood flow velocities in corresponding posterior ciliary or ophthalmic arteries (Ward et al. 1995; Wolf et al. 1987; Wong et al. 1998). In general, lower flow values represent compromised blood flow proximal to the point of sampling by CDI or increased resistance distal to the sampling point (Costa et al. 1999; Geroulakos et al. 1996; Hu et al. 1995; Lee and Fu 1997; Mawn et al. 1997).

Hosten et al. 1999; Imamura et al. 1998; Ishiguchi et al.1996),and renal tumors (Chen et al.1998; Lewis and James 1989; Polascik et al. 1999; Ramos et al. 1988) and are helpful in their differential diagnosis. Histopathologically, tumor vessels are often primitive vascular channels lacking smooth muscle, consisting only of an endothelial layer and connective tissue (Paweletz and Knierim 1989).Low resistance to flow is expected, since in most neoplasms the vessels lack normal arteriolar smooth muscle, the recognized site of peripheral vascular resistance.

High sensitivity in detecting even minimal flow is necessary to allow detection of fine tumor vascularity (Fig. 1.4a,b). Lesions that can simulate uveal melano-

1.1.6

Intraocular Tumors

Effective vasculature is essential for all tumor growth. It is formed by newly sprouted, ingrowing vessels and by incorporation of existing host vessels into the tumor mass. Other than the qualitative information provided by intravenous fluorescein angiography, to date no technique is available to assess the tumorassociated blood flow in the eye and orbit.

Recently,severalgroupsreportedtheirresultsusing conventional Duplex scanning and CDI on intraocular tumors (Guthoff et al. 1989, 1991a; Hirai et al. 1998; Lieb et al. 1990b; Pineda et al. 1998; WolffKormann et al. 1992a,b). They were able to demonstrate flow within the intraocular tumors and noted a decrease in Doppler shift after therapy. Abnormal Doppler signals have been reported for many tumors (Shimamoto et al. 1987) such as breast carcinomas (Cosgrove et al. 1990; Raza and Baum 1997), hepatomas (Bartolozzi et al. 1997; Goldberg et al. 1993;

a

b

Fig. 1.4.a Power spectrum display of a large choroidal melanoma with fan-shaped vasculature throughout the tumor (TU). R, retina; N, optic nerve. b Spectrum analysis of tumor vessels demonstrating neoplastic vasculature with low resistance flow characteristics

mas,such as large subretinal hemorrhages,usually do not have a distinctive blood supply. Therefore, they can be differentiated from melanomas on the basis of the absence of Doppler flow. Further improvement in the detection of low blood flow in neoplasms was achieved with ultrasound contrast agents such as Levovist (Albrecht et al. 1998; Bauer et al. 1999; Bogers et al. 1999; Brown et al. 1998; Cennamo et al. 1994; Kim et al. 1998; Pugh et al. 1996; Rizzatto et al. 1997; Schlief 1991; Uggowitzer et al. 1999) and the power Doppler mode (Kurjak et al. 1998; Silverman et al. 1999).

1.1.7

Orbital Disorders

In the orbit CDI has been used to study orbital vascular lesions such as orbital varices, carotid cavernous sinus fistulas (CCF), as well as orbital mass lesions. Flaharty et al. (1991) as well as Kotval et al. (1990) have reported their experience with CDI in the diagnosis and monitoring of carotid cavernous sinus fistulas. In all cases of CCF studied, CDI was able to demonstrate the dilated, arterialized SOV with high velocity blood flow towards the transducer (Fig. 1.5a,b). CDI further depicted the dilated preseptal high blood flow shunts and the secondary extraocular enlargement of muscles characteristic of this entity (Aung et al. 1996; Costa et al. 1997; Martin et al. 1995; Nagy et al. 1995). The recent report from Soulier-Sotto et al. (1992) confirmed the findings and stressed the point that this technique can be also used to monitor CCF noninvasively to assess their spontaneous course or effects of embolization or balloon occlusion. In contrast to CCSF, orbital varices, when studied with CDI, demonstrate relatively low blood flow velocities, and the dynamic evaluation depicts venous inflow and outflow into the varix during inspiration and Valsalva maneuvers (Kawaguchi et al. 1997; Lieb et al. 1990; Wildenhain et al. 1991).When studying orbital mass lesions, CDI adds a new dimension to their evaluation.

Whereas computed tomography (CT) scanning and magnetic resonance imaging (MRI) give a good topographic display of those lesions and some indications of their vascularity, when studying contrast enhancement or signal intensities, CDI directly displays active flow in those lesions (Fig. 1.6). Several tumors have been studied by Jain et al. (1992), but in their report they were unable to attribute specific vascularity patterns to individual tumors. We found that cavernous hemangiomas of the orbit usually

a

b

Fig. 1.5.a High flow carotid cavernous sinus fistula. The flow in the dilated superior ophthalmic vein (SOV) is reversed and therefore displayed in red, and the spectrum shows a characteristic high flow shunt pattern with high end-diastolic flow velocities. b After spontaneous partial thrombosis of the SOV, there is a blunted spectrum within the vessel as a sign of less flow

show only very little to almost no flow and extremely low venous flow velocities throughout the lesion. In contrast to this, lymphomas and metastatic lesions contain large arterial and venous vessels supplying the tumor (Lieb et al. 1992a). Compression of the CRA has been shown in a case of amaurosis caused by a cavernous hemangioma. The authors (Knapp et al. 1992) demonstrated clearly that CDI is able to substantiate hemodynamic changes caused by the tumor, which in their case caused intermittent amaurosis by vascular compression of the central retinal vessels. Several groups have found the information obtained by CDI quite helpful and supplementary to CT and MRI in planning the surgical approach to a tumor (Ivekovic et al. 2000; Zuravleff and Johnson

1997) and in patients with thyroid ophthalmopathy (Benning et al. 1994; Nakase et al. 1994).

In our opinion, CDI provides useful information in the orbit:

1.for the evaluation of the normal orbital and ocular vasculature,

2.for the evaluation of other orbital vessels displaced by a mass lesion or tumor

3.for the primary evaluation and follow-up of orbital vascular lesions as varices, arteriovenous malformations, and CCFs

4.for the assessment of the vascularity pattern of orbital or intraocular mass lesions,

5.for the differentiation of intraocular tumors from hemorrhage, and vitreous bands from retinal detachment.

Studies evaluating the role of CDI investigating hemodynamic changes of orbital vessels indicate that this technique provides additional useful information in CRV occlusions, diabetic retinopathy, arterial occlusions, and perhaps in glaucoma (Chiou et al. 1999; Evans et al. 1999a,b; Harris et al. 1995; Sergott et al. 1994).

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Fig.1.6.Well-circumscribed tumor (TD) in the anterior orbit with significant vascularity. Histologically shown to be a hemangiopericytoma

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