Morley / Donald / Sanders MD Ultrasonic Sectional Anatomy

1. Auflage 2013
ISBN: 978-1-4832-8006-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 254 Seiten

Ultrasonic Sectional Anatomy centers on the imaging processes, methodologies, and approaches employed in sectional anatomy. The selection first offers information on the brain and cerebral ventricles, eye and orbit, and the thyroid and adjacent soft tissues of the neck. The book also examines the breast, heart, and abdominal muscles and skeletal boundaries. Topics include anterior abdominal wall, pelvic muscles, diaphragm, recording the cross-sectional echocardiogram, and echography of the normal breast. The text elaborates on the upper abdominal viscera and the kidneys, including renal anomalies, spleen, pancreas, adrenal glands, and gall bladder and bile ducts. The manuscript then takes a look at the gastrointestinal tract and peritoneal cavity and viscera of the lower abdomen and pelvis. Discussions focus on scrotum and penis, urinary bladder, ureter, seminal vesicles, and prostate, and peritoneal recesses. The selection is a dependable reference for readers interested in ultrasonic sectional anatomy.

Autoren/Hrsg.

Morley, Patricia

Donald, Gabriel

Sanders MD, Roger C.

Leseproben

2

THE EYE AND ORBIT

Malcolm LeMay

Publisher Summary

Ultrasonic echography of the eye can be performed using a probe in direct contact with the eyelid or cornea or with a water bath as an acoustic coupling between the probe and the subject. When the direct-contact method is used, the structures of the anterior segment tend to be lost in the near field of the ultrasonic probe because the probe is refractory to echoes returning after a short interval as it is still vibrating following the main excitation pulse. The eye is protected by the bony orbit in the other meridian, and even in the sagittal orbital plane, the bone limits examination to a sector or linear scan. Ophthalmic examination is usually concerned with high resolution of static structures, and the compound scan is produced with the eye immobile during the few seconds necessary to complete each scan. Real-time scanning is useful when abnormalities of the vitreous are being explored as these are best seen on eye movement. Real-time scanning techniques currently being used produce a sector scan. The sector scans produced in this way show movements of the retina and vitreous, demonstrating fibrosis and points of adhesion between these structures.

Ultrasonic scanning of the eye was first described by Mundt & Hughes in 1956.

In examination of the eye and orbit it is necessary for sound to penetrate less than 6 cm into the tissues to image all structures from the eyelids to the orbital apex. Focusing of the probe is necessary if maximum resolution is to be obtained. Probes designed for use in ophthalmology are often focused at a distance corresponding to the posterior wall of the eye in an attempt to maximise resolution at this level.

The anatomical structures are mostly small and imaging of what is largely a hollow organ is concerned with detail in and around the walls of the eye. Higher frequencies of ultrasound can therefore be used. The loss of power at depth is not a problem and the improved resolution is a positive advantage in imaging small structures. In the eye resolution of structures smaller than 200 µm is therefore possible. This is an improvement over the value of 500 µm given by Oksala. For these reasons a frequency of 8 MHz or 10 MHz is typically used for the eye. Frequencies from 5 MHz to 20 MHz are often available for orbital and anterior segment imaging respectively (Coleman et al 1969).

Ultrasonic echography of the eye can be performed with the probe in direct contact with the eyelid or cornea, or with a water-bath as acoustic coupling between the probe and the subject. When the direct contact method is used the structures of the anterior segment tend to be lost in the near field of the ultrasonic probe. This is because the probe is refractory to echoes returning after a short interval as it is still vibrating following the ‘main bang’ of the ultrasonic pulse. There are therefore considerable advantages in using some type of water-bath to enable the probe to be held a few centimetres from the eye, and to facilitate free movement of the probe in compound scanning. If the examination is done with the eyes closed, the cornea is not visible as it tends to be inseparable from the images of the eyelids. It is therefore normal practice to perform ultrasonic examination of the eye with the eye open. The water-bath consists of a polythene sheet with a central aperture for the eye. The polythene is stuck to the face around the eye using a double-sided adhesive disc. The cornea is in direct contact with the liquid of the water bath, which should be sterile and isotonic. With practice the water-bath becomes easy to erect and is well tolerated by patients. Irrigation or injection quality saline or compound sodium lactate solution is used. This liquid should be as near body temperature as possible to standardise the velocity of sound and to minimise disturbance due to thermal currents within the eye. Because high resolution of small structures is required there appears to be an additional advantage to examination with the eye open: if the sound beam passes through the eyelids the thickness of the lids causes sufficient scattering of sound to degrade the final resolution of the image (Coleman et al 1977).

Scans in the transverse orbital plane are the rule in ophthalmology and compound scanning is used or the lateral walls of the globe are not well demonstrated (Coleman et al 1977). The eye; is protected by the bony orbit in the other meridia and even in the sagittal orbital plane the bone limits examination to a sector or linear scan. If it is necessary to scan in more than one plane a repeat scan with the eye in a different position of gaze is indicated. Coronal scans of the orbit can be used (Fig. 2.14) but these are constructed by special electronic techniques (Restorie & Wright 1977).

Fig. 2.14 Orbital ‘C’ scan.
A coronal scan of the orbit produced by the technique of Restorie & Wright (1977). This picture shows a coronal section of the orbital fat pad immediately behind the globe. The optic nerve is clearly seen.

Ophthalmic examination is usually concerned with high resolution of static structures and the compound scan is produced with the eye immobile during the few seconds necessary to complete each scan. Real-time scanning is useful when abnormalities of the vitreous are being explored as these are best seen on eye movement (McLeod et al 1977). Real-time scanning techniques currently used produce a sector scan. The sector scans produced in this fashion show movements of the retina and vitreous, demonstrating fibrosis and points of adhesion between these structures. (The equipment currently in use at Moorfields Eye Hospital, London, produces a real-time linear scan.) However overall resolution tends to be less satisfactory where detailed visualisation of small structures is concerned and so both static and dynamic scanning have an important place in ophthalmic ultrasound.

The scans in the following sections are all compound scans produced during the routine examination of hospital patients at an ultrasonography clinic. Most of these scans are produced with a 10 MHz probe and the water bath technique. The equipment used is the Sonometrics 100 Ophthalmoscan. The equipment is fully described elsewhere (Coleman et al 1969).

Fig. 2.1 Eye and orbit. Transverse scan in the plane of the orbits. Left eye.
The overall appearance of the eye and orbit are seen. The normal axial length is 24 mm. The velocity of sound is some 10% higher through the lens and the lens is therefore a little foreshortened in a B scan. The medial and lateral rectus muscles are demonstrated. The apex of the orbit is not well visualised. The optic nerve is only demonstrated posteriorly and lies superior to the plane of examination.

Fig. 2.2 Anterior segment.
The cornea is about 0.6 mm thick centrally and up to 1 mm thick at the periphery. The anterior chamber is bounded by the cornea anteriorly and by the iris and the anterior surface of the lens posteriorly. The shape and depth of the anterior chamber vary, a shallow anterior chamber being formed by an anteriorly more convex lens/iris diaphragm. The shape of the lens will vary slightly with accommodation. The structures of the anterior chamber angle are complex and are not separately demonstrated. The angle of the anterior chamber between the iris and the cornea can be demonstrated and may become closed in some types of acute and chronic glaucoma. In this situation the lens/iris diaphragm appears anteriorly inserted with respect to the external limbus. In this illustration the anterior surface of the lens is not demonstrated. This is partly due to the convex shape and consequent divergence of reflected sound. The anterior lens surface is however easily demonstrated in an eye with a dilated pupil.

Fig. 2.3 Lateral part of anterior segment.
The area of transition of cornea to sclera is shown. The iris is the anterior part of the uveal layer of the eye and this merges into the structures known as the ciliary body. The ciliary body is posteriorly continuous with the choroid. The angle where the steeper curve of the cornea changes to the curve of the eye is known as the limbus. This is a zone about 1 mm wide. Posterior to this, the ciliary body extends back nasally approximately 6 mm and temporally approximately 7 mm. The ciliary body therefore becomes the choroid some 7 or 8 mm posterior to the limbus as seen externally. The ciliary body itself consists of two parts: posteriorly is the pars plana; anteriorly, the inner surface presents about 70 longitudinal ridges and is thicker than the main mass of the ciliary body. This area is some 2 mm wide and is the pars corona ciliaris. This latter area is referred to as the ‘ciliary body’ in common usage.

Fig. 2.4 Horizontal section of the globe to show the posterior wall.
In this view the anterior chamber structures are less well seen and the scan has been performed to define the posterior wall. This consists primarily of the sclera which is about 1 mm thick at the posterior pole and becomes thinner further forwards. The globe lies in a thin fibrous capsule (Tenon’s capsule) which separates the eye from the orbital contents.

Fig. 2.5 Horizontal section of the globe to show the...