Color Doppler Imaging

Core Messages

¥The CDI technique is used to measure blood velocities in the retrobulbar arteries: Ophthalmic artery, central retinal artery, nasal and temporal posterior ciliary arteries.

¥Widely used variables are peak-systolic, end-diastolic and mean ßow velocities, as well as resistance index.

¥The advantages of the CDI technique are: (1) not dependent on optic media,

(2) non-invasive, (3) good reproducibility with an experienced user.

¥It can be used in a number of vascularrelated ocular pathologies, from glaucoma to central vein occlusion.

I. Stalmans, M.D. Ph.D. (*)

Department of Ophthalmology, University Hospitals Leuven, Kapucijnenvoer 33, Leuven 3000, Belgium e-mail: ingeborg.stalmans@uz.kuleuven.ac.be

S. OrgŸl, M.D.

Department of ophthalmology, University of Basel Eye Clinic,

Mittlere-Strasse 91, Basel CH-4031, Switzerland e-mail: sorguel@uhbs.ch

L. Schmetterer, Ph.D.

Department of Clinical Pharmacology, Center of Medical Physics and Biomedical Engineering, Medical University of Vienna, Weahringer Guertel 18-20, Vienna

A-1090, Austria

e-mail: leopold.schmetterer@meduniwien.ac.at

¥Caution should be used when comparing results obtained with different ultrasound machines/probes, as data may not be interchangeable.

¥Velocity should not be read as ßow, as this correlation implies vessel diameter. Such variable has not been reproducible using the current technology.

8.1 Principles

Color Doppler imaging (CDI) is a technique that combines B-scan ultrasonography for the conventional imaging of tissue with velocity extraction based on the acoustic Doppler effect. In the eye, this technique can be employed for the visualization of blood velocities in the retrobulbar vessels including the ophthalmic artery (OA), the posterior ciliary arteries (PCAs), and the central retinal artery (CRA). With CDI, ultrasound waves with frequencies of several MHz are used. Analysing the time elapsed between emission and return can lead to a quantiÞcation of the reßecting structure depth. B-scan ultrasound can then be used to produce gray-scale images of the structures of the human eye. If a structure within the scattering volume of the ultrasound is moving, a Doppler shift is induced (Fig. 8.1). If the reßecting object is moving toward the transducer, the frequency of the returning sound wave is increased as compared to the emitted sound wave. If the reßecting object is, however, moving away from the ultrasound probe, the frequency of

Flow velocity

Fig. 8.1 Doppler principles

the returning sound wave is smaller than that of the emitted sound wave. In tissue, the main source of Doppler shifting is related to the movement of red blood cells within the tissue. Hence, blood ßow in retrobulbar vessels can be visualized, and blood velocity can be extracted. The Doppler shift Df depends on the velocity of the moving erythrocytes (v), the frequency of the incoming ultrasound waves f0, the velocity of the ultrasound wave within the tissue (C), and the Doppler angle. The velocity of the blood can be calculated as

v = f .C / (2 × f0.cosθ)

From this equation, it becomes clear that the highest Doppler shift is induced when the transducer is parallel to the vessel and the angle q = 0 because in this case cos q = 1. When the vessel is, however, perpendicular to the incoming sound wave, no Doppler shift is detected because cos q =0. In practice, measurement of Doppler frequencies are usually done at angles between 30¡ and 60¡ [36]. When the angle q is close to 0¡, problems arise from total reßection of the sound wave at the vessel wall. When the angle q is, however, close to 90¡, the frequency shift becomes largely dependent on the Doppler angle. In this situation, only small errors in measuring the Doppler angle may induce large errors in the frequency.

In ultrasound, there is an inverse relation between penetration depth and resolution. The higher the frequency of the ultrasound, the higher the resolution within the tissue. Since the amount of attenuation per unit distance is also increasing with tissue, higher frequencies are, however,

8.2Instrumentation

Nowadays, commercial CDI machines use pulsedwave Doppler. Pulsed-wave ultrasound probes emit series of pulses. These sound pulses are transmitted in the tissue, and the time until the reßected pulse wave is detected by the same ultrasound probe is measured. If a moving structure is present, there is not only a frequency shift in the returning ultrasound wave according to the Doppler effect but also a relative phase shift. Normally, the latter effect is used to extract the velocity data. The maximum velocity that can be detected with a pulsed Doppler probe depends on the pulse repetition rate, the vessel depth, the transmitter frequency, and the Doppler angle. If the velocity is too high, aliasing may occur, resulting in erroneous velocity extraction, but given that retrobulbar vessel is relatively small, this is not a limitation in ophthalmic use of CDI.

Since the delay of the incoming and the reßected wave is related to the depth of the reßecting interface, velocities can be measured at different depths within tissues. Combing pulsed-wave Doppler with B-scan imaging is called Duplex imaging. This technique of overlapping the Doppler signal with the gray-scale reßectance image allows for anatomical allocation of the velocity information. The direction of ßow is normally translated into a color scale on the ultrasound image (CDI). With this technique, the colored ßow information is visible in parallel to the gray-scale reßection image. Usually, ßow toward the probe is depicted in red and is arterial, while ßow away from the probe is depicted in blue and is venous. This means that blood ßowing from the heart shows up in red, whereas blood ßowing toward the heart shows up in blue.

Since CDI allows for relatively high time resolution [50], the blood velocity can be displayed as a function of time. Given the pulsatile nature of blood velocity in retrobulbar vessels, the systolic and the diastolic parts of the velocity signal can easily be identiÞed.

CDI uses a linear array transducer consisting of linearly arranged, sequentially excited piezoelectric elements. As mentioned above, the frequency of the Doppler probe is chosen as a compromise between resolution and penetration depth. A typical transducer for retrobulbar CDI has a frequency of 7.5 MHz, but some investigators have used up to 12.5 MHz, thereby providing better resolution but also weaker Doppler signals.

8.3Procedure

When the patient is examined in a lying position, legs should be uncrossed to avoid inßuences on venous return. The patient is instructed to look straight while the eyelids are closed. The examiner is seated behind the head of the patient while the base of the examinerÕs hand rests on the patientÕs forehead, with a Þnger is placed on the patientÕs cheek (Fig. 8.2). The tip of the probe is covered with a sufÞcient amount of acoustic coupling gel to provide adequate contact between the probe and the skin. The probe needs to be gently positioned on the closed upper eyelid in order to avoid mechanical force on the eyeball. This may increase intraocular pressure and thereby perfusion pressure, leading to a change in perfusion pressure as discussed later in this chapter.

The anatomy of the eye and the optic nerve head are identiÞed using the gray scale images in the B-scan mode (Fig 8.3a). Color Doppler is used to visualize the ßow within the vessels and allows

for identiÞcation of the appropriate vessels (Fig 8.3b). The sample volume is placed in the center of the vessel, and the angle is set parallel to the vessel to account for the Doppler angle (Fig 8.3c and d).

In order to obtain reliable and reproducible measurements using CDI, it is important to have a thorough knowledge of the retrobulbar vascular anatomy, as well as the characteristic waveforms of the different vessels, and the speciÞc locations that are conventionally chosen for measurement [51] (Fig. 8.4). The central retinal artery (CRA) and its corresponding vein lie close together in the middle of the optic nerve and cannot be measured separately by CDI. Therefore, a double waveform is obtained with a distinct pulsatile arterial waveform above the zero line and a gentle sinusoidal venous variation below the zero line (Fig 8.5a). The nasal and temporal short posterior ciliary arteries (NPCA and TPCA) are located on both sides of the optic nerve and should be measured at a position that is close to the optic nerve and as anterior as possible without receiving interference from the choroid. It is important to realize that individual short posterior ciliary vessels cannot be distinguished by CDI. Therefore, the obtained waveform represents the mass effect produced by a bundle of vessels rather than from individual ciliary vessels. These arteries produce a more uniform arterial pulse without a venous wave (Fig 8.5b). The ophthalmic artery (OA) is situated deeper in the orbit, and by convention should be measured on the nasal side of the optic nerve, immediately after it crosses the optic nerve (Fig 8.5c).

be distinguished by CDI because of their small size. Hence, bundles of vessels are examined, and the number of arteries contributing to the signal remains uncertain. Some authors also do not distinguish between nasal and temporal PCAs.

When measuring blood velocities using CDI, it needs to be considered that the Doppler angle inßuences the results. The direction of the sampling gate therefore has to be in good alignment with the angle of the measured vessel. If this is not the case, a measurement error is introduced, which is the more severe the larger the Doppler angle. In the CRA and the OA, the angle can usually be determined easily. In the PCA, however, the prob-

lem is more severe because of their smaller size, the more tortuous course, and the uncertain number of vessels within the probing volume.

The RI offers the advantage that it is independent of the Doppler angle. It is a dimensionless parameter that can take values between 0 and 1. The relation between RI and vascular resistance is, however, not entirely clear. Particularly, it seems that the value is largely dependent of the vascular compliance, which is deÞned as the ability of a vessel to distend and increase intravascular volume with increasing transmural pressure. In an in vitro model, where liquid was moved using a pulsatile pump, RI was independent of

Fig. 8.4 Anatomy retrobulbar vessels

Arteria lacrimalis

Arteria ciliaris posterior longa

Arteria ciliaris posterior brevis

Arteria ophthalmica

Fig. 8.5 IdentiÞcation of retrobulbar vessels

Arteria dorsalis nasi

Arteria ethmoidalis anterior

Arteria ethmoidalis posterior

Arteria centralis retinae