Dr. Neil Friedman takes a look at what is state of the art in ophthalmological ultrasound and provides some tips on choosing the right ultrasound technology for your practice.
Ultrasonography has wide application in ophthalmology. Two types of devices, A-scan and B-scan, are used diagnostically.
A-scan ultrasound refers to a one dimensional amplitude modulation scan. It is most commonly used for measurement of axial length (AL), which is critical for intraocular lens (IOL) calculations prior to cataract surgery. Another application of A-scan is to determine the ultrasonic properties (i.e., high internal reflectivity, low internal reflectivity) and size of posterior segment masses, which is used in conjunction with B-scan imaging to differentiate these lesions.
B-scan ultrasound refers to a two dimensional, cross-section brightness scan. It is typically used to evaluate posterior segment and orbital pathology, particularly when the ocular media are cloudy and a direct view is not possible. High resolution B-scan or ultrasound biomicroscopy (UBM) uses higher frequency probes (20-50 MHz vs. the standard 10 MHz) to provide detailed images of anterior segment structures, especially the angle, iris, and ciliary body. Accurate measurement of the eye’s AL is essential for attaining excellent refractive outcomes after cataract surgery. This is especially important in the modern era of advanced technology IOLs and femtosecond laser-assisted cataract surgery.
The most common reason for a postoperative refractive surprise is an error in axial length because a 1 mm error in AL measurement produces approximately a 3 D error in postoperative refraction. A myopic surprise is caused by an erroneously short AL measurement, and a hyperopic surprise occurs with a falsely long AL value. Although optical biometry with instruments such as the IOLMaster 500 Biometer from Carl Zeiss Meditec and LENSTAR LS 900 Optical Biometer from Haag-Streit has become a more popular method for AL determination, A-scan is still required in some patients, such as those with mature or dense posterior subcapsular cataracts and central corneal scars.
Ultrasound systems are available as single mode devices or combination systems with dual or multi-functionality. Here is a comparison of some of the available products:
A-Scan Ocular Ultrasound Systems:
Eye-Scan A-Scan from Sonogage:
This is a basic, portable A-scan for biometry and IOL calculations. It uses the latest electronic and transducer technologies for excellent accuracy and reliability.
300A+ PacScan Plus from Sonomed:
This portable, digital A-scan has a large color touchscreen and USB interface for EMR archiving. It features built-in immersion capability and up to nine IOL calculation formulas including 3 for post-refractive eyes.
DGH 6000 A-Scan Scanmate A from DGH:
This device is a portable A-scan unit combined with the processing power, data storage and connectivity advantages of a PC. Patient records can be exported in a format compatible with EMR/EHR systems. The IOL calculator includes modern formulas, a post-refractive calculator, and lens constants can be optimized automatically.
B-Scan Ocular Ultrasound Systems:
EZ Scan B-5500+ from Sonomed:
This instrument is a high resolution, portable B-scan with touch screen operation that features real time display, simultaneous vector selectable A-scan, and gray scale or color imaging.
DGH 8000 B-Scan Scanmate from DGH:
This self-contained, portable B-scan unit is a multi-mode ultrasound imaging system that can interface with most computers via a USB 2.0 port. High resolution video loops and still images are obtained rapidly, and post processing features include playback, gain adjustment, contrast and image intensity variation, distortion-free zoom, and dual digital calipers for distance measurements.
Combination A-Scan & B-Scan Ocular Ultrasound Systems:
EZ Scan AB5500+ from Sonomed:
This compact, portable, digital A-scan and B-scan instrument is a combination of the popular EZ B-5500+ and the 300A+ devices.
Ultrasound US-4000/500 from NIDEK:
This compact, lightweight system performs A-scan, B-scan, and pachymetry. The A-scan and pachymetry modes allow automatic or manual measurements. It can be configured as a single or combination device. There is a tiltable color LCD touch-screen monitor so no PC is required.
ODM-2100 Ultrasonic A/B Scan from Meda Co.:
This combined A/B scan device has a 10-inch monitor that displays real time image, allows playback for 56 images, and has dual image in one picture. The A-scan features manual and automatic measurement for normal, aphakic, special, and dense cataracts, and comes with 6 IOL calculation formulas. It is also portable and PC compatible. The 2100S model has a 7-inch monitor and the 2200 model has a color LCD display.
Compact Touch 3-in-1 Ultrasound System from Quantel:
This instrument performs A-scan, B-scan, and pachymetry. It is a compact, portable, lightweight unit that has a touch-screen and does not require a PC. Data transfer is accomplished via 2 USB ports, 1 Ethernet port and connection to medical servers with EMR. The A-scan is equipped with Probeam for faster measurements. There is automatic detection of the scleral spike, and the velocity is specific and adjustable for each tissue media. It measures all eye types (normal, dense cataract, long eye, aphakic, pseudophakic in PMMA, acrylic and silicone), has special modes for silicone filled and post-refractive eyes, and includes 6 implant calculation formulas and 6 post-refractive formulas. The high quality B-scan distinguishes globe and orbit structures, and the B-scan biometer offers automatic axial length measurement using a B-scan image so it is ideal for cases of advanced myopia or staphyloma. Post-processing measuring tools include calipers, areas, angles, markers, and comments. The pachymetry module allows for glaucoma and refractive surgery screening. There are four measurement methods: central measurement or cartographic map (automatic, continuous and scanning mode), and intraocular pressure and corneal thickness correlation tables are included.
Eye Cubed from Ellex:
This instrument has A-scan, B-scan, and UBM functions. The A-scan mode offers immersion or contact biometry with an internal fixation light. Scan acquisition is automatic or manual, and there is built in pattern recognition with automatic scleral echo detection. IOL calculations can be performed with 4 different formulas. The B-scan modes are the standard 10 MHz posterior segment and the 40 MHz UBM wide-field anterior segment. The former has a 25 frame/sec image acquisition rate and 10 second movie capability while the latter has a 13 frame/sec image acquisition rate and 20 second movie capability. Image depth for the B-scan is 45 mm and that of the UBM is 12.5 mm. Images can be stored on the removable one-terabyte hard drive, and the system has a built-in DVD burner, 5 USB 2.0 ports, and full network and printer capabilities.
With so many ultrasound devices on the market, choosing the appropriate instrument for your practice depends upon which features you need (i.e., modality (A-scan, B-scan, combination), size, portability, connectivity, etc).
As an anterior segment surgeon in a major metropolitan area, my primary use for this technology is for cataract surgery, and I have both ultrasound (immersion A-scan) and optical (IOLMaster) biometry. While I mainly perform biometry with the IOLMaster, I do find that 1-2% of my preoperative cataract patients require an A-scan to measure the length of their eye. I do not need B-scan capability because I very rarely have to order such a test, and when I do, there are several nearby posterior segment practices and academic centers that can perform this test for me. Therefore when I selected a device, I chose a small, portable A-scan instrument.