The customer does intra-vascular diagnostic ultrasound. In this novel technique, an ultrasonic catheter is guided through a patient's blood vessel. Throughout its helical or corkscrew-like path, the ultrasonic transducer-tipped catheter emits bursts of ultrasonic energy. Reflections from the vessel wall return into the transducer and can be used to characterize such features as plaque deposits.
The customer wishes to resolve features as small as 3 mm. The amount of time, D t, required for ultrasound to reflect off an interface that is a distance x away and return to the transducer is:
D t = 2x/v
The velocity of sound, v, in the blood vessel wall can be approximated as the velocity of sound in water, which is roughly v H2O = 1.5 km/sec = 1.5 mm/ms. A reflection from an interface 3 mm from the transducer, therefore, requires D t = 2x(3 mm) / (1.5 mm/ms) = 4 ns.
To determine the required sampling frequency, we simply take the reciprocal of 4 ns, since Frequency = 1/Time. In this case, Frequency = 1 / 4 ns = 250 MS/s.
This calculation shows that in order to achieve a 3 mm spatial resolution, the customer will have to sample in excess of 250 MS/s. The customer wants to capture reflected echoes from as deep as 3 mm for a maximum capture time of 2 x 3 mm/(1.5 mm/ms) = 4 ms.
Another important requirement of the application is derived from the way the catheter position is determined. The catheter moves at a uniform rate along its helical path and its position is determined by counting the number of repetitive ultrasonic pulses that occur. It is of paramount importance, therefore, that every ultrasonic pulse is captured by the A/D system. If the A/D system cannot re-arm itself before the next trigger occurs, that trigger will be missed and knowledge of the catheter's position will be lost. The ultrasonic pulses occur at a rate of 14 kHz. An entire scan of a patient's blood vessel consists of 20,000 ultrasonic captures and takes a few seconds. The customer will run the A/D system for optimal performance in an application written in C.
The GaGe solution is a CompuScope 8500 in an industrial grade GaGePC 586 with a compact PIA-740 chassis. This 7" x 9" x 16" chassis can be transported as carry-on luggage to different test sites. The CS8500 is a single-slot PCI 8 bit A/D card capable of digitizing one signal channel into a buffer of up to 8 Megasamples of on-board memory at up to 500 MS/s.
The customer's experimental set-up is shown in Figure 1 below. The analog output signal from the customer's ultrasonic pulser/receiver provides the 50 Ohm input to the CS8500. The pulser/receiver also has a synchronized trigger output that is used as the external trigger input to the CS8500.
The customer will operate the CS8500 in conventional memory mode, wherein data records are sequentially captured into the CS8500's on-board memory and then downloaded to PC RAM. Since the customer requires a capture time of 8 ms and will sample at 500 MS/s, each data record will have a length of 4 ms x 500 MS/s = 2 kS. The 20,000 data records will require a PC RAM buffer of 40 MB.
The qualified PCI bus of the GaGePC 586 can indefinitely sustain PCI transfers at 100 MB/s. For the 8-bit CS8500, one sample is equal to one byte, so that the transfer time required to download the 2 kS = 2 kB data record is 2 kB / 100 MB/sec = 20 ms.
The total time required to handle a data record is the sum of the capture time, the transfer time and the overhead time. The overhead time consists of such intervals as, for instance, the trigger re-arm time and the PCI transfer set-up time. The overhead time for the CS8500 is 30 ms.
The total time required to handle a data record is therefore 4 ms + 20 ms + 30 ms = 54 ms. This total time is the minimum pulse repeat interval (PRI), below which triggers may be missed. With these capture parameters, therefore, the CS8500 can keep up with a pulse repeat frequency (PRF) of 1 / 54 ms = 18.5 kHz, which exceeds the customer's requirement of 14 kHz.
The customer developed a simple test in order to establish that the CS8500 could continuously capture data records at high PRF without missing a single record. These tests were reproduced and are presented here in order to illustrate the capabilities of the CS8500.
A slow 300 Hz triangle wave serves as the input to the CS8500. A separate function generator provides a square wave with adjustable PRF to the external trigger input of the CS8500. The CS8500 captures sequential 1024 sample records at 500 MS/s. During the ~2 m s capture time, the slow triangle wave signal hardly changes. The average DC level from sequential records, therefore, should change by the slope of the triangle wave multiplied by the pulse repeat interval. Any other change in voltage between sequential data records indicates missed triggers.
The maximum achievable PRF is easily calculated as before from the capture, transfer and overhead times. The minimum pulse repeat interval is (1024 / 500 MS/s) + (1024 / 100 MS/s) + 30 ms = 42 ms for a maximum PRF of 24 kHz.
Capture of 1024 Sample records with a PRF of 4 kHz is plotted in Figure 2:
Clearly, no triggers have been missed and the expected 4 kHz / 300 Hz = 13.3 captures occur for every period of the triangle wave.
In Figure 3, the PRF has been raised to 12 kHz and exactly 12 kHz / 300 Hz = 40 captures occur for every triangle wave period:
Figure 4 shows capture conditions near the limit of the PCI bus capability and that with a PRF as high as 20 kHz, there is no loss of data records:
The customer now routinely captures thousands of records at a PRF of 14 kHz in this cutting edge medical ultrasound application. While the 500 MS/s maximum sampling rate is fast, the rapid PCI transfer capability is CompuScope 8500's most impressive feature.
We encourage you to contact us and discuss your medical application in more detail with our engineering team. GaGe can provide tailored custom data acquisition hardware and software solutions to meet specific application requirements.