A sneak peek into the signal chain that enables accurate measurement of blood flow deep within the body

BY
EVAN SAWYER, Product Marketing Engineer
Texas
Instruments
www.ti.com

Medical
ultrasound is a non-invasive method for imaging the body’s internal structures
like organs. It processes by transmitting high-frequency sound waves into the
body and measuring the reflections that occur at various boundaries such as
those between bone and muscle.

There
are several different types of ultrasound, such as brightness modulation
(B-mode), pulsed-waveform (F-mode), and continuous waveform Doppler (CW
Doppler). Each has its own benefits and drawbacks, including what can be imaged
within the body and the level of penetration depth.

In
this article, we will take a closer look at CW Doppler and how a high-accuracy
signal chain enables accurate measurement of blood flow deep within the body.

How CW Doppler works
In
the medical world, CW Doppler ultrasound is used to determine the flow of blood
through key veins or structures. In cases where blood flow is abnormal, perhaps
within a vein or around a potential tumor location, ultrasound can be used to
diagnose the cause and help medical practitioners determine the best method of
treatment.

Fig.
1 shows the method using CW Doppler
to measure the rate of blood flow in a vein.

Special_Medical_TI_Fig1

 

Fig. 1: CW Doppler measurement of blood
flow.

In
CW Doppler, half of the transducer array (transmitter/receiver array)
continuously transmits a high-frequency audio sine wave, denoted by Tx in Fig. 1. A portion of this signal is reflected by the blood cells
flowing within a targeted vein. This reflected signal is received by the second
half of the transducer, denoted by Rx.

When
the signal reflects off of the flowing blood cells, it experiences a shift in
frequency related to the velocity of the cells. The change in frequency — known as the Doppler Effect or shift — can be compared to the known frequency of the
transmitted signal to determine the velocity of the blood cells.

Because
the orientation of the vein within the body and the direction of blood flow can
be unknown to the ultrasound operator, the received signal is fed through a
mixer and low-noise summer to demodulate the I and Q signals — the real and imaginary portions of the
received signal.

These
signals are simultaneously measured by a high-precision data acquisition system
and finally processed by a host — for example, an
FPGA or a DSP — to determine the velocity
of the cells. Fig. 2 shows a block diagram of the Rx
portion of the CW Doppler signal chain. This signal chain is duplicated for
both the I and Q signals.

Special_Medical_TI_Fig2

Fig. 2: Block diagram of a CW Doppler
signal-conditioning circuit replicated for both I and Q measurement.

Enter the ADC
While
the generation of the transmitted signal is a critical part of an ultrasound
machine, the receive system will be the focus of this article because of the
difficulty in accurately measuring the reflected signal.

In
comparison to Tx, which can range from ±2.5 to ±100 V, the Rx signal can be as
low as ±10 µV and as high as ±500 mV. Accurately measuring Rx at the lower end
of its signal range and amidst noise created by the transmitter or by undesired
reflections in the body requires a high-precision analog-to-digital converter
(ADC).

Because
the ADC is handling the conversion of the I and Q signals, selecting one that
is high-performance is crucial to maintaining system accuracy. Due to the high
range of the received signal (10 µV to 500 mV) in the CW Doppler system, the
ADC must have a very high dynamic range, which is comparable to its
signal-to-noise ratio (SNR).

For
the system to provide an accurate result in a timely manner, the ADC should
have low latency — time between the
beginning and end of conversion — and
provide a raw conversion, the one without filtering. Finally, the ADC must have
a high enough resolution so that it is capable of measuring the change in the
received signal down to only several µV.

Today,
the successive approximation register (SAR) ADC has become a preferred choice
for CW Doppler systems because it meets each of the requirements outlined
above. Additionally, a signal chain using a SAR ADC can be customized to
optimize performance in the system.

For
example, it can be designed to be used with a specific transducer or measure
the blood flow at a certain depth within the body. Being able to customize the
signal chain for a specific function can result in highly accurate
measurements. Select SAR ADCs also integrate a portion of the signal chain,
such as the voltage reference driver shown in Fig. 3.

Special_Medical_TI_Fig3

Fig. 3: External vs. internal voltage
reference buffer.

A more stable reference point
There
are several benefits to integrating a voltage reference driver. During a SAR
ADC’s conversion cycle, the input signal is compared to a reference voltage to
determine the value of the input. During this comparison process, the ADC will
draw current from the reference to charge a bank of switched capacitors.

While
this is occurring, the reference voltage is prone to voltage droop on the
output if not supplied with sufficient current, and this can result in an
inaccurate conversion. The voltage reference buffer is used to supply the
current needed by the ADC so as to avoid a voltage droop.

Integrating
the voltage reference buffer into the ADC reduces the overall system size by
eliminating a discrete amplifier, which is critical for portable or
high-channel-count CW Doppler systems. Moreover, the buffer is designed
specifically for the ADC, further improving overall system performance.

The
integrated buffer enables the use of a single voltage reference with multiple
ADCs, further reducing board space and cost for high-channel-count systems.

Because
ultrasound is non-invasive and capable of imaging a variety of systems inside a
body, it is becoming more widely accepted as a fundamental piece of medical
equipment. Today, it can be found anywhere from metropolitan hospitals to
remote locations with limited, if not zero, sources of power.

To enable better imaging of a body and identification
of the best medical procedure, it is imperative for the ultrasound machine to
have a high accuracy signal chain that incorporates an ADC capable of precisely
measuring the desired signal.