Health care is a hot topic today. The high-performance digital-to-analog converter (DAC) AD5791 delivers uncompromising performance.
Governments have invested a large amount of money in health care research and system building to protect the well-being of the people and ensure their physical and mental health. Require active prevention of disease, rather than passive response, and the correct diagnosis of some conditions.
In this situation, medical imaging systems are playing an important role. With the help of images, doctors can observe patients in more detail and understand the disease without surgery. In addition, the image can help the surgeon study the case before starting the surgery.
A wide variety of imaging methods are available today, such as computed tomography, X-ray, ultrasound and magnetic resonance. Various systems have their advantages and disadvantages. They can be used to generate still images of a certain part or organ of the human body, and can also be used to generate dynamic images for doctors to verify or study the activity of organs. Motion pictures are also used in some operations.
There are also differences in the imaging capabilities of different systems. X-ray technology is ideal for diagnosing bone diseases. Ultrasound uses sound waves to monitor the fetus, imaging the blood flow in organs and in the atria, ventricles, and blood vessels. MRI is suitable for imaging soft tissue. For all of the above medical imaging systems, Analog Devices has a corresponding technical solution. This article focuses on a new high-resolution DAC developed for high-performance applications such as magnetic resonance imaging (MRI).
Magnetic resonance imaging
MRI is mainly used to produce high-quality images of the human body, which can be used to detect diseases and distinguish tumors from normal tissues. 70% of the human body is fat and water, both of which contain hydrogen atoms. MRI utilizes magnetic imaging of hydrogen atoms.
MRI requires a strong homogeneous magnetic field. The unit of magnetic field strength is Tesla (T). 1 Tesla is equal to 10,000 Gauss, and the Earth's magnetic field strength is about 0.5 Gauss. Current MRI systems use a magnetic field strength of 1.5 T to 3 T, sometimes even reaching 7 T. Such a strong magnetic field is generated by the superconducting coil magnet and the patient is in a magnetic field. Figure 1 shows the positional relationship of the patient to the MRI scanner coil.
Figure 1. Positional relationship between patient and MRI coil
For a 1.5T system, the applied frequency is approximately 64 MHz, and for a 3T system it is 128 MHz. This will cause the protons inside the body to spin, parallel or anti-parallel to the direction of the magnetic field, and thus in a low energy state or a high energy state. The higher the magnetic field strength, the greater the energy difference between the two spin states. After the applied magnetic field is removed, the protons forward the magnetic energy and the forwarded magnetic energy is measured by the receiving coil or antenna. Designed with sensitive preamplifiers, gain blocks, and high resolution ADCs, these antennas meet the overall dynamic range requirements of 120 dB to 140 dB. Since we are only interested in imaging small faults in the human body, we need to add a gradient to this homogeneous magnetic field.
Figure 2. High resolution gradient control loop
A large coil is used to transmit this gradient signal (magnetization vector) to provide a response from a single fault we are interested in. Figure 2 shows the gradient control loop implemented in an MRI system. The signal sent to the gradient coil is generated by an amplifier with an output power of several megawatts. The frequency range is quite low, so the key requirements are stability, high linearity and low drift. This is the characteristic of the 20-bit DAC AD5791.
Why use a 20-bit DAC?
As mentioned above, the power required to drive the gradient coils of the MRI system is in megawatts. If a 2 MW amplifier is driven with only 16-bit accuracy, 1 LSB will be equivalent to a minimum of 30 W steps! This is why you need a higher resolution DAC. If properly designed, a 20-bit DAC can achieve system performance levels of 2 W/LSB.
The frequency of the gradient signal is only a few hundred Hz, so high stability, low short-term drift and low noise are necessary to meet the overall requirements. To design an ultra-low noise low frequency system, the device used must be carefully examined. The filter adds noise and phase shift, so the selected signal chain device must be able to achieve good DC performance and low noise in the low frequency band close to DC. The AD5791 combines high resolution, high stability and low noise, making it the perfect choice for this application.
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