To ensure patient safety during an ultrasound, machines display standardized indices that provide feedback to the operator. One of these is the Thermal Index (TI), which relates to the potential for the ultrasound beam to warm the tissues being examined. Understanding this index helps clarify how patient safety is monitored during a medical imaging procedure.
What is the Thermal Index in Ultrasound?
The Thermal Index is a real-time value displayed on the ultrasound screen that estimates the maximum potential temperature increase in tissue. It is calculated as the ratio of the machine’s current acoustic power output to the power needed to raise tissue temperature by 1°C. The TI is not a direct measurement of tissue temperature, but rather a standardized guide that helps the sonographer assess and manage potential thermal effects throughout the scan.
Ultrasound works by sending sound waves into the body, and as these waves travel through tissues, their energy is absorbed and converted into heat. While this effect is generally very small in diagnostic imaging, prolonged exposure or higher-intensity applications could theoretically lead to a significant temperature rise. The TI provides a consistent way for operators to be aware of the thermal output of the equipment in relation to the tissue being imaged.
The Thermal Index is part of a broader safety framework and exists alongside the Mechanical Index (MI), which relates to non-thermal effects. By monitoring these indices, sonographers can make informed decisions to adjust settings as needed. This ensures they capture a clear image while maintaining a focus on patient safety.
Different Types of Thermal Index
The way heat is generated and distributed by an ultrasound beam changes based on the type of tissue it encounters. There are three different Thermal Index models used to provide a more accurate estimation of thermal risk depending on the clinical situation. The ultrasound machine automatically displays the most appropriate TI based on the exam type selected by the sonographer.
The most common model is the Thermal Index for Soft Tissue (TIS). This model is used when the ultrasound beam is traveling through uniform soft tissues, such as during an abdominal scan or in the very early stages of pregnancy. The TIS calculation assumes that bone is not present near the focal point of the ultrasound beam, providing a safety guide for many common examinations.
When bone is present, the Thermal Index for Bone (TIB) is used. Bone absorbs significantly more acoustic energy than soft tissue, meaning it can heat up more quickly. The TIB model accounts for this by assuming that bone is at or near the beam’s focus, making it the appropriate index for scans like fetal ultrasounds in the second and third trimesters, when the fetus has developed a bony skeleton.
A third, more specialized model is the Thermal Index for Cranial Bone (TIC). This index is designed for transcranial exams, where the ultrasound beam must pass through the skull bone near the surface to image the brain. Because the bone is close to the transducer and has a strong effect on heating, the TIC provides a specific safety calculation for this unique application.
Thermal Index Values: Safety and Potential Effects
The numerical value of the Thermal Index provides a straightforward estimate of potential heating; a TI of 1.0 suggests a possible temperature rise of up to 1°C under specific conditions. Regulatory bodies and professional organizations have established guidelines for these values to ensure patient safety during diagnostic procedures.
For most examinations, sonographers adhere to the ALARA principle, which stands for “As Low As Reasonably Achievable.” This means they use the lowest ultrasound output power and shortest exposure time necessary to obtain a diagnostic-quality image. While the U.S. Food and Drug Administration (FDA) sets upper limits for TI, in practice, operators keep the index well below these maximums, often under 1.0 for the majority of scans.
Concerns about thermal effects are most pronounced in fetal imaging. Animal studies have shown that a sustained temperature increase of over 4°C for five minutes or more can be linked to developmental abnormalities. However, a temperature increase of less than 1.5°C has not been associated with any adverse effects on an embryo or fetus.
Sonographers have several methods to manage the TI value during an exam. They can reduce the acoustic output power, adjust the imaging mode, or avoid dwelling on a single spot for an extended period. Some applications, like Doppler ultrasound, generate higher TI values to function correctly. In these cases, the operator is especially mindful of scan duration, balancing the need for diagnostic information with patient safety.