How Are Temperature Sensors Used in PET/CT Scanners?

Have you wondered why type of sensors are used in medical imaging instruments?

Sensors used in electronic medical instruments convert various stimuli into electrical signals for monitoring and diagnosis. Medical Sensors hunt physical, biological, and chemical signals and provide a way for those signals to be recorded and measured. Medical technology relies on these tiny modules to improve the effectiveness and safety of medical instruments and make them easier to operate. You may have noticed the presence of medical imaging tools like X-rays, CT scanners, MRIs, and PET scanners. These tools integrate sensors for an accurate diagnosis.

So, what type of sensors are used in medical imaging instruments like CT and PET scanners? A variety of them, to be precise.  These sensors measure the patient's body parameters and keep a performance check of the medical instrument for accurate and reliable performance. Temperature, pressure, humidity, image, sound level, and radiations are some physical properties that are sensed and further processed for diagnosis by these instruments. In this Tech Spotlight, we will focus on the importance of temperature sensors in medical scanning devices. You will understand the usage, characteristics, and types of temperature sensors used in medical imaging equipment. Some practical examples of temperature sensors found in medical imaging equipment are also discussed  at the end.

Overview to CT and PET Scanners

Computed Tomography (CT) and Positron emission tomography (PET) scanners are categorized under medical imaging instruments that generate visual representations of the interior of a body for clinical analysis. These instruments send a signal into the body and ascertain its reaction to the signal. They also learn how that reaction affects the original signal or a return signal. Both technologies are non-invasive and are used to create images of the body's tissues, organs, and bones. We will consider the purpose and use of these techniques before delving into the sensor technology utilized in such scanners.

• CT Scanner: A Computed tomography (CT) machine appears similar to a large box with a central tunnel. You lie on a sliding table that slides you in and out of this tunnel as the machine rotates around you and produces cross-section images of your body. This action generates three-dimensional (3-D) images of your internal body structures using complex x-ray and computer-aided tomographic imaging techniques. CT scanners are used to visualize bones, internal organs, and blood vessels precisely. Upper body parts like the brain, neck, spine, chest, and sinuses are commonly scanned.

• PET Scanner: Positron emission tomography (PET) imaging systems construct 3-D medical images by detecting gamma rays emitted after the patient is injected with radioactively doped sugars (radiotracer). A PET scan reveals metabolic changes in an organ or tissue at the cellular level or earlier, whereas a CT detects changes a little later after the disease begins to cause changes in the organs or tissues structures. PET scans are often partnered with CT scans, X-rays, or MRI scans to obtain a clear idea of how tissue and organs function in real-time. These scans are used to detect cancer, heart problems, brain disorders, and other central nervous system disorders.

Let's dig a little deeper: Temperature Sensing in PET/CT Scanners

We will now discuss the significance of temperature sensors in these systems and why they are crucial for medical instruments.

Proper temperature is critical to electronic system operation and must satisfy particular specifications. Since Tomography scanners generate significant heat during the use of coils and focusing electrons, the risk of overheating and damage of internal components is notably increased. This may also influence the acquired data by the addition of further noise. Also, the tomography exam room operating temperature should be kept as close to 72° F as possible (never to exceed 75° F or fall below 64° F).

Temperature sensors are a mandatory component of image scanning devices, as shown in Figure 1. The temperature monitoring system senses changes in the instrumental environment and immediately transmits a signal to the system brain (microcontroller/processor) for corrective measures or to alarm the system. Medical imaging instruments are expensive pieces of equipment, requiring proper system monitoring and maintenance. Physical sensors, like temperature sensors in the instruments, safeguard the system from further damage and increase the instrument's life span.

Figure 1.Sample Block diagram of a CT imaging system.

What should be the characteristics of a temperature sensor used in medical equipment like CT and PET scanners? Is it identical to those used for general applications? Or are the characteristics of temperature sensors different for medical instruments? Medical systems need real-time, reliable, and accurate diagnostic results. Temperature sensors embedded in CT and PET scanners must have the following attributes for uninterrupted services:

• High Accuracy
• High Precision
• High Sensitivity
• Linearity
• Reproducibility
• Low Power Consumption
• Wide Operating Temperature Range

RTD Temperature sensors

Temperature Sensors are available in different sizes, materials, and characteristics depending upon their respective applications. RTD (Resistance Temperature Detector) temperature sensors are widely used in medical imaging scanners for health monitoring. The attributes of RTD makes it ideal to be used in CT and PET scanners.

An RTD (Resistance Temperature Detector) is a temperature sensor in which the resistance depends on temperature; the change of sensor resistance correlates to temperature changes. The most stable RTDs are made of platinum. RTDs typically have a ceramic or glass core with a thin winding of platinum metal.  The platinum metal is chosen for its stability. Alternative configurations use different insulation and winding materials, which results in various performances and temperature ranges.

Resistive temperature detectors have positive temperature coefficients, and their output is extremely linear. These detectors produce and yield highly accurate temperature measurements. A typical RTD (PT100) has a base resistance of about 100Ω at 0^oC, with a standard operating temperature range between -200 to +600^oC. These sensors are unaffected by corrosion or oxidation over many years, and can be easily replaced. Cumulatively, all these traits of high stability, high accuracy, linearity, and noise immunity make RTD a perfect choice for monitoring temperature in medical scanners.

The RTDs, however, are passive elements, and they should be connected to some control or monitoring equipment to measure temperature. A 2-wire, 3-wire, and 4-wire (Wheatstone) are conventional wiring methods.

Which type of temperature sensor would you is most commonly used in medical imaging devices? Take our poll at the end of this article or leave a comment below.

Omega's RTD Sensors

RTD elements come in several types conforming to different standards, capable of varying temperature ranges, with various sizes and accuracies. The Omega RTD-830 series makes a perfect choice for measuring temperature variations in certain parts of medical scanning devices. It is a surface mount RTD offering high stability and minimum electrical interference. The aluminum housing allows fast transfer of heat to/from the sensing element. The housing contains a clearance hole and can be bolted or cemented into place.

Figure 2. RTD-830 Surface RTD Sensor

The sensor is a 100 ohm Class A DIN Platinum element and hence furnishes accuracy of ±0.06% at 0°C (DIN Class A Standard). The temperature coefficient variation of RTD-830 series is 00385 Ohm/Ohm/°C, and the sensor can operate between a temperature range of -60 to +230^o

Temperature Transmitter for RTD Probes and Sensors

Since RTD temperature sensors are purely resistive devices, they also generate imperceptible signals. This implies the need for some additional accessories for signal conditioning to further inter-connect the analog signals with data acquisition, monitoring, or control systems. A temperature transmitter in such instances answers the purpose. Omega's 2-wire SPRTX-SS Series temperature transmitters convert RTD signal to a 4-20 mA output signal and are the ideal solution for many temperature measurement applications. Such temperature transmitters are high-performance, low-cost, industrial transmitters designed for direct connection to most CIP (clean-in-place) sanitary Pt100 probes and sensors that incorporate an M12 style connection. The signal conditioner converts the resistive change of a 100 Ω, 0.00385 RTD probe or sensor into an industry-standard 2-wire, 4 to 20 mA analog output across a dedicated temperature range. This unique probe is ideal for areas with space limitations where traditional head connections are too big to fit.

Examples of RTD Sensors