A medical imaging system produces images that allow accurate and timely diagnoses and improves evaluation of such images using protocols of increasing quality and standardizing [1]. In particular, Magnetic Resonance Imaging (MRI) is a highly sophisticated imaging modality commonly used in clinical diagnoses [2]. It is a non-invasive technique which provides images of internal tissues without applying ionizing radiations [3, 4]. MRI scanners combine three different electromagnetic fields, i.e., static magnetic field (typically symbolized by B0), radiofrequency (RF) field (B1), generated by coils, that can operate in transmit and receive mode with high signal-to-noise ratio and wide field homogeneity [5], and magnetic field gradients in the three spatial directions, to select the region of interest for spatial encoding of image. The possibility that some hazards for patient can be associated with performing MRI diagnostic images concerns, above all, B0, RF (B1) and magnetic field gradient.
It is necessary to establish regular and adequate Quality Assurance (QA) procedures to guarantee the maintenance of consistent image quality over the imaging equipment lifetime and to ensure safe and accurate operation of the whole process with respect to patients, workers and population.
Every QA program should include periodic tests to identify any degradation in image quality [6] reducing the ability to detect and correctly interpret abnormal findings that could imply a decrease in diagnostic accuracy.
Such tests, known as Quality Control (QC) tests, play a key role within the QA procedure because they enable a complete evaluation of system status and image quality [7, 8].
MRI QA programs for images evaluation are well established [9,10,11,12,13], whereas evaluation of proper operating of hardware components of scanners, such as coils, bed, is more difficult, because no specific standardized procedures and guidelines are available. In this context, every responsible person, expert in charge, shall set up his own program.
About the coils, the RF pulses generated for image production are transmitted through free space from coils to the patient.
Such pulses can induce electrical currents in conducting materials and in human body can heat tissues quite dramatically, resulting in superficial skin burning if there is a malfunction or malposition [14].
There are unquestionable evidence of MRI-related reports of patients’ burns (thermal injuries or incidents) that strongly indicate the need for increased awareness, education and understanding concerning this rare, but real, MRI-related hazard [15].
Only for specific absorption rate (SAR) evaluation, the safety of RF exposure to clinical MRI is regulated by the US Food and Drug Administration and the International Electrotechnical Commission’s guidelines for RF exposure adopted in Europe [16, 17]. The SAR, which describes potential heating of the patient’s tissue due to RF, is automatically calculated by the system when is set a sequence to acquisition.
In all cases, coils should be periodically checked before use on patient to ensure the absence of frayed insulation, exposed wires and other hazards [18].
Many different techniques have been proposed for coil efficiency estimation [19, 20].
Some of these use methods that produce images directly whereas probe techniques generate B1 map from different points in the space. The perturbing sphere method has recently been applied to map the RF fields from MRI coils but it can provide accurate efficiency measurements only when the electric and magnetic field components are well separated in space [21, 22].
In this paper, a simple method that use non-contact Infra-Red (IR) thermography is presented in order to evaluate the proper operating of coils.
This method involves the use of an IR Camera to detect if a temperature variation occurs during MRI scans.
It is well known that all objects with temperature above the absolute zero emit electromagnetic radiation known as thermal radiation [23,24,25].
The wavelength range of this radiation is (0.7–350) µm [26]; this range can be subdivided in three bands: near infrared (NIR), medium infrared (MIR) and far infrared (FIR). According to thermal radiation theory, blackbody is considered as a hypothetical object that absorbs all incident radiations and radiates a continuous spectrum according to Plank’s law [25]. The total emissive power of a blackbody is described by Stefan–Boltzmann’s law (Eq. 1):
where E is the total emissive power, σ is the Stefan–Boltzmann’s constant and T represents the absolute temperature in kelvin.
For real bodies, Eq. (1) is modified in the following Eq. (2):
$$E=\varepsilon \sigma {T}^{4}$$
(2)
where ε represents the emissivity of the emitting surface.
In this experiment, thermography is used during MRI phantom tests, to evaluate the heat delivered by coils during scans, to inspect, without interference non-destructive manner, eventual surface crack in materials and to individuate superficial abnormal behavior. The local temperature increase, besides, can be correlated to electric field around the coil [27].
The thermal camera detected the infrared (IR) energy emitted by the investigated coils and created electronic images based on information about the temperature differences [28]. In fact, due to thermal conductivity and specific heat, each object or its region has its own temperature; a thermal camera can detect all the field of view, and the objects appear as distinct in a thermal image, although less detailed [29,30,31].
