Imaging-based internal body temperature measurements: The journal Temperature toolbox

Figures & data

Figure 1. A simplified scheme of the PRFS phenomenon on atomic level. Left: A hydrogen atom in a water molecule is in an external magnetic field B₀. The electron cloud of the water molecule induces an opposite magnetic field Bind which has considerably smaller magnitude than B0. The induced field reduces the effect of the external magnetic field on the hydrogen nucleus (the proton) hereby the local field experienced by the proton is: B_loc = B₀ – B_ind. The electron cloud thus causes a small shielding effect. However, the adjacent water molecules are attracted to each other by a hydrogen bond (green dotted line) which has an effect on the electron cloud. This causes a decrease of Bind and so reduces the shielding effect Right: As the temperature rises the hydrogen bonds stretch, bend, and break due to thermal motion and the hydrogen bond does not interfere the electron cloud as strongly anymore. Therefore, the shielding effect on the hydrogen nucleus increases and the local field is reduced

Figure 2. A color scaled PRF-based temperature map acquired using phase imaging from pelvic area during MRI-guided high-intensity focused ultrasound treatment of uterine fibroid. Maximum heating can be seen in the focal point of ultrasound beam at the center of uterine fibroid

Figure 3. An example of the temperature dependence of the frequency difference between water and fat. A test tube containing 50 ml of margarine (fat content 82%, approximately the same as a mixture of white and brown adipocytes in human supraclavicular adipose tissue) was heated from 20°C to 32°C while MR spectra were measured from a single voxel (size: 10 × 10 x 30 mm³). Simultaneously an external thermometer was used for acquiring the temperature. Left: Few spectra at different phases of heating. Right: The frequency difference between water and methylene signal of fat plotted against temperature to show the correlation between the frequency difference and temperature. Reprinted with permission from [13]

Figure 4. The most important effects that temperature change has on ultrasound parameters. The change in the speed of sound appears as a shift of an echo, yet thermal expansion causes echo shifts as well [139]. Attenuation arises from ultrasound absorption and scattering [64]. The backscatter energy changes due to the change in the speed of sound and the density of sub-wavelength scatterers [140]

Figure 5. The density of an object affects directly how much radiation passes through the object

Figure 6. A simple demonstration where 300 ml of heated water was cooled down from 53°C to 33°C. During the cooling, a CT image was acquired every 2 minutes and the temperature of the sample was measured with an external thermometer. Top: CT images of the water at different phases of cooling. The color scale has been set from 1.2 to 10 HU. A non-colored image of the container has been set on the background. Bottom: CT-numbers plotted against the temperature to show the correlation between CT-number and temperature

Figure 7. A near-infrared light source transmits light into the target tissue and an optical receiver detects the light reflected from the tissue. Tissue temperature can be estimated based on the temperature-dependence of the water absorption spectrum

Figure 8. In microwave radiometry, a passive receiver measures the intensity of thermally generated microwave electromagnetic noise. The measurement depth can be altered by changing the frequency of the receiver as lower frequencies originate from deeper tissues

Figure 9. A pulsed laser light travels through tissue in which optically absorbing substance, such as hemoglobin absorbs the energy of the light. This makes the temperature of the object rise rapidly and the object experiences a thermoelastic expansion. This in turn creates a shockwave which travels through the tissue and can be detected at the surface with a piezoelectric sensor such as in an ultrasound transducer

Table 1. Comparison of internal temperature imaging methods

Method	Absolute or Relative Thermometry	Spatial Resolution	Tissue Penetration	Availability	Cost	Harm to Target Tissue	Implementable using current clinical equipment	Applicable for bedside use
MR	Absolute^a and Relative^b	High	High	Widely	High	Heat	Yes	No
Ultrasonography	Relative	Moderate	High	Widely	Moderate	Heat	Yes^c and No	Yes
Computed Tomography	Relative	High	High	Widely	High	Ionizing Radiation	Yes	No
Near-Infrared Spectroscopy	Relative	Moderate	Low	Moderately	Moderate	None	Not available	Yes
Microwave Radiometry	Absolute	Moderate	Low	Moderately	Moderate	None	Not available	Yes
Photoacoustic Thermometry	Relative	Moderate	Low	Moderately	Moderate	None	Not available	Yes

^aSpectroscopy, Proton Density, and Temperature-Sensitive Contrast Agents.

^bPRF, T1 relaxation time, T2 relaxation time, and Diffusion.

^cFew clinical ultrasound systems allow implementation of thermography applications.

Table 2. List of technical terms

Chemical shift	Resonance frequency of a nucleus in relation to the frequency of a standard chemical compound. See Shielding.
Hydrogen bond	Electrostatic force between molecules in which hydrogen is bound to an electronegative atom, e.g. oxygen. The charge of these molecules is polarized and therefore the negative end of the molecule attracts to the positive end of a similar molecule.
Brownian motion	Rapid, irregular movement.
Shielding	The effect of an electron cloud on the resonance frequency of an atom. In an external magnetic field the electron cloud induces an opposing magnetic field which reduces the local magnetic field of the atom leading to reduced resonance frequency (and greater chemical shift). The higher the electron density the stronger the shielding is.
RF-spoiled gradient echo sequence	Magnetic resonance imaging sequence in which echo is produced using a bipolar gradient and a phase of the radiofrequency pulses is systematically incremented in order to reduce coherence artifacts and repetition time.
Gyromagnetic ratio	The ratio of the magnetic momentum in a particle or system to its angular momentum.
Magnetic susceptibility	The ratio of the magnetization of a material and an external magnetic field. A measure of how much a material will become magnetized in a magnetic field.
Dipole-dipole interaction, Dipolar interaction	Interaction between two magnetic dipoles. Most important mechanism responsible for relaxation of the magnetization in biological tissues
Larmor frequency	The rate of precession of the magnetic moment of the proton around the external magnetic field.
Variable flip angle gradient echo sequence	Magnetic resonance imaging sequence in which echo is produced using a bipolar gradient and sequence is repeated with variable flip angles.
B1 inhomogenity	Non-uniform excitation of the atoms resulting in faulty flip angles within the object.
Transverse relaxation time, spin-spin relaxation time, T2 relaxation time	Time constant for decay of transverse magnetization after radiofrequency pulse.
Turbo-spin echo readout	Magnetic resonance imaging technique in which the phase-encoding gradient is changed between echoes and therefore multiple lines of k-space (i.e., phase-encoding steps) can be acquired within a given repetition time which may reduce imaging time significantly.
Diffusion gradient	A strong pulsed gradient that is used as pairs in order to cause a signal loss from moving spins.
Paramagnetic	A material including oxygen and metal ions and having a small positive susceptibility. Decreases relaxation times.
Shear modulus	The ratio of shear stress to the shear strain.
Scattering cross-section	A quantity which is proportional to the rate of radiation–target interaction.
Echogenicity	The ability of a material to return an echo in ultrasound examinations.
Cathode	A negative electrode from which electrons are emitted toward a positive electrode, anode.
Compton scattering	A phenomenon in which a photon strikes an electron and scatters. The direction and energy of the photon are changed.
Radiolength	Wavelength. In a periodic wave, the length of one complete wave pattern, i.e. the distance from any point to the corresponding point on the next repetition of the wave shape.
brightness temperature	Brightness temperature or radiance temperature is the temperature of a black body in thermal equilibrium with its surroundings, in order to duplicate the observed intensity of a gray body object at a frequency
Lossy body	A medium in which a significant amount of the energy of a propagating electromagnetic wave is absorbed per unit distance traveled by the wave.
Piezoelectric sensor	A device that measures pressure by piezoelectric effect. An electric charge is created when pressure is applied to a piezoelectric material.
Thermoelastic expansion	Volume expansion due to temperature rise.
Isothermal compressibility	Measure of relative volume change as a response to pressure change.

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