Tumor ablation by using radio-frequency energy has begun to receive increased attention as an effective minimally invasive approach for the treatment of patients with a variety of primary and secondary malignant neoplasms. To date, these techniques have been used to treat tumors located in the brain, musculoskeletal system, thyroid and parathyroid glands, pancreas, kidney, lung, and breast; however, liver tumor ablation has received the greatest attention and has been the subject of a large number of published reports. In this article, the authors review the technical developments and early laboratory results obtained with radio-frequency ablation techniques, describe some of the early clinical applications of these techniques, and conclude with a discussion of challenges and opportunities for the future.

Tumor ablation with radio-frequency energy.

Continuous thermometry during a hyperthermic procedure may help to correct for local differences in heat conduction and energy absorption, and thus allow optimization of the thermal therapy. Noninvasive, three-dimensional mapping of temperature changes is feasible with magnetic resonance (MR) and may be based on the relaxation time T(1), the diffusion coefficient (D), or proton resonance frequency (PRF) of tissue water. The use of temperature-sensitive contrast agents and proton spectroscopic imaging can provide absolute temperature measurements. The principles and performance of these methods are reviewed in this paper. The excellent linearity and near-independence with respect to tissue type, together with good temperature sensitivity, make PRF-based temperature MRI the preferred choice for many applications at mid to high field strength (>/= 1 T). The PRF methods employ radiofrequency spoiled gradient-echo imaging methods. A standard deviation of less than 1 degrees C, for a temporal resolution below 1 second and a spatial resolution of about 2 mm, is feasible for a single slice for immobile tissues. Corrections should be made for temperature-induced susceptibility effects in the PRF method. If spin-echo methods are preferred, for example when field homogeneity is poor due to small ferromagnetic parts in the needle, the D- and T(1)-based methods may give better results. The sensitivity of the D method is higher that that of the T(1) methods provided that motion artifacts are avoided and the trace of D is evaluated. Fat suppression is necessary for most tissues when T(1), D, or PRF methods are employed. The latter three methods require excellent registration to correct for displacements between scans.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/1522-2586%28200010%2912%3A4%3C525%3A%3AAID-JMRI3%3E3.0.CO%3B2-V

Magnetic resonance temperature imaging for guidance of thermotherapy

We present a novel temperature-sensing technique using thermoacoustic and photoacoustic measurements. This noninvasive method has been demonstrated using a tissue phantom to have high temporal resolution and temperature sensitivity. Because both photoa- coustic and thermoacoustic signal amplitudes depend on the tempera- ture of the source object, the signal amplitudes can be used to monitor the temperature. A temperature sensitivity of 0.15°C was obtained at a temporal resolution as short as 2s , taking the average of 20 signals. The deep-tissue imaging capability of this technique can potentially lead us to in vivo temperature monitoring in thermal or cryogenic

Thermoacoustic and photoacoustic sensing of temperature

To improve the safety and efficacy of thermal therapy, it is necessary to map tissue temperature in real time with submillimetre spatial resolution Accurate temperature maps may provide the necessary control of the boundaries of the heated regions and minimize thermal damage to surrounding normal tissues Current imaging modalities fail to monitor tissue temperature in real time with high resolution and accuracy We investigated a non-invasive optoacoustic method for accurate, real-time monitoring of tissue temperature during thermotherapy In this study, we induced temperature gradients in tissue and tissue-like samples and monitored the temperature distribution using the optoacoustic technique The fundamental harmonic of a Q-switched Nd : YAG laser (λ = 1064 nm) was used for optoacoustic wave generation and probing of tissue temperature The tissue temperature was also monitored with a multi-sensor temperature probe inserted in the samples Good agreement between optoacoustically measured and actual tissue temperatures was obtained The accuracy of temperature monitoring was better than 1°C, while the spatial resolution was about 1 mm These data suggest that the optoacoustic technique has the potential to be used for non-invasive, real-time temperature monitoring during thermotherapy

/pdf/real-time-optoacoustic-monitoring-of-temperature-in-tissues-4ysil4fkuf.pdf

Real-time optoacoustic monitoring of temperature in tissues

MRI-based temperature imaging that exploits the temperature-sensitive water proton resonant frequency shift is currently the only available method for reliable quantification of temperature changes in vivo. Extensive pre-clinical work has been performed to validate this method for guiding thermal therapies. That work has shown the method to be useful for all stages of the thermal therapy, from resolving heating below the threshold for damage to ensuring that the thermal exposure is sufficient within the target volume and protecting surrounding critical structures and to accurately predicting the extent of the ablated volume. In this paper, these validation studies will be reviewed. In addition, clinical studies that have shown this method feasible in human treatments will be overviewed.

Quantitative MRI-based temperature mapping based on the proton resonant frequency shift: Review of validation studies

Open magnetic resonance (MR) scanners allow MR-guided targeting of tumors, as well as temperature monitoring of radio frequency (RF) ablation. The proton frequency shift (PFS) technique, an accurate and fast imaging method for temperature quantification, was used to synthesize thermal maps after RF ablation in an open 0.5 T MR system under ex vivo and in vivo conditions. Calibration experiments with 1.5% agarose gel yielded a chemical shift factor of 0.011 +/- 0.001 ppm/ degrees C (r2 = 0.96). Three gradient echo (GRE) pulse sequences were tested for thermal mapping by comparison with fiberoptic thermometer (Luxtron Model 760) readings. Temperature uncertainty decreased from high to low bandwidths (BW): +/-5.9 degrees C at BW = 15.6 kHz, +/-1.4 degrees C at BW = 3.9 kHz, and +/-0.8 degrees C at BW = 2.5 kHz. In vitro experiments (N = 9) in the paraspinal muscle yielded a chemical shift factor of 0.008 +/- 0.001 ppm/ degrees C. Temperature uncertainty was determined as +/-2.7 degrees C (BW = 3.9 kHz, TE = 19.3 msec). The same experiments carried out in the paraspinal muscle (N = 9) of a fully anesthetized pig resulted in a temperature uncertainty of +/-4.3 degrees C (BW = 3.9 kHz, TE = 19.3 msec), which is higher than it is in vitro conditions (P < 0.15). Quantitative temperature monitoring of RF ablation is feasible in a 0.5 T open-configured MR scanner under ex vivo and in vivo conditions using the PFS technique.

Temperature quantification using the proton frequency shift technique: In vitro and in vivo validation in an open 0.5 tesla interventional MR scanner during RF ablation

BACKGROUND AND OBJECTIVE: Lymphangiomas of the tongue and neck are uncommon benign congenital lymphatic tumors. These vascular lesions are difficult to treat, frequently recur, and can cause patients significant morbidity. Treatment may also be complicated by adjacent vital anatomic structures. Magnetic resonance (MR)-controlled laser-induced interstitial thermotherapy (LITT) has been proven to be a noninvasive safe treatment. Real-time monitoring of tissue temperature with thermosensitive sequences allows controlled coagulation necrosis. STUDY DESIGN/MATERIALS AND METHODS: LITT was performed in a lymphangioma specimen ex vivo. In four patients (eight procedures) with lymphangiomas of the tongue and neck, MR-guided LITT was performed with a percutaneous approach in a multiapplicator technique. The laser system consisted of a titanium catheter and a protective catheter. The dome of the fiber end had a diameter of 1.4 mm with an active length of 20 mm. Temperature sensitive sequences were used in a 0.5 T open-configured MR scanner with the proton frequency shift technique to map the spatial and temporal distribution of Nd:YAG laser effects (7 Watts, 30 pulses per second, 10 minutes/location). Postoperative MR follow-up was performed at 1 week and at 3 months. In three patients, partial resection of the tumor was performed 6 months after LITT. RESULTS: In three patients, MR clearly showed a diminished tumor volume. All four patients reported subjective amelioration and in three patients former functional problems, such as speech and swallowing were improved. MR thermometry allowed accurate demarcation of changes by heat and distinction of affected tumor volume (3.0 cm +/- 0.3 cm). The histology of the patients 6 months after LITT showed laser-induced fibrosis of former lymphatic tissue. CONCLUSION: The results suggest that LITT can be performed safely with tissue preserving of vital structures and can be effective in the treatment of deep tumors, such as lymphangiomas. However, given the nature of the lesion, the potential for recurrence exists no matter what modality is chosen.

Temperature mapping of magnetic resonance-guided laser interstitial thermal therapy (LITT) in lymphangiomas of the head and neck.

The purpose of this study was to evaluate interactive MR-guided joint puncture with intra-articular application of contrast agent MR-guided arthrography of the shoulder joint was successfully performed in three patients using an interactive guidance system implemented in an open-configuration MR system Visualization of the needle pathway and contrast inflow was comparable to that with conventional X-ray fluoroscopy The position of the intra-articular needle tip was accurately confirmed and subsequent MR arthrography was diagnostic in all cases

Real-time MR-guided joint puncture and arthrography: preliminary results

PURPOSE: To evaluate the feasibility and accuracy of monitoring radio-frequency (RF) ablation with an open-configuration, 0.5-T magnetic resonance (MR) imager. MATERIALS AND METHODS: Thirty-six in vivo RF ablation experiments were performed in porcine paravertebral muscle (n = 24) and liver (n = 12). A 90 degrees C tip temperature was applied for 3-9 minutes. MR images were acquired after continuous or during intermittent RF application. Temperature changes were monitored as signal intensity and proton-frequency-shift (PFS) alterations in two T1-weighted gradient-echo sequences. An update image was obtained every 2.5 seconds (20/10 [repetition time msec/echo time [TE] msec]) or every 5.0 seconds (40/20). A color-coded subtraction technique enhanced the signal intensity and PFS changes. Macroscopic coagulation size was compared with MR image lesion size. RESULTS: The RF application mode had no significant effect on coagulation size in muscle or liver (P > .05). Twenty-two of 24 coagulative lesions in muscl...

Radio-frequency-induced thermoablation: monitoring with T1-weighted and proton-frequency-shift MR imaging in an interventional 0.5-T environment.

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Temperature quantification using the proton frequency shift technique: In vitro and in vivo validation in an open 0.5 tesla interventional MR scanner during RF ablation

Temperature mapping of magnetic resonance-guided laser interstitial thermal therapy (LITT) in lymphangiomas of the head and neck.

Real-time MR-guided joint puncture and arthrography: preliminary results

Radio-frequency-induced thermoablation: monitoring with T1-weighted and proton-frequency-shift MR imaging in an interventional 0.5-T environment.