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Diagnostic imaging plays a key role in all steps of radiofrequency (RF) tumor ablation:

Since the late 1980s, magnetic resonance imaging (MRI) has been added to ultrasound (US) and computed tomography (CT) as a cross-sectional imaging tool that can be used to guide the interventional diagnosis and treatment of a variety of disorders. Due to its superior soft tissue contrast, multiplanar capabilities, lack of ionizing radiation, and, most importantly, ability to image tissue function and temperatures, MRI has been suggested as the ideal tool to guide minimally invasive therapies (1,2). Nevertheless, when the concept of percutaneous MRI-guided tumor ablation was first introduced, restricted patient access and limited spatial and temporal resolution with conventional high- and mid-field MRI systems limited its widespread use. Technical advances in both open-configuration magnet design and the development of fast gradient-echo pulse sequences have contributed substantially to an increasing interest in MRI-guided interventions (1,2).

Recent trends in the care of cancer patients emphasize minimizing invasiveness while improving the effectiveness of treatment by utilizing medical resources in a cost-effective manner. This has been evident in the emerging and steadily evolving utilization of interventional radiology over the past quarter century.

The physical and biologic principles of localized high-temperature thermal therapy are well understood. If the targeted tissue volume is heated beyond 57° to 60°C, the threshold for protein denaturation, then coagulation necrosis occurs. This type of thermal treatment results in irreversible cell damage in both normal and neoplastic tissues. Since heat energy deposited above this critical level is not selective, thermal ablation is more comparable to surgery than to the more selective hyperthermia. In the case of cryoablation, the underlying physical and biologic principles are less well understood; nevertheless multiple freezings at a relatively low temperature also result in cell death.

Percutaneous ethanol injection therapy (PEIT) is one of the most widely used local procedures to treat hepatocellular carcinoma (HCC). Local-regional therapies are specific ablation modalities that introduce a damaging agent directly into the neoplastic tissue percuta-neously. These techniques are capable of destroying the tissue chemically, such as with sterile ethanol acetic acid, or thermally by laser, microwave, or radiofrequency (1–3). Percutaneous ethanol injection therapy was the first method to be proposed (4). It was conceived independently at the University of Chiba in Japan and at the Vimercate Hospital in Milan, Italy.

The goal of radiofrequency (RF) ablation is to induce thermal injury to the tissue through electromagnetic energy deposition. The term refers to the alternating electric current that oscillates in the range of high frequency (200–1200kHz). In RF ablation, the patient is part of a closed-loop circuit that includes an RF generator, an electrode needle, and a large dispersive electrode (ground pads). An alternating electric field is created within the tissue of the patient. Because of the relatively high electrical resistance of tissue in comparison with the metal electrodes, there is marked agitation of the ions present in the target tissue that surrounds the electrode, since the tissue ions attempt to follow the changes in direction of alternating electric current. The agitation results in frictional heat around the electrode. The discrepancy between the small surface area of the needle electrode and the large area of the ground pads causes the generated heat to be focused and concentrated around the needle electrode ().

As treatment for hepatocellular carcinoma (HCC), surgical resection, transcatheter arterial embolization (TAE), and ultrasound (US)-guided local treatment have been performed alone or in combination. Surgical resection is not a viable option for all patients due to poor liver function induced by hepatic cirrhosis, and TAE sometimes is ineffective because of angioneogenesis in small HCCs. For these reasons, US-guided percutaneous local treatment has been adopted independently or in combination with TAE.

Thermal destruction by microwaves has been used effectively for many years for ablation of both small liver metastases and primary lesions. Microwaves produce effective ablation without islands of viable cells in a rapid and reproducible fashion. The microwave region of the electromagnetic spectrum is well suited to such a role due to the efficient conversion of electromagnetic energy to heat. This translation of energy is a result of the strong interaction between polar molecules and microwaves that causes oscillation of molecules, which is expressed as heat.

Percutaneous laser therapy is one of the general percutaneous ablation therapy strategies. In principle, laser therapy is possible using fibers or by laser-induced thermotherapy (LITT). The LITT procedure currently is performed at laparotomy, by laparoscopic control intraoperatively, or percutaneously. Monitoring is essential for percutaneous interventions; magnetic resonance imaging (MRI) has proven to be the most reliable thermal measure, while ultrasound has been unable to show an adequate estimation of the thermal changes induced by the applied energy. Laser-induced thermotherapy makes available a photothermal tumor destruction technique that allows solid tumors within parenchymal organs to be destroyed. The laser energy is transmitted via thin optic fibers and causes a well-defined area of coagulative necrosis. This effect results in destruction of tissue by direct heating, while limiting damage to surrounding structures. Magnetic resonance imaging has proven to be an ideal clinical instrument to define the exact position of the optical fibers in the target area, provides real-time monitoring of the thermal effects, and the subsequent evaluation of the extent of coagulative necrosis.

Cold temperatures have been used to decrease inflammation and relieve pain since the time of the ancient Egyptians. James Arnott (1797–1883), an English physician, is credited with being the first to use cold to destroy tissue, using a combination of ice and salt to produce tissue necrosis (). Because this cryogen needed to be applied topically to tumors, he was able to treat only tumors of the cervix and breast; he reported decreases in tumor size and palliation of pain in patients treated by cryoablation ().