Laser Induced Interstitial Therapy LITT

Interstitial laser-induced hyperthermia is based on the conversion of laser light into heat. This is typically performed using a Nd:YAG laser at a wavelength of 1064 nm. The technique involves the invasive insertion of a laser catheter into the lesion using a light-conducting quartz fiber. To avoid early carbonization around the tip of the optic fiber, a closed cooling system based on circulating saline solution decreases the temperature peak around the applicator tip [23]. The success of the treatment is highly dependent upon the optimal positioning of the laser applicator in the center of the lesion. Hence, precise monitoring during therapy is mandatory. The conversion of laser light into heat in LITT results in cell death followed by coag-

ulative necrosis and secondary degeneration. Subsequent atrophy results in tumor shrinkage with minimal damage to surrounding structures. During the procedure, monitoring of thermo-induced signal changes in the treated tissue is possible on the basis of T1-weighted fast low angle GRE sequences in which a rise of the temperature is represented by a signal decrease [24].

The size of the heated volume depends on the power of the laser, the irradiation time and the optical and thermal characteristics of the treated tissue. Frequently, lesions with a diameter of up to 5 cm can be treated successfully [21].

T2-weighted MR images acquired approximately two to three months after LITT intervention typically reveal a hyperintense lesion surrounded by a hypointense rim. No enhancement of the lesion should be seen on T1-weighted equilibrium phase images acquired after injection of a Gd-chelate (Fig. 6).

Fig. 6a-f. Patient with metastases from colorectal cancer treated by LITT intervention. The T2-weighted HASTE images in axial (a) and coronal (b) orientation demonstrate a hyperintense lesion (arrow), surrounded by some hypointense tissue which again is demarcated from the normal liver parenchyma by a hyperintense rim. On the corresponding T1-weighted (c) and T1-weighted fat suppressed images (d) the more peripheral parts of the lesion show signs of hemorrhage (arrowin d). On T1-weighted (e) and T1-weighted fat suppressed (f) images, acquired during the equilibrium phase after contrast agent injection, the lesion is distinguished from surrounding liver tissue by a hyperintense rim. The absence of contrast agent uptake in the center of the lesion indicates complete tumor destruction

Fig. 6a-f. Patient with metastases from colorectal cancer treated by LITT intervention. The T2-weighted HASTE images in axial (a) and coronal (b) orientation demonstrate a hyperintense lesion (arrow), surrounded by some hypointense tissue which again is demarcated from the normal liver parenchyma by a hyperintense rim. On the corresponding T1-weighted (c) and T1-weighted fat suppressed images (d) the more peripheral parts of the lesion show signs of hemorrhage (arrowin d). On T1-weighted (e) and T1-weighted fat suppressed (f) images, acquired during the equilibrium phase after contrast agent injection, the lesion is distinguished from surrounding liver tissue by a hyperintense rim. The absence of contrast agent uptake in the center of the lesion indicates complete tumor destruction

Local tumor recurrence should be considered if T2-weighted images reveal nodular areas of medium to high signal intensity within the hypointense rim surrounding the lesion, or if nodular regions within the hyperintense necrosis, which differ from the signal intensity of the necrosis itself, can be detected. The same applies for contrast-enhanced T1-weighted images in which recurrent tumor growth is indicated if enhancement is seen in central areas of the lesion or if enhancing peripheral nodular structures are detected [2,23,25].

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