Hans-Joachim Schwarzmaier

MRI-guided LASER-induced interstitial thermotherapy of cerebral neoplasms

Objective Laser-induced interstitial thermotherapy (LITT) using a neodymium:yttrium aluminum garnet (Nd: YAG) laser is a new therapeutic approach in the treatment of brain tumors. The purpose of our study was to determine the value of MRI in monitoring LITT. Materials and Methods Eight patients with intracerebral tumors were treated with LITT. The light guide was inserted via an applicator sheath that was implanted stereotaxically with CT guidance. The laser irradiation was performed within the MR unit and monitored by repetitive measurements of a T1-weighted 2D-FLASH sequence. Results During therapy in all patients, typical changes of signal intensity were seen. A gradually increasing central zone of high signal intensity was surrounded by an increasing peripheral area of reduced signal intensity. The diameter of an enhancing rim at the outer border of the peripheral area after Gd-DTPA was considered as the total lesion size. The lesion size as determined on 2D-FLASH scans during LITT accounted for 88–100% (mean 93.5%) of total lesion size on T1-weighted images after Gd-DTPA acquired immediately after therapy. On T2-weighted images the signal intensities of the two zones were vice versa. Follow-up studies showed a decrease of total lesion size (15–87%). Conclusion Our results demonstrate that MRI is feasible and effective in monitoring LITT. However, the role of LITT in the therapeutic workup of brain tumors still has to be defined in future clinical studies.

Laser-induced thermal lesions in the human brain: short- and long-term appearance on MRI.

PURPOSE The purpose of our study was to investigate with MRI the development of thermal lesions in the human brain up to almost 4 years after laser-induced interstitial thermotherapy (LITT). METHOD Eighteen patients with brain tumors who underwent LITT entered the study. RESULTS In all patients the acute lesion comprised five concentric zones that showed reverse signal intensities on T1- versus T2-weighted images. Lesion development over time was uniform in 89% of the lesions. In two cases variations were observed. CONCLUSION The results of our MR follow-up studies showed that post-LITT, laser-induced lesions will shrink exponentially after an initial expansion without any pseudocystic effects.

Laser-induced interstitial thermotherapy of liver metastases in an interventional 0.5 Tesla MRI system: Technique and first clinical experiences

Laser‐induced interstitial thermotherapy (LITT) surveyed by magnetic resonance imaging (MRI) has been shown to be effective in various applications. The laser treatment of colorectal liver metastases usually requires a separate device (e.g., ultrasound or CT) to position the laser applicator. In this study, we used an interventional 0.5 T MRI system, allowing both the navigation to the target tissue and on‐line thermometry. Laser irradiation was performed using a near‐infrared laser source combined with a cooled laser light guide. We treated 20 patients exhibiting a total of 58 colorectal liver metastases. Clinically relevant complications did not occur. No residual tumor was observed after laser irradiation in all metastases with a diameter below 2 cm. Metastases with a mean diameter between 2 and 3 cm demonstrated total necrosis in 71%, while in larger tumors this proportion decreased to 46% (diameter, 3–4 cm) and 30% (diameter, >4 cm), respectively. We conclude that LITT, guided by the employed interventional MRI system, is feasible and safe. The results suggest a more aggressive treatment, especially for larger metastases. J. Magn. Reson. Imaging 2001;13:729–737.

Optical properties of blood in the near-infrared spectral range

We determine the optical properties of whole blood samples in the near infrared spectral range from double integrating sphere measurements using an inverse Monte Carlo technique. The measured values included the diffuse reflectance, the total transmittance, and the collimated transmittance. From these data, the absorption coefficient, the scattering coefficient, and the anisotropy factor were derived. The spectral range investigated extended from 700 nm to 1200 nm. It was found that the optical properties of blood were substantially different from the respective data for other relevant human tissues known so far. In addition, we analyzed the effect of the scattering phase function approximation on the resulting estimates of the optical properties. The Henyev-Greenstein and the Gegenbauer kernel phase functions were considered. The calculated angular distributions of scattered light were compared with goniophotometric measurements performed at the wavelength of 633 nm. The data presented in this study prove that the variations of the employed scattering phase function approximation can cause large discrepancies in the derived optical properties. This leads to the conclusion that the exact knowledge of the scattering phase function is required for the precise determination of the optical constants from the double integrating sphere measurements.

MR‐guided laser irradiation of recurrent glioblastomas

We treated two patients with recurrent glioblastoma multiforme using Nd:YAG laser irradiation in the framework of a salvage therapy. The underlying concept is to achieve cytoreduction by partial coagulation of the tumor. Magnetic resonance imaging (MRI) follow‐up examinations revealed a volume reduction of the laser‐irradiated areas, while the untreated parts of the tumor exhibited a progression. The survival time after the diagnosis of the recurrence was 16 and 20 months, respectively, which is substantially (about four times) longer than the natural history of the disease would suggest. In conclusion, cytoreduction by laser irradiation may be a promising option for patients suffering from recurrent glioblastoma multiforme. Future work should optimize the therapeutic regimen and evaluate this treatment approach in controlled clinical trials. J. Magn. Reson. Imaging 2005.

Relation between ventricular late endocardial activity during intraoperative endocardial mapping and low-amplitude signals within the terminal QRS complex on the signal-averaged surface electrocardiogram

Noninvasive recording of ventricular late potentials and intraoperative endocardial mapping at 36 sites were performed in 24 patients with left ventricular aneurysm and drug-resistant sustained ventricular tachycardia due to coronary artery disease. Their mean age was 55 +/- 8 years. Mean ejection fraction was 28 +/- 12%. For detection of late potentials on the signal-averaged QRS complex, 3 different algorithms were used. Late potentials were found in 54, 67 and 67% of the patients, respectively. In patients with a late potential on the signal-averaged electrocardiogram (ECG), delayed local activation (greater than 40 ms beyond the QRS complex on the intraoperative surface ECG) was recorded at 5.5, 5.5 and 5.6 endocardial sites. In patients without a late potential, this type of delayed local activation was detected at 2.4, 1.1 and 0.9 of 36 endocardial sites, respectively (p less than 0.05; p less than 0.01; p less than 0.002). The mean delay of local endocardial activity was 38, 35 and 37 ms in patients with a late potential on the body surface recording versus 20, 19 and 11 ms, respectively, in patients without a late potential (p less than 0.05; p less than 0.05; p less than 0.002). There was no correlation between the duration or amplitude of the late potential, if present, and the number of endocardial sites exhibiting delayed activity (r = -0.23, r = -0.05, r = 0.21; correlation not significant for each) or the mean duration of the endocardial delayed activity (r = -0.25, r = -0.14, r = -0.07; correlation not significant for each). These results indicate that the presence of late potentials on the signal-averaged surface ECG is related to the mean duration of endocardial late activity as well as to the number of endocardial sites exhibiting a given degree of delayed activation. Thus, it is dependent on the mass of slowly activated tissue. However, a direct conclusion from the duration or the amplitude of a late potential to the amount of delayed activation or the extent of endocardial time delay does not seem possible.

Basic Principles of Laser Induced Interstitial Thermotherapy in Brain Tumors

Summary This work reviews the physical basics and technical prerequisits of laser-induced interstitial thermotherapy (LITT) of brain structures. Laser-tissue interaction in the framework of LITT is predominantly based on absorption mediated thermal effects. Therefore, the optical and thermal properties of irradiated tissue are of major importance. These data are employed to predict the irradiation effects. In addition, they allow to develop appropriate applicator devices. Finally, the detailed knowledge about the irradiation process allows to define suitable monitoring techniques for LITT.

Different phase-function approximations to determine optical properties of blood: a comparison

We investigated the impact of the scattering phase function approximation on the optical properties of whole human blood determined from integrating sphere measurements using an inverse Monte Carlo technique. The diffuse reflectance Rd and the total transmittance Tt ((lambda) equals 633nm) of the whole blood samples were measured with a double integrating sphere equipment. The experimental scattering phase functions of the highly diluted blood samples were measured with a goniophotometer. We approximated the experimental scattering phase function with Mie, Gegenbauer kernel (GKPF), and Henyey-Green (HGPF) phase functions to pre-set the anisotropy factor (mu) for the inverse problem. We have employed HGPF, GKPF, and MPF approximations in the inverse Monte Carlo procedure to derive the absorption coefficient (mu) a and the scattering coefficient (mu) s. The results show significant difference in the final estimates of (mu) s. 12

Photodynamic cancer therapy: fluorescence localization and light absorption spectra of chlorophyll-derived photosensitizers inside cancer cells

The first prerequisite for an optimum effect of photodynamic therapy with chlorophyll-derived photosensitizers is irradiation at the S1 absorption maximum in the red spectral region. This absorption maximum changes its position due to molecular association by 20 to 100 nm depending on the subcellular environment, and must be determined by direct absorption spectrometry in the region of subcellular sensitizer localization. Fluorescence-intensifying video microscopy allows for localization of the sensitizer storage site at or near the Golgi apparatus of OAT 75 small-cell lung carcinoma cells. The absorption maximum at 760 nm taken from spectra of single cells and cell layers determines the postulated optimum condition for dye laser irradiation with bacteriopheophorbide a methyl ester as the sensitizer.

Caries selective ablation by pulsed lasers

The aim of the study was to find conditions for selective ablation of carious dentine. Absorption characteristics of healthy enamel, dentine, and carious lesions were studied by means of microspectrophotometry. It was found that in the spectral range of 330 to 520 nm optical density of carious dentine is higher by about a factor of 4 as in respect to healthy dentine. In the second part of the study, ablation thresholds were studied for different tissues at 355 nm (frequency tripled Q-switched Nd:YAG-Laser). By means of a piezoelectric detector, ultrasonic waves generated by ablation and subthreshold thermoelastic mechanisms were detected. Employing a fiber-optic delivery system, selective ablation of carious dentine occurred for pulse energies exceeding 0.4 Jcm-2 up to 1.2 Jcm-2.