Radiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Mueller-Lisse, U. G.
Right arrow Articles by Reiser, M. F.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Mueller-Lisse, U. G.
Right arrow Articles by Reiser, M. F.
(Radiology. 1999;210:373-379.)
© RSNA, 1999

Vascular and Interventional Radiology

Coagulative Interstitial Laser-induced Thermotherapy of Benign Prostatic Hyperplasia: Online Imaging with a T2-weighted Fast Spin-Echo MR Sequence—Experience in Six Patients

Ullrich G. Mueller-Lisse, MD1,2, Martin Thoma, MD1, Sonja Faber, PhD1, Andreas F. Heuck, MD1, Rolf Muschter, MD2, Peter Schneede, MD2, Ernst Weninger, MD3, Alfons G. Hofstetter, MD2 and Maximilian F. Reiser, MD1

1 Departments of Diagnostic Radiology (U.G.M.L., M.T., S.F., A.F.H., M.F.R.)
2 Urology (U.G.M.L., R.M., P.S., A.G.H.)
3 Anaesthesiology (E.W.), Klinikum Grosshadern, University of Munich "Ludwig Maximilian," Germany.


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To determine if hypointense lesions clearly outline on T2-weighted fast spin-echo (SE) magnetic resonance (MR) images obtained during coagulative interstitial laser-induced thermotherapy (LITT) of a prostate with benign hyperplasia.

MATERIALS AND METHODS: In six patients with benign prostatic hyperplasia (BPH), 12 LITT treatments were followed online with repetitive axial T2-weighted fast SE imaging (repetition time, 3,700 msec; echo time, 138 msec; acquisition time, 19 seconds). Development, time course, correlation with interstitial tissue temperature, and diameters of hypointense lesions around the laser diffusor tip were investigated. Lesion diameters on T2-weighted images acquired during LITT were compared with diameters of final lesions on T2-weighted images and unperfused lesions on enhanced T1-weighted SE images obtained at the end of therapy.

RESULTS: Hypointense lesions developed within 20–40 seconds of LITT. Average correlation coefficients between interstitial temperature development and signal intensity development were 0.92 during LITT and 0.90 after LITT. Regression slopes were significantly steeper during LITT (0.67% signal intensity change per degree Celsius) than after LITT (0.47% per degree Celsius; P = .038). Lesions remained visible after LITT for all procedures. Average maximum diameters of lesions were 1–3 mm larger during LITT than after LITT (P = .0006–.019).

CONCLUSION: Repetitive T2-weighted fast SE MR imaging during interstitial coagulative LITT of BPH demonstrates the development of permanent hypointense prostate lesions. However, posttherapeutic lesion diameters tend to be overestimated during LITT.

Index terms: Lasers, interstitial therapy, 844.1269 • Magnetic resonance (MR), guidance, 844.316 • Magnetic resonance (MR), rapid imaging, 844.121416 • Prostate, hyperplasia, 844.316


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Interstitial thermotherapy, predominantly carried out with lasers as the source of thermal energy (ie, laser-induced thermotherapy [LITT]), has recently evolved as a minimally invasive treatment for benign prostatic hyperplasia (BPH) (1–5). Magnetic resonance (MR) imaging lends itself to online imaging of thermal processes in various tissues because MR parameters, including the spin-lattice relaxation time T1 (6–23), the proton resonance frequency (24,25), diffusion coefficients (26,27), and equilibrium magnetization, correlate with tissue temperature. The drawbacks of temperature-sensitive MR imaging in the monitoring of thermotherapy the intended purpose of which is tissue coagulation lie in the impermanence of the temperature-dependent effects (12,15,19,28) and in the impossibility of direct, immediate delineation of the margins of coagulation in LITT procedures (16,19). However, posttherapeutic MR imaging after LITT of the prostate has shown that coagulated tissue delineates well from viable surrounding tissue not only on contrast material–enhanced T1-weighted images (19,29,30) but also on T2-weighted images, where low signal intensity indicates areas of coagulation (29,31).

We applied a rapid T2-weighted fast spin-echo (SE) sequence to the online MR imaging of LITT in patients with BPH to determine whether hypointense lesions clearly outline during the procedure. If they did outline, we further sought to determine whether these lesions are permanent and whether their diameters correspond with lesion diameters measured on posttherapeutic T2-weighted fast SE images and contrast-enhanced T1-weighted SE images of the prostate obtained at the end of LITT.


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Patients
Six patients (age range, 58–83 years) with symptomatic BPH underwent coagulative interstitial LITT of the prostate during MR imaging with a T2-weighted fast SE sequence. All patients gave written informed consent to undergo MR imaging–guided LITT and were aware of the experimental nature of the LITT treatment. The LITT procedure was approved by the local hospital ethics committee.

LITT Procedure
LITT was performed with use of general anaesthesia in two patients and with spinal epidural anesthesia in the other four patients, depending on the patient's preference and the anesthetist's recommendation. The MR imaging room was completely furnished with MR-suited anesthetic equipment for narcosis, including ventilation and monitoring.

During preparation for the LITT procedure, the patient lay on the patient table outside the MR imager in supine position. With the help of an assistant, the legs were manually kept in a low lithotomy position; this allowed transurethral endoscopic access to the prostate and bladder. Before the first LITT treatment, a suprapubic catheter was placed in the bladder under endoscopic guidance to collect both urine and irrigational fluid (1,2,4). The laser waveguide was forwarded through the operating channel of the endoscope and its quartz glass–clad diffusor tip inserted into the hyperplastic tissue of the left or right prostatic lobe. The endoscope was then retracted from the urethra, covered with sterile wrapping, and left at the foot of the MR table for the period of MR imaging. The patient's legs were put down straight, and the patient table was moved into the MR imager for the LITT treatment. After LITT inside the MR imager, the patient table was pulled out of the imager. The laser waveguide was retracted and eventually relocated in the opposite prostatic lobe under endoscopic guidance as above, and the patient was returned to the imager for repeated MR imaging and LITT.

All patients were treated with the LITT method described by Muschter and Hofstetter (4,5). During MR imaging, one patient received four LITT treatments with a medical Nd:YAG laser operating at 1,064-nm wavelength and a laser waveguide (diameter, 600 µm) with a quartz glass–clad cylindric diffusor fiber tip of 20 mm length and 1.9 mm diameter (Fibertom model 4060 N and waveguide ITT H-6190-I; Dornier Medical Systems, Germering, Germany). The laser diffusor tip emits a conically configurated array of beams that induces cylindric or ovoid lesions. Each LITT treatment consisted of a 3-minute power-formatted laser sequence in continuous wave mode (20 W over 30 seconds, 15 W over 30 seconds, 10 W over 30 seconds, 7 W over 90 seconds). During three of the four LITT treatments, MR images were obtained with the T2-weighted fast SE sequence described in the next section.

The other five patients each received two LITT treatments with a medical diode laser (IDL 830 E; Indigo Medical, Muenster, Germany) (median wave length, 830 nm) during MR imaging with the T2-weighted fast SE sequence. The interstitial thermotherapy waveguide of the diode laser (diameter, 600 µm) is also covered with a quartz-clad cylindric diffusor fiber tip (length, 20 mm; diameter, 1.9 mm). However, it carries an additional outer sheath of thermally resistant silicon that also includes a thermoluminescence thermometer at the tip. The diffusor leads to a cylindrically shaped volume of coagulation. The diode laser includes a temperature feedback control system that downregulates laser power settings from an initial setting of 15 W according to temperature measured at the diffusor tip. The maximum interstitial tissue temperature allowed at the tip was set either to 85°C (four LITT treatments in two patients) or to 90°C (six LITT treatments in three patients). Irradiation from one fiber tip position per prostatic lobe consisted of a 3-minute sequence of declining laser power settings in continuous wave mode as determined by the feedback control system. Temperature data for correlation with MR imaging data were gained during diode laser LITT in nine treatments, since the feedback system displayed current intraprostatic tissue temperature at the diffusor tip on a monitor. One diode laser treatment could not be evaluated due to artifact caused by bowel movement during MR imaging.

Laser sources were placed outside the MR imaging room, and the respective waveguides were led into the room via a shielded metal tube penetrating the wall.

MR Imaging
Patients were examined with the circularly polarized four-element body phased-array coil commercially available for the 1.5 T whole-body MR imager applied (Magnetom Vision; Siemens, Erlangen, Germany). Before each LITT treatment, the position of the laser diffusor tip inside the prostate was confirmed on sagittal and axial T2-weighted fast SE images (4,700/99 [repetition time msec/echo time msec], echo train length of 11 lines, 13–23 sections, 3–5-mm section thickness, no intersection gap, 188 x 250-mm or 250 x 250-mm field of view, matrix of 192 x 256 or 256 x 256 pixels, one signal acquired). Correction of diffusor tip position was made where necessary.

MR imaging was performed during LITT with a rapid T2-weighted fast SE sequence with the following parameters: 3,700/138, echo train length of 29 lines, 116 x 256-pixel matrix, 225 x 300-mm field of view, and a total measurement time of 19 seconds for three axial sections with a section thickness of 5 mm, intersection gap of 0 or 2.5 mm, and one signal acquired. The center section of the fast SE sequence was positioned such that it included the center of the laser diffusor tip. At time intervals of 20 seconds, the fast SE sequence was repeated 21 times, over a total duration of 420 seconds. After repeated baseline images over a period of 60–100 seconds (three to five MR imaging cycles) during preparation of the laser source for LITT, the LITT treatment was initiated with the beginning of the next MR imaging cycle of 20 seconds and imaged over 180 seconds (nine imaging cycles), and the immediate post-LITT phase was imaged over the remaining 140–180 seconds (seven to nine imaging cycles). At the end of the entire LITT procedure in each patient, axial T2-weighted fast SE images of the prostate (4,700/99, other parameters as above) were acquired after removal of the laser waveguide from the prostate. Then, contrast medium (gadopentetate dimeglumine, Magnevist; Schering, Berlin, Germany) was injected intravenously at a standard dose (0.1 mmol per kilogram of body weight) before axial T1-weighted SE images (714/14, 13–19 sections, 3–5-mm section thickness, no intersection gap, 188 x 250-mm or 250 x 250-mm field of view, matrix of 192 x 256 or 256 x 256 pixels, one signal acquired) were obtained of the prostate to demonstrate laser-induced lesions by their lack of enhancement (19,30).

Data Evaluation
Serial images from rapid fast SE measurements during LITT were searched for lesion areas of decreased signal intensity around the laser diffusor tip that were considered to be indicative of coagulation (19,28,30). Where clearly outlined on the MR images, anteroposterior and left-to-right lesion diameters in the axial plane were measured in the center sections with standard evaluation software integrated into the MR imaging system. The diameters were compared with lesion diameters measured in the same manner in the T2-weighted fast SE images and contrast-enhanced T1-weighted SE images obtained at the end of the LITT procedure.

Signal intensities from rapid fast SE measurements were determined in selected regions of interest (ROIs) of rectangular shape on each image of a series. The first ROI, including 3 x 6 pixels (5.8 x 7.0 mm), was centered around the laser tip (1.9 mm diameter), and subsequent ROIs, including 3 x 3 pixels each (5.8 x 3.5 mm), were placed side by side in a row, immediately adjacent to one another (Fig 1). Signal intensity–time curves were plotted for each LITT treatment to determine if signal intensity develops in characteristic ways before, during, and after LITT. Signal intensity development was evaluated relative to the average of the baseline signal intensities prior to LITT, to account for possible interindividual differences of baseline signal intensity. Signal intensity–tissue temperature regressions were calculated for in vivo LITT treatments carried out with the diode laser, to determine if characteristic correlations exist on T2-weighted fast SE images.



View larger version (16K):
[in this window]
[in a new window]
  		Figure 1. Schematic representation of a cross-sectional image of the prostate. Demonstrated are the positions of the laser diffusor tip with integrated thermometer (6), central region of interest around laser diffusor tip (1), and peripheral regions of interest (2–5) for data analysis are demonstrated.

 
Statistical evaluation, including regression analysis and calculation of coefficients of linear correlation, was carried out on a separate personal computer with STATVIEW R software (Abacus Concepts, Berkeley, Calif).


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Altogether, 12 LITT treatments were monitored with the rapid T2-weighted fast SE sequence. The laser diffusor tip at the active end of the waveguide was clearly recognized before all treatments by a signal void on the baseline rapid fast SE images obtained over 60–100 seconds before initiation of the LITT treatment, even when signal intensities within the central area of the prostate were inhomogeneous (Fig 2a). After initiation of LITT, a round or slightly ovoid hypointense area spreading from the diffusor tip was first noted within two imaging cycles after initiation of laser irradiation (Fig 2b). In 10 of the 12 treatments, the hypointense area demarcated clearly from surrounding prostate tissue throughout the entire treatment, whereas in two treatments, lesion margins were not clearly outlined on some images obtained during LITT. However, lesion demarcation was sufficient for evaluation in all treatments. Lesion demarcation was best at an average of 60 seconds (three imaging cycles) after initiation of LITT. In all treatments, the diameters of the hypointense areas increased during LITT. At average, the largest diameters of the hypointense lesions recognized during LITT were 1–2 mm larger than the diameters of the final unperfused lesions as demonstrated by the contrast-enhanced axial T1-weighted SE images (P = .0187, Wilcoxon matched pairs signed rank test) and 2–3 mm larger than the final diameters after LITT on T2-weighted axial fast SE images (P = .0006) (Table; Fig 2c–2e). During and immediately after the LITT procedure, there was no apparent edema of the prostate or surrounding tissues in any patient.



View larger version (182K):
[in this window]
[in a new window]
  		Figure 2a. (a) Rapid T2-weighted axial fast SE image (3,700/138) of prostate with BPH before LITT. Laser diffusor tip is recognized by a signal void (arrowhead). Prior position of laser waveguide in right prostatic lobe shows high signal intensity (open arrow). (b) Rapid T2-weighted axial fast SE image (3,700/138) of prostate obtained during LITT (seventh imaging cycle, 140 seconds of laser irradiation). A hypointense area of round or slightly ovoid shape (arrows) spreads from the laser diffusor tip and demarcates from surrounding tissue. (c) Rapid T2-weighted axial fast SE image (3,700/138) obtained immediately after LITT (10th imaging cycle, 20 seconds after laser irradiation). Arrows demarcate outlines of laser-induced lesions in right and left prostatic lobes. (d) T2-weighted axial fast SE image (4,700/99) obtained at the end of the procedure (similar level as in c). Hypointense lesions of LITT demarcate from surrounding prostatic tissue (solid arrows). Prior positions of laser waveguides show high signal intensity (open arrows), possibly from interstitial fluid filling the waveguide track. No evidence of periprostatic edema. (e) Contrast-enhanced axial T1-weighted SE image (714/14) obtained at the end of the procedure shows the same laser-induced lesions (similar level as in c and d). Tissue necrosis in the hypointense lesions (short arrows) is suggested by lack of perfusion. Long arrow demarcates prostatic urethra at entry site of laser waveguide tips into prostatic tissue.

 


View larger version (183K):
[in this window]
[in a new window]
  		Figure 2b. (a) Rapid T2-weighted axial fast SE image (3,700/138) of prostate with BPH before LITT. Laser diffusor tip is recognized by a signal void (arrowhead). Prior position of laser waveguide in right prostatic lobe shows high signal intensity (open arrow). (b) Rapid T2-weighted axial fast SE image (3,700/138) of prostate obtained during LITT (seventh imaging cycle, 140 seconds of laser irradiation). A hypointense area of round or slightly ovoid shape (arrows) spreads from the laser diffusor tip and demarcates from surrounding tissue. (c) Rapid T2-weighted axial fast SE image (3,700/138) obtained immediately after LITT (10th imaging cycle, 20 seconds after laser irradiation). Arrows demarcate outlines of laser-induced lesions in right and left prostatic lobes. (d) T2-weighted axial fast SE image (4,700/99) obtained at the end of the procedure (similar level as in c). Hypointense lesions of LITT demarcate from surrounding prostatic tissue (solid arrows). Prior positions of laser waveguides show high signal intensity (open arrows), possibly from interstitial fluid filling the waveguide track. No evidence of periprostatic edema. (e) Contrast-enhanced axial T1-weighted SE image (714/14) obtained at the end of the procedure shows the same laser-induced lesions (similar level as in c and d). Tissue necrosis in the hypointense lesions (short arrows) is suggested by lack of perfusion. Long arrow demarcates prostatic urethra at entry site of laser waveguide tips into prostatic tissue.

 


View larger version (181K):
[in this window]
[in a new window]
  		Figure 2c. (a) Rapid T2-weighted axial fast SE image (3,700/138) of prostate with BPH before LITT. Laser diffusor tip is recognized by a signal void (arrowhead). Prior position of laser waveguide in right prostatic lobe shows high signal intensity (open arrow). (b) Rapid T2-weighted axial fast SE image (3,700/138) of prostate obtained during LITT (seventh imaging cycle, 140 seconds of laser irradiation). A hypointense area of round or slightly ovoid shape (arrows) spreads from the laser diffusor tip and demarcates from surrounding tissue. (c) Rapid T2-weighted axial fast SE image (3,700/138) obtained immediately after LITT (10th imaging cycle, 20 seconds after laser irradiation). Arrows demarcate outlines of laser-induced lesions in right and left prostatic lobes. (d) T2-weighted axial fast SE image (4,700/99) obtained at the end of the procedure (similar level as in c). Hypointense lesions of LITT demarcate from surrounding prostatic tissue (solid arrows). Prior positions of laser waveguides show high signal intensity (open arrows), possibly from interstitial fluid filling the waveguide track. No evidence of periprostatic edema. (e) Contrast-enhanced axial T1-weighted SE image (714/14) obtained at the end of the procedure shows the same laser-induced lesions (similar level as in c and d). Tissue necrosis in the hypointense lesions (short arrows) is suggested by lack of perfusion. Long arrow demarcates prostatic urethra at entry site of laser waveguide tips into prostatic tissue.

 


View larger version (184K):
[in this window]
[in a new window]
  		Figure 2d. (a) Rapid T2-weighted axial fast SE image (3,700/138) of prostate with BPH before LITT. Laser diffusor tip is recognized by a signal void (arrowhead). Prior position of laser waveguide in right prostatic lobe shows high signal intensity (open arrow). (b) Rapid T2-weighted axial fast SE image (3,700/138) of prostate obtained during LITT (seventh imaging cycle, 140 seconds of laser irradiation). A hypointense area of round or slightly ovoid shape (arrows) spreads from the laser diffusor tip and demarcates from surrounding tissue. (c) Rapid T2-weighted axial fast SE image (3,700/138) obtained immediately after LITT (10th imaging cycle, 20 seconds after laser irradiation). Arrows demarcate outlines of laser-induced lesions in right and left prostatic lobes. (d) T2-weighted axial fast SE image (4,700/99) obtained at the end of the procedure (similar level as in c). Hypointense lesions of LITT demarcate from surrounding prostatic tissue (solid arrows). Prior positions of laser waveguides show high signal intensity (open arrows), possibly from interstitial fluid filling the waveguide track. No evidence of periprostatic edema. (e) Contrast-enhanced axial T1-weighted SE image (714/14) obtained at the end of the procedure shows the same laser-induced lesions (similar level as in c and d). Tissue necrosis in the hypointense lesions (short arrows) is suggested by lack of perfusion. Long arrow demarcates prostatic urethra at entry site of laser waveguide tips into prostatic tissue.

 


View larger version (185K):
[in this window]
[in a new window]
  		Figure 2e. (a) Rapid T2-weighted axial fast SE image (3,700/138) of prostate with BPH before LITT. Laser diffusor tip is recognized by a signal void (arrowhead). Prior position of laser waveguide in right prostatic lobe shows high signal intensity (open arrow). (b) Rapid T2-weighted axial fast SE image (3,700/138) of prostate obtained during LITT (seventh imaging cycle, 140 seconds of laser irradiation). A hypointense area of round or slightly ovoid shape (arrows) spreads from the laser diffusor tip and demarcates from surrounding tissue. (c) Rapid T2-weighted axial fast SE image (3,700/138) obtained immediately after LITT (10th imaging cycle, 20 seconds after laser irradiation). Arrows demarcate outlines of laser-induced lesions in right and left prostatic lobes. (d) T2-weighted axial fast SE image (4,700/99) obtained at the end of the procedure (similar level as in c). Hypointense lesions of LITT demarcate from surrounding prostatic tissue (solid arrows). Prior positions of laser waveguides show high signal intensity (open arrows), possibly from interstitial fluid filling the waveguide track. No evidence of periprostatic edema. (e) Contrast-enhanced axial T1-weighted SE image (714/14) obtained at the end of the procedure shows the same laser-induced lesions (similar level as in c and d). Tissue necrosis in the hypointense lesions (short arrows) is suggested by lack of perfusion. Long arrow demarcates prostatic urethra at entry site of laser waveguide tips into prostatic tissue.

 

View this table:
[in this window]
[in a new window]
  		Lesion Size Development on Rapid T2-weighted Fast SE MR Images during LITT of the Prostate versus Lesion Diameters on T2-weighted Fast SE and Contrast-enhanced T1-weighted SE Images Obtained after LITT
 
Relative baseline signal intensity variability over three to five rapid fast SE measurements before LITT was within ±2.8% (range, 0%–27.0%; n = 60 ROIs in 12 LITT procedures). Maximum signal intensity decrease in the central ROI around the laser diffusor tip was reached within seven to 10 imaging cycles (140–200 seconds) after initiation of LITT (Fig 3). Lowest signal intensity during LITT averaged 58% of baseline signal intensity (range, 39%–80%). Temperature increase at the laser diffusor tip at the time of maximum signal intensity change averaged 52.4°C (range, 48.7°C–54.7°C, n = nine procedures with the diode laser). In eight of 12 treatments, final signal intensity during online monitoring with the rapid T2-weighted fast SE sequence in the central and adjacent ROI clearly remained lower than baseline signal intensity (average for the central ROI, 68% of baseline signal intensity; range, 60%–73%), even once the laser had been switched off and interstitial tissue temperature returned toward normal body temperature (Fig 3). In four diode laser treatments in two patients (maximum interstitial temperature of 85°C in two and 90°C in the other two treatments), however, signal intensity in the central ROIs approached baseline with a steeper slope after LITT, such that final signal intensity at average was only -14% lower than baseline (range, -7% to -20%).



View larger version (16K):
[in this window]
[in a new window]
  		Figure 3. Signal intensity development (traces 1–6) before, during, and after LITT of the prostate. Signal intensity–baseline signal intensity (%) and interstitial tissue temperature at the laser diffusor tip (°C) are recorded over time. After 60 seconds of baseline imaging at normal body temperature, the laser is switched on and tissue temperature quickly reaches a plateau at 90°C (trace 6). Signal intensity around the laser diffusor tip (trace 1) decreases by approximately 45% and also shows a plateau. After the laser has been switched off at 240 seconds, signal intensity in the central region of interest (trace 1) increases to a second plateau at about 68% of baseline signal intensity, while interstitial tissue temperature decreases toward normal body temperature. Signal intensity alteration in the first peripheral region of interest (trace 2) is less pronounced, possibly due to both temperature gradient between tissue close to the laser and peripheral prostatic tissue, and partial volume effect between tissue included in the laser-induced lesion and unaffected peripheral tissue. Still, a permanent effect is suggested by trace 2. More distant regions of interest (traces 3–5) do not show signal intensity change.

 
During interstitial laser-induced temperature increase, the correlation coefficient between signal intensity alteration with the rapid, T2-weighted fast SE sequence and temperature change at the laser diffusor tip averaged 0.92 (range, 0.86–0.97; coefficients calculated with the lease squares fit method). During the cooling phase after LITT, coefficients averaged 0.90 (range, 0.81–0.97).

Signal intensity alteration relative to average individual baseline signal intensity and to interstitial temperature increase averaged -0.67% per degree Celsius during laser energy deposition in the prostate (range, -0.41% to -0.96% per degree Celsius). During the cooling phase after LITT, the average slope of the relative signal intensity over temperature regression was more gentle at an average of -0.47% per degree Celsius (range, -0.23% to -0.81% per degree Celsius), (P = .0382, Wilcoxon matched-pairs signed rank test).

The T2-weighted fast SE MR images obtained immediately after completion of LITT procedures did not show intraprostatic, periprostatic, perivesical, perirectal, or presacral edema in any of the patients.


   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
In earlier studies of posttherapeutic MR imaging effects of laser-induced thermotherapy of the prostate carried out with lasers with sidefire fibers (30,32,33) or interstitial fibers (19,29), unperfused lesions of low signal intensity were demonstrated on contrast-enhanced T1-weighted images obtained after therapy. In interstitial LITT treatments, lesions of low signal intensity were found in identical locations on T2-weighted images (19,29,31). While it remained unclear in these studies whether the posttherapeutic lesions visible on T2-weighted images clearly demarcated during the ongoing laser treatment or with some time delay, our results suggest that hypointense lesions on T2-weighted fast SE images are an immediate effect of interstitial laser thermotherapy the intended purpose of which is tissue coagulation. The effect was observed during LITT in all 12 treatments investigated here and became visible within two imaging cycles (ie, within 40 seconds) after initialization of laser irradiation.

In contrast to hypointense areas that can be observed on T1-weighted MR images obtained during interstitial laser treatments in various body tissues, including liver (20,28), head and neck malignancies (15,34,35), and prostate (19), the hypointense lesions observed on T2-weighted MR images in our study remained visible beyond the return of tissue temperature to normal body temperature. Temperature-dependent changes of T1 relaxation cannot explain the persistence of the low signal intensity in the lesion areas on T2-weighted images. Since the LITT method applied here has been demonstrated to produce interstitial tissue coagulation necrosis (1,2,4) and persistent, hypointense lesions on T2-weighted MR images that decrease in size over weeks and months (29,31), the low signal intensity within lesions on T2-weighted images may reflect the result of both tissue coagulation and decrease of the bulk water–hydration water ratio within the lesion. Both rapid, persistent tissue dehydration under the influence of laser-induced heat (which would result in a persistent local decrease of proton density rather than in a temperature-dependent, transient change of equilibrium magnetization) and increase of irrotational water molecule bonds with newly exposed sites of proteins whose secondary and tertiary structure has been altered or destroyed by temperature-induced denaturation could be accountable for this phenomenon.

The diameters of hypointense lesions observed on the T2-weighted images during LITT increased by an average of 3–4 mm between the first images showing a lesion and the images showing the largest lesion. Average deviations between maximum lesion diameters measured during and after LITT amounted to 1–3 mm. In the majority of cases, diameters of laser-induced lesions were overestimated on T2-weighted fast SE images obtained during LITT. This overestimation may be attributed to the temperature-dependent influence of both equilibrium magnetization, which decreases during LITT, and T1 relaxation inherent even on T2-weighted sequences, which increases during LITT. Both effects may overlay a more permanent T2 or proton-density effect.

Since LITT was the only therapy for BPH that the patients received in this study, histologic specimens and pathologic correlation of lesions detected with MR imaging were not obtained. However, a recent study compared lesion sizes in prostates treated with a laser with a sidefire fiber as determined on contrast-enhanced T1-weighted MR images obtained before radical prostatectomy and in the prostatic specimens at pathologic work-up. Results showed a 96% correlation (30). With respect to these results (30), our data suggest a reliable, if slightly overestimated, reflection of the actual lesion development in the prostate with rapid T2-weighted MR images obtained during laser-induced thermotherapy.

Signal intensity development correlated well with interstitial tissue temperature development in all treatments, which may be attributed to the influence of T1 relaxation inherent even with a strongly T2-weighted MR sequence. A strong linear correlation between T1 relaxation and related MR imaging parameters and temperature has been demonstrated in earlier studies both in vitro (8,11,15–19) and in vivo (12,14,22,23). However, the slopes of the regression curves calculated for interstitial temperature development in the prostate and signal intensity development with the rapid T2-weighted fast SE sequence applied for MR imaging before, during, and after LITT were significantly steeper during LITT than during tissue cooling immediately after LITT. Accordingly, contrast between the lesion and adjacent prostate tissue was lower at the end of the post-LITT cooling phase than during LITT. While signal intensity in the lesion remained about 32% lower than that in surrounding tissue in four patients (eight LITT treatments), it increased to 14% below baseline in the other two patients (four LITT treatments). Still, at the end of the LITT procedure, lesions of low signal intensity were delineated on T2-weighted images in the same locations as lesions that had developed during LITT. An earlier study (31) of the postoperative time course of intraprostatic changes induced by the same LITT method demonstrated similar hypointense lesions over up to 6 months after LITT. The lesions decreased in size over time (31). This, again, may indicate that a tissue parameter other than T1 relaxation undergoes an irreversible or merely partially reversible change during LITT. It may also reflect the interstitial tissue coagulation caused by the LITT method applied (1,2), since nonenhancing lesions that represent necrosis (30) were observed in identical positions on contrast-enhanced T1-weighted images obtained after LITT.

During and immediately after the LITT procedures, we did not observe substantial intra- or periprostatic edema. Since an earlier study of 18 patients who had received LITT of the prostate with the same laser method applied here demonstrated moderate to severe edema within 24–48 hours after therapy (31), it seems likely that edema develops with a prolonged time course. Constant bladder irrigation during the procedure as applied here could be one reason for delayed development of edema. The finding of prostatic and periprostatic edema at intermittent MR imaging between transurethral laser light application, by means of sidefire fibers, to the prostate (32) differs from our observation. This may be due to the different laser technique applied, different patterns of irrigation, or different time courses of the operations.

In contrast to the assessment of lesion size development on the basis of T1-weighted MR imaging (6–23), diffusion-weighted MR imaging (26, 27), or MR phase contrast imaging (24,25) performed during thermotherapies the intended purpose of which is tissue coagulation, online MR imaging with a T2-weighted sequence as performed here does not seem to require the intermediate steps of interstitial tissue temperature derivation from signal or phase alterations and calculation of tissue exposure time to a certain interstitial temperature (36) to determine the extent of coagulated lesions. However, while the application of T2-weighted sequences may reduce the technical expenditure of online monitoring of thermotherapies the intended purpose of which is tissue coagulation, this happens at the expense of reliable information on tissue temperature, at least after tissue coagulation. On the other hand, rapid imaging with T2-weighted sequences may provide a viable solution to the problem of online monitoring of coagulative thermotherapy in dedicated low-field-strength MR imaging systems that are not suited for phase contrast or diffusion studies. It remains to be shown, however, how far our observations at 1.5 T can be validated at lower field strengths.

In the prostate, the clinical value of online monitoring of coagulative thermotherapy with T2-weighted MR sequences appears to lie in the control of lesion extension in treatments of BPH, particularly in locations close to the prostatic capsule, prostatic apex, and neurovascular bundles. The method may lend itself to coagulative thermal treatments of prostate cancer for the same reasons. It remains to be shown, however, whether hypointense lesions caused by thermotherapy can be distinguished from hypointense lesions of prostate cancer.

In conclusion, online MR imaging of coagulative LITT of the prostate with a rapid T2-weighted fast SE sequence allows reliable estimations of lesion development during treatment, although final postoperative lesion diameters tend to be slightly overestimated during LITT. Further studies will have to show if reliable assessments of lesion development on the basis of T2-weighted sequences are possible in other tissues and at lower magnetic field strengths. While T2-weighted MR imaging may suffice to monitor thermal therapies the intended purpose of which is tissue coagulation and spare the inclusion of external data computation to derive the size of coagulated lesions, the application of other MR sequences seems to be necessary whenever precise estimations of interstitial temperature are required.


   Footnotes
 
U.G.M.L. supported by a grant from Friedrich-Baur-Stiftung, Munich, Germany.

Address reprint requests to U.G.M.L., Department of Radiology, University of California, San Francisco, 505 Parnassus Ave, San Francisco, CA 94143-0628.

Abbreviations: BPH = benign prostatic hyperplasia LITT = laser-induced thermotherapy ROI = region of interest SE = spin echo

Author contributions: Guarantor of integrity of entire study, U.G.M.L.; study concepts and design, U.G.M.L., M.T., A.F.H., R.M., E.W.; definition of intellectual content, U.G.M.L., A.G.H., M.F.R.; literature research, U.G.M.L., S.F., A.F.H.; clinical studies, U.G.M.L., A.F.H., R.M., P.S., E.W.; data acquisition and analysis, U.G.M.L., M.T., S.F.; statistical analysis, U.G.M.L., M.T.; manuscript preparation, U.G.M.L.; manuscript editing, U.G.M.L., M.T.; manuscript review, U.G.M.L., A.F.H., M.F.R.

Received December 9, 1997; revision requested February 27, 1998; revision received July 2, 1998; accepted September 2, 1998.
   References
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

  1. Hofstetter A. Interstitielle thermokoagulation (ITK) von prostatatumoren (editorial). Lasermedizin 1991; 7:179.
  2. Muschter R, Hofstetter A, Hessel S. Laser induced thermotherapy of benign prostatic hyperplasia: fundamentals and clinical experiences. Minimal Invasive Medizin-MedTech 1994; 5:51-54[German].
  3. Roggan A, Handke A, Miller K, Mueller G. Laser induced interstitial thermotherapy of benign prostatic hyperplasia: basic investigations and first clinical results. Minimal Invasive Medizin-MedTech 1994; 5:55-63[German].
  4. Muschter R, Hofstetter A. Technique and results of interstitial laser coagulation. World J Urol 1995; 13:109-114.[Medline]
  5. Muschter R, Hofstetter A. Interstitial laser therapy outcomes in benign prostatic hyperplasia. J Endourol 1995; 9:129-135.[Medline]
  6. Anzai Y, Lufkin RB, Saxton RE, et al. Nd:YAG interstitial laser phototherapy guided by magnetic resonance imaging in an ex vivo model: dosimetry of laser-MR-tissue-interaction. Laryngoscope 1991; 101:755-760.[Medline]
  7. Bleier AR, Jolesz FA, Cohen MS, et al. Real time magnetic resonance imaging of laser heat deposition in tissue. Magn Reson Med 1991; 20:132-137.
  8. Blueml S, Schad LR, Stepanow B, Lorenz WJ. Spin-lattice relaxation time measurement by means of a TurboFLASH technique. Magn Reson Med 1993; 30:289-295.[Medline]
  9. Castro JD, Saxton RE, Layfield LJ, et al. Interstitial laser phototherapy assisted by magnetic resonance imaging: a new technique for monitoring laser-tissue-interaction. Laryngoscope 1990; 100:541-547.[Medline]
  10. Cline HE, Schenck JF, Watkins RD, Hynynen K, Jolesz FA. Magnetic resonance-guided thermal surgery. Magn Reson Med 1993; 30:98-106.[Medline]
  11. Dickinson RJ, Hall AS, Hind AJ, Young IR. Measurement of changes in tissue temperature using MR imaging. J Comput Assist Tomogr 1986; 10:468-472.[Medline]
  12. El Ouahabi A, Guttmann CRG, Hushek SG, et al. MRI guided interstitial laser therapy in a rat malignant glioma model. Lasers Surg Med 1993; 13:503-510.[Medline]
  13. Fan M, Ascher PW, Schrottner O, Ebner F, Germann RH, Kleinert R. Interstitial 1,064 nm Nd:YAG laser thermotherapy for brain tumors under real-time monitoring of MRI: experimental study and phase I clinical trial. J Clin Laser Med Surg 1992; 10:355-361.[Medline]
  14. Hall AS, Prior PV, Hand JW, Young IR, Dickinson RJ. Observation by MR imaging of in vivo temperature changes induced by radio frequency hyperthermia. J Comput Assist Tomogr 1990; 14:430-436.[Medline]
  15. Jager L, Mueller-Lisse GU, Gutmann R, Feyh J, Thoma M, Reiser M. Initial results with MRI-controlled laser-induced interstitial thermotherapy of head and neck tumors. Radiologe 1996; 36:236-244.[Medline]
  16. Jolesz FA, Bleier AR, Jakab P, et al. MR imaging of laser-tissue-interactions. Radiology 1988; 168:249-253.[Abstract/Free Full Text]
  17. Matsumoto R, Mulkren RV, Hushek SG, Jolesz FA. Tissue temperature monitoring for thermal interventional therapy: comparison of T1-weighted MR sequences. JMRI 1994; 4:65-70.
  18. Matsumoto R, Oshio K, Jolesz FA. Monitoring of laser and freezing-induced ablation in the liver with T1-weighted MR imaging. JMRI 1992; 2:555-562.
  19. Mueller-Lisse GU, Heuck A, Stehling MK, et al. MRI monitoring before, during, and after interstitial laser-induced hyperthermia of benign prostatic hyperplasia: first clinical experience. Radiologe 1996; 36:722-731.[Medline]
  20. Pushek T, Farahani K, Saxton RE, et al. Dynamic MRI-guided interstitial laser therapy: a new technique for minimally invasive surgery. Laryngoscope 1995; 105:1245-1252.[Medline]
  21. Tracz RA, Wyman DR, Little PB, et al. Magnetic resonance imaging of interstitial laser photocoagulation in brain. Laser Surg Med 1992; 12:165-173.
  22. Youl BD, Hawkins CP, Morris JK, DuBoulay EPHG, Tofts PS. In vivo T1 values from guinea pig brain depend on body temperature. Magn Reson Med 1992; 24:170-173.[Medline]
  23. Young IR, Hand JW, Oatridge A, Prior M. Modeling and observation of temperature changes in vivo using MRI. Magn Reson Med 1994; 32:358-369.[Medline]
  24. De Poorter J, De Wagter C, De Deene Y, Thomsen C, Stahlberg F, Eric A. Noninvasive MRI thermometry with the proton resonance frequency (PRF) method: in vivo results in human muscle. Magn Reson Med 1995; 33:74-81.[Medline]
  25. Ishihara Y, Calderon A, Watanabe H, et al. A precise and fast temperature mapping using water proton chemical shift. Magn Reson Med 1995; 34:814-823.[Medline]
  26. MacFall J, Prescott DM, Fullar E, Samulski TV. Temperature dependence of canine brain tissue diffusion coefficient measured in vivo with magnetic resonance echo-planar imaging. Int J Hyperthermia 1995; 11:73-86.[Medline]
  27. Delannoy J, Chen CN, Turner R, Levin RL, LeBihan D. Noninvasive temperature imaging using diffusion MRI. Magn Reson Med 1991; 19:333-339.[Medline]
  28. Vogl TJ, Mueller PK, Hammerstingl R, et al. Malignant liver tumors treated with MR imaging–guided laser-induced thermotherapy: technique and prospective results. Radiology 1995; 196:257-265.[Abstract/Free Full Text]
  29. Heuck A, Mueller-Lisse U, Muschter R, et al. Einsatz der magnetresonanz-tomographie für die darstellung der interstitiellen laserkoagulation bei der benignen prostatahyperplasie: erste erfahrungen. Lasermedizin 1995; 11:27-33.
  30. Huch Boni RA, Sulser T, Jochum W, Romanowski B, Debatin JF, Krestin GP. Laser ablation–induced changes in the prostate: findings at endorectal MR imaging with histologic correlatation. Radiology 1997; 202:232-236.[Abstract/Free Full Text]
  31. Mueller-Lisse UG, Heuck AF, Schneede P, et al. Postoperative MRI in patients undergoing interstitial laser coagulation thermotherapy of benign prostatic hyperplasia. J Comput Assist Tomogr 1996; 20:273-278.[Medline]
  32. de Souza NM, Flynn RJ, Coutts GA, et al. Endoscopic laser ablation of the prostate: MR appearances during and after treatment and their relation to clinical outcome. AJR 1995; 164:1429-1434.[Abstract/Free Full Text]
  33. Tazaki H, Nakashima J, Nakagawa K. MRI evaluation of cavitation induced by laser prostatectomy. J Endourol 1995; 9:171-173.[Medline]
  34. Castro JD, Lufkin RB, Saxton RE, et al. Metastatic head and neck malignancy treated using MRI guided ILP: an initial case report. Laryngoscope 1992; 102:26-32.[Medline]
  35. Vogl TJ, Mack MG, Mueller P, et al. Recurrent nasopharyngeal tumors: preliminary clinical results with interventional MR imaging-controlled laser-induced thermotherapy. Radiology 1995; 196:725-733.[Abstract/Free Full Text]
  36. Boulnois JC. Photophysical processes in recent medical laser developments: a review. Las Med Sci 1986; 1:47-66.



This article has been cited by other articles:


Home page
 Am. J. Roentgenol.Home page
S. N. Goldberg, G. S. Gazelle, and P. R. Mueller
Thermal Ablation Therapy for Focal Malignancy: A Unified Approach to Underlying Principles, Techniques, and Diagnostic Imaging Guidance
Am. J. Roentgenol., February 1, 2000; 174(2): 323 - 331.
[Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Mueller-Lisse, U. G.
Right arrow Articles by Reiser, M. F.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Mueller-Lisse, U. G.
Right arrow Articles by Reiser, M. F.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
RADIOLOGY RADIOGRAPHICS RSNA JOURNALS ONLINE