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Journal ArticleOPEN ACCESS

Use of depth dependent attenuation characteristics of thermal energy deposition to increase spatial uniformity of HIFU ablation in large uterine fibroids

  • Ismail S
  • Bilgen M
Medical Instrumentation (2013) 1(1) 1
DOI: 10.7243/2052-6962-1-1
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Abstract

Purpose : In high intensity focused ultrasound (HIFU) treatment of large fibroid, depth dependent attenuation of thermal deposition leads to insufficient temperature rise for necrosis in deeper tissue. The purpose is to determine the relationship between temperature rise (T) and sonication parameters focal depth (F), applied energy (E) and spot size for inducing more uniform heat distribution throughout the fibroid. Materials and methods : Measurements were performed on 10 patients with large fibroids and a phantom using magnetic resonance guided focused ultrasound (MRgFUS) system ExAblate 2000 ® . Peak temperatures were measured from various size sonication spots placed at different depths and empirically modeled using linear regression T(F,E) = a*F+ b*E+c , where (a,b) are constant coefficients and c is zero crossing. Results : Phantom study yielded T(F,E) = -0.3458 *F +0.03653 *E +57.2. For the fibroids, a varied from -0.396 to -0.162 and b varied from 0.009 to 0.038 while c ranged between 27.81 and 103.22, depending on the selected spot size. The values for the coefficient of determination were close to 1 in all cases. This indicated that the model represented the measurements reasonably well and could reliably be used for predicting necessary sonication energy to produce temperature rise required at any desired depth. Based on this, a simple algorithm was outlined to estimate the constants (a,b,c) from only three sets of measurements (T,F,E) for a given sonication condition. Conclusion : In clinics, predicting temperature profile prior to treatment enables compensating attenuation via increasing applied sonication energy, planning optimal strategies with a primary goal of ensuring more uniform distribution of thermal ablation in large fibroids, and thereby improving therapeutic efficiency.

Figures

  • Figure 1. Sagittal MR image of a large uterine fibroid from one of the patients recruited in this study.
  • Figure 2. Four selected spots displayed on a background image of Daily Quality Assurance phantom. The target spots were identical in size (5.6 mm x 27.0 mm) and placed in the phantom at depths (a) 125.3 mm, (b) 144.7 mm, (c) 159.8 mm (d) 182.8 mm away from the transducer’s front face.
  • Figure 3. a) Spatial distribution of the temperature rise following 20 s sonication of the third spot at 159.8 mm depth, as shown in (Figure 2). The temperature rise was represented by a color code overlaid on the background sagittal and coronal phase images. b) Temporal profile of the temperature rise at the indicated location. The red curve represents the temperature at a selected pixel and the green curve is the average of the readings within a 3x3 kernel centering that pixel.
  • Table 1. Demographics of recruited patients. Fibroid dimension was given by maximum distances measured between its walls viewed in different orientations using the digital caliper of the ExAblate2000 software. The anterior and posterior distance was measured in the sagittal plane. Similarly, width and height measurements were made in the axial and coronal planes, respectively. The fibroid volume was estimated from the 3D volume data by the software using automatic segmentation.
  • Figure 4. Sagittal fast spin echo T2-weighted images of the large uterine fibroids from the remaining 9 patients recruited in this study. The image acquisition parameters were TR/TE= 6,467 ms/111 ms, slice thickness= 4 mm, number of averages=2 and echo train length=24.
  • Figure 5. Four selected spots displayed on a background anatomical abdominal image. The target spots of identical size (4.9 mm in diameter x 24.4 mm in length) were placed within the fibroid at (a) 5.9 mm, (b) 21.0 mm, (c) 35.9 mm and (d) 51.8 mm depth from the skin line.
  • Figure 6. a) The measurements of treatment temperature from Daily Quality Assurance phantom at four target spots shown in (Figure 2) against the energy and focal depth of the sonication when the spot size was 5.6 mm in diameter and 27 mm in length. The plots in b) and c) are the projections of the data into 2 D planes. The data shows nonuniform behavior, meaning the temperature rise is lesser in the deeper fibroid tissue. But, increasing the applied energy proportionally elevates the temperature rise. This trend was empirically expressed by the formula T(F,E) = -0.346*F+ 0.037*E+57.2. The solid lines in the plots denote the linear fits to the measurements according to this equation.
  • Figure 7. The measurements of the treatment temperature similar to those in (Figure 6) but from a fibroid when the spot size was 4.9 mm in diameter and 24.4 mm in length. The solid lines are the predictions by the formula T(F,E) = -0.162*F+ 0.013*E+67.72.

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APA

Ismail, S. Z., & Bilgen, M. (2013). Use of depth dependent attenuation characteristics of thermal energy deposition to increase spatial uniformity of HIFU ablation in large uterine fibroids. Medical Instrumentation, 1(1), 1. https://doi.org/10.7243/2052-6962-1-1

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