Limits...
Treatment envelope evaluation in transcranial magnetic resonance-guided focused ultrasound utilizing 3D MR thermometry.

Odéen H, de Bever J, Almquist S, Farrer A, Todd N, Payne A, Snell JW, Christensen DA, Parker DL - J Ther Ultrasound (2014)

Bottom Line: We present two different types of treatment envelopes.The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating.Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring.

View Article: PubMed Central - HTML - PubMed

Affiliation: Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, Utah 84108, USA ; Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA.

ABSTRACT

Background: Current clinical targets for transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone. Non-central targets will have to be reached to treat a wider variety of neurological disorders and solid tumors. Treatment envelope studies utilizing two-dimensional (2D) magnetic resonance (MR) thermometry have previously been performed to determine the regions in which therapeutic levels of FUS can currently be delivered. Since 2D MR thermometry was used, very limited information about unintended heating in near-field tissue/bone interfaces could be deduced.

Methods: In this paper, we present a proof-of-concept treatment envelope study with three-dimensional (3D) MR thermometry monitoring of FUS heatings performed in a phantom and a lamb model. While the moderate-sized transducer used was not designed for transcranial geometries, the 3D temperature maps enable monitoring of the entire sonication field of view, including both the focal spot and near-field tissue/bone interfaces, for full characterization of all heating that may occur. 3D MR thermometry is achieved by a combination of k-space subsampling and a previously described temporally constrained reconstruction method.

Results: We present two different types of treatment envelopes. The first is based only on the focal spot heating-the type that can be derived from 2D MR thermometry. The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating. This utilizes the full 3D MR thermometry data achieved in this study.

Conclusions: It is shown that 3D MR thermometry can be used to improve the safety assessment in treatment envelope evaluations. Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring. The results presented in this study highlight the need for 3D MR thermometry in tcMRgFUS.

No MeSH data available.


Related in: MedlinePlus

Treatment envelopes based on 2D MRTI. Three orthogonal views of the treatment envelopes based only on the focal spot heating for the phantom (a) and lamb (b) studies. Near-field temperatures are not considered for this type of envelope. Temperature rises of at least 10°C can be seen in large parts of the intracranial volume in both cases. This is the type of envelope that can be derived from 2D MRTI. In both (a) and (b), the temperature maps are overlayed on magnitude images. In (a) and (b), the three views are color coded, with the colored frame and arrows indicating each view’s position in the other two views.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC4199783&req=5

Figure 6: Treatment envelopes based on 2D MRTI. Three orthogonal views of the treatment envelopes based only on the focal spot heating for the phantom (a) and lamb (b) studies. Near-field temperatures are not considered for this type of envelope. Temperature rises of at least 10°C can be seen in large parts of the intracranial volume in both cases. This is the type of envelope that can be derived from 2D MRTI. In both (a) and (b), the temperature maps are overlayed on magnitude images. In (a) and (b), the three views are color coded, with the colored frame and arrows indicating each view’s position in the other two views.

Mentions: Figure 2 shows results from the hydrophone scans with and without the plastic and lamb skulls in the US beam path. In (a–b) and (e–f), 2D plots are shown for the plastic skull and lamb skull studies when electronically steering the US focus 5 mm in x and y. In (c–d) and (g–h), line plots through the focal spot in the x- and y-directions for the two studies are shown. All plots are normalized to the peak intensity measured in corresponding scans in the water bath only (i.e., without the skull in the US beam path). Intensity losses of about 85% (for the phantom study) and 70% (for the lamb study) were observed when focusing through the skulls. Only slight beam aberrations were observed in both cases, but in the phantom, a slight positional shift of the focus (~1 mm) in the anterior-posterior direction was observed.In Figure 3a,b, three orthogonal views of superpositions of the hottest time frame for all 29 and 32 sonications in the phantom and the lamb studies are shown. The focal spots can be seen to experience only slight beam aberration. The near field can further be seen to have more substantial temperature rise in the lamb study than in the phantom study. This is also highlighted in Figure 4, which shows transverse 2D thin-slab maximum intensity projections (MIP) of the tissue in the near field inside the skull (i.e., not including any focal spot heating, but just near-field voxels). The phantom, which can be placed closer to the transducer due to the larger acoustic window, experiences temperature rises in the near field of approximately 2°C–4°C, spread over a relatively large area (approximately 6 × 5 cm). For the lamb, the energy entering the skull is spread over a considerably smaller area (approximately 1 × 1.5 cm) because the transducer was located further from the skull, resulting in temperature rises of approximately 20°C–30°C.The temporal characteristics of the sonications are demonstrated in Figure 5 as the temperature rise as a function of time for the focal spot and the near field (mean of ten hottest voxels). Data for both the phantom and the lamb studies, and for both the most distal and the most proximal planes relative to the transducer, are shown.Figures 6 and7 show the two types of treatment envelopes derived in this study. Figure 6 is based only on the focal spot temperatures, and Figure 7 is based on the relative near-field heating as the ratio between the focal spot and corresponding near-field heating, made possible by 3D MRTI. In Figure 6, temperature rises greater than 10°C can be seen in large parts of the brains for both the phantom and the lamb, suggesting that therapeutic levels of FUS can be delivered to these areas. Figure 7 shows that relative near-field heating ratios of up to 3 can be achieved in the phantom study, whereas for the lamb study, the majority of the brain has a ratio less than 1, indicating that the near field will experience more heating than the focal spot when attempting to treat these areas.


Treatment envelope evaluation in transcranial magnetic resonance-guided focused ultrasound utilizing 3D MR thermometry.

Odéen H, de Bever J, Almquist S, Farrer A, Todd N, Payne A, Snell JW, Christensen DA, Parker DL - J Ther Ultrasound (2014)

Treatment envelopes based on 2D MRTI. Three orthogonal views of the treatment envelopes based only on the focal spot heating for the phantom (a) and lamb (b) studies. Near-field temperatures are not considered for this type of envelope. Temperature rises of at least 10°C can be seen in large parts of the intracranial volume in both cases. This is the type of envelope that can be derived from 2D MRTI. In both (a) and (b), the temperature maps are overlayed on magnitude images. In (a) and (b), the three views are color coded, with the colored frame and arrows indicating each view’s position in the other two views.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4199783&req=5

Figure 6: Treatment envelopes based on 2D MRTI. Three orthogonal views of the treatment envelopes based only on the focal spot heating for the phantom (a) and lamb (b) studies. Near-field temperatures are not considered for this type of envelope. Temperature rises of at least 10°C can be seen in large parts of the intracranial volume in both cases. This is the type of envelope that can be derived from 2D MRTI. In both (a) and (b), the temperature maps are overlayed on magnitude images. In (a) and (b), the three views are color coded, with the colored frame and arrows indicating each view’s position in the other two views.
Mentions: Figure 2 shows results from the hydrophone scans with and without the plastic and lamb skulls in the US beam path. In (a–b) and (e–f), 2D plots are shown for the plastic skull and lamb skull studies when electronically steering the US focus 5 mm in x and y. In (c–d) and (g–h), line plots through the focal spot in the x- and y-directions for the two studies are shown. All plots are normalized to the peak intensity measured in corresponding scans in the water bath only (i.e., without the skull in the US beam path). Intensity losses of about 85% (for the phantom study) and 70% (for the lamb study) were observed when focusing through the skulls. Only slight beam aberrations were observed in both cases, but in the phantom, a slight positional shift of the focus (~1 mm) in the anterior-posterior direction was observed.In Figure 3a,b, three orthogonal views of superpositions of the hottest time frame for all 29 and 32 sonications in the phantom and the lamb studies are shown. The focal spots can be seen to experience only slight beam aberration. The near field can further be seen to have more substantial temperature rise in the lamb study than in the phantom study. This is also highlighted in Figure 4, which shows transverse 2D thin-slab maximum intensity projections (MIP) of the tissue in the near field inside the skull (i.e., not including any focal spot heating, but just near-field voxels). The phantom, which can be placed closer to the transducer due to the larger acoustic window, experiences temperature rises in the near field of approximately 2°C–4°C, spread over a relatively large area (approximately 6 × 5 cm). For the lamb, the energy entering the skull is spread over a considerably smaller area (approximately 1 × 1.5 cm) because the transducer was located further from the skull, resulting in temperature rises of approximately 20°C–30°C.The temporal characteristics of the sonications are demonstrated in Figure 5 as the temperature rise as a function of time for the focal spot and the near field (mean of ten hottest voxels). Data for both the phantom and the lamb studies, and for both the most distal and the most proximal planes relative to the transducer, are shown.Figures 6 and7 show the two types of treatment envelopes derived in this study. Figure 6 is based only on the focal spot temperatures, and Figure 7 is based on the relative near-field heating as the ratio between the focal spot and corresponding near-field heating, made possible by 3D MRTI. In Figure 6, temperature rises greater than 10°C can be seen in large parts of the brains for both the phantom and the lamb, suggesting that therapeutic levels of FUS can be delivered to these areas. Figure 7 shows that relative near-field heating ratios of up to 3 can be achieved in the phantom study, whereas for the lamb study, the majority of the brain has a ratio less than 1, indicating that the near field will experience more heating than the focal spot when attempting to treat these areas.

Bottom Line: We present two different types of treatment envelopes.The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating.Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring.

View Article: PubMed Central - HTML - PubMed

Affiliation: Utah Center for Advanced Imaging Research, Department of Radiology, University of Utah, Salt Lake City, Utah 84108, USA ; Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA.

ABSTRACT

Background: Current clinical targets for transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone. Non-central targets will have to be reached to treat a wider variety of neurological disorders and solid tumors. Treatment envelope studies utilizing two-dimensional (2D) magnetic resonance (MR) thermometry have previously been performed to determine the regions in which therapeutic levels of FUS can currently be delivered. Since 2D MR thermometry was used, very limited information about unintended heating in near-field tissue/bone interfaces could be deduced.

Methods: In this paper, we present a proof-of-concept treatment envelope study with three-dimensional (3D) MR thermometry monitoring of FUS heatings performed in a phantom and a lamb model. While the moderate-sized transducer used was not designed for transcranial geometries, the 3D temperature maps enable monitoring of the entire sonication field of view, including both the focal spot and near-field tissue/bone interfaces, for full characterization of all heating that may occur. 3D MR thermometry is achieved by a combination of k-space subsampling and a previously described temporally constrained reconstruction method.

Results: We present two different types of treatment envelopes. The first is based only on the focal spot heating-the type that can be derived from 2D MR thermometry. The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating. This utilizes the full 3D MR thermometry data achieved in this study.

Conclusions: It is shown that 3D MR thermometry can be used to improve the safety assessment in treatment envelope evaluations. Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring. The results presented in this study highlight the need for 3D MR thermometry in tcMRgFUS.

No MeSH data available.


Related in: MedlinePlus