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Publication Date: 2016-Aug-19
Document File: 5 page(s) / 278K

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The Prior Art Database

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CEM43 based RF dose monitoring method and UI

Problem description

MRI systems deposit significant RF energy in the human body. Apart from Specific Absorption Rate (SAR), the total deposited energy (SA or SED) is a relevant parameter to monitor. This is part of current MR systems and user interfaces.

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A known problem related to SED is that it continually increases with time, whereas thermal burden may decay if the RF exposure is paused for a while. In other words, SED cannot account for dissipated energy. Actual temperature evaluation is needed to deal with pauses and dissipation effects.

Another problem is that SED accumulates Whole Body (WB) SAR, whereas thermal discomfort or risks are primarily associated with local SAR (causing hot spots versus systemic temperature rise from WB SAR). There is an interest to provide a more accurate control mechanism based on the local SAR hot spot location, which can vary with table position (i.e. patient position relative to the body coil) or using advanced RF pulse schemes with each having another hotspot location (but similar overall WB SAR).

The effectiveness of dissipation (body cooling) and therefore the actual thermal burden to a patient resulting from deposited energy depends on the patient state (age, disease), and on the SAR level. Thus, information is needed if the patient can thermo-regulate normally or is compromised (e.g. diabetic or under certain drug treatment).

Temperature increase has been calculated from electromagnetic simulations of 10g peak spatial SAR (psSAR10g) distribution in different human models, and the Pennes Bio-Heat equation, taking into account cooling through blood flow (Murbach et al, 2013). Transient and steady state temperature were evaluated for different RF power input levels (characterized by WB SAR values), and correlated with both WB SAR and psSAR10g. Using this model, based on multiple numerical human models (Neufeld et al, 2015), worst case temperature increase can be calculated for each episode of RF exposure. This is given by the formula ΔT = d.SQRT(ΔTnonreg), with d = 1.5 for (realistically) impaired thermoregulation, and ΔTnonreg = c.psSAR10g.

The risk for thermal damage has been investigated extensively for cancer treatment with hyperthermia. It appears that a parameter called CEM43, the cumulative equivalent minutes at 43 degrees, relates well with cell damage. Thresholds as currently proposed are

CEM43 is given by

With R=2 if T>43°C, and R=4 if T≤43°C.

This CEM43 metric is highly non-linear, and depends on starting temperature, and rate of temperature increase induced by RF exposure. During non-RF exposure episodes, temperature may decrease, but CEM43 continues to increase. Like temperature, CEM43 can be evaluated locally (hotspots in the body).

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Also, RF exposure from a scan at low start temperature (at the beginning of the exam) may have negligible effect in terms of CEM43, while the same scan at hig...