MRI-based anatomical model of the human head for specific absorption rate mapping

Abstract

In this study, we present a magnetic resonance imaging (MRI)-based, high-resolution, numerical model of the head of a healthy human subject. In order to formulate the model, we performed quantitative volumetric segmentation on the human head, using T1-weighted MRI. The high spatial resolution used (1 x 1 x 1 mm(3)), allowed for the precise computation and visualization of a higher number of anatomical structures than provided by previous models. Furthermore, the high spatial resolution allowed us to study individual thin anatomical structures of clinical relevance not visible by the standard model currently adopted in computational bioelectromagnetics. When we computed the electromagnetic field and specific absorption rate (SAR) at 7 Tesla MRI using this high-resolution model, we were able to obtain a detailed visualization of such fine anatomical structures as the epidermis/dermis, bone structures, bone-marrow, white matter and nasal and eye structures.

Fig. 1

Representative coronal (left) and sagittal (right) sections comparing the segmentation results of the CMA head with the Schnitzlein and Murtagh (1985) head atlas [43]. Key: 1 epidermis, 2 ear/pinna, 3 nasal structures, 4 SC tissue, 5 connective tissue, 6 soft tissue, 7 aqueous humor, 8 vitreous humor, 9 cornea, 10 lens/iris, 11 R/C/S, 12 outer table, 13 diploe, 14 inner table, 15 dura, 16 bone, 17 teeth, 18 air (resp./diges./sinus), 19 mastoid/air cells, 20 adipose, 21 orbital fat, 22 SC fat/muscle, 23 muscle, 24 tongue, 25 nerve, 26 spinal cord, 27 blood/vessel, 28 CSF-subarachnoid

Fig. 2

Electrical structural entities. Maps of density ρ (kg m⁻³) (top), conductivity σ (S m⁻¹) (center), and permittivity ε_r (bottom), at 300 MHz for the different ASEs segmented. The maximum scale was fixed at ε_r = 70 and σ = 1 for illustrative purposes. These values were used for FDTD simulations at 7 T MRI

Fig. 3

Results of segmentation of the head model in eight representative pairs of coronal sections. In each pair, the segmented image is shown on the left and the image with the color-coded structural entities (SEs) is on the right (brain SEs are masked in black). The complete segmentation can be found at the following URL: http://www.cma.mgh.harvard.edu/head_model/brain.php

Fig. 4

3-D view of head model. a Epidermis; b muscle (red), outer table (white) and eye regions (lens, cornea and vitreous humor); c dura mater (gold), blood vessels (red), and bone (white); d dura mater (gold), blood vessels (red) and eye regions; (e) dura mater (gold), blood vessels (red), eye regions, tongue (light red), ears and nasal structures (pink) and bone (white); f outer table, bone (white) and blood vessels (red); g cerebral cortex (green), blood vessels (red), and eye regions; h cerebral white matter (offwhite), blood vessels (red) and eye regions

Fig. 5

Results of FDTD simulations. The FDTD simulations implemented with the proposed head model allowed for precise EM computation and mapping. (Right) MRI data on the human subject at 7 T showing the characteristic central brightening effect. (Center) simulated B field mapped on the numerical model. (Left) related SAR distribution in the human head

Fig. 6

The wealth of anatomical tissue classes make the 3D-rendering of the model an ideal tool for anatomical visualization and SAR mapping (in W/kg), as shown in this 3D view of the epidermis/dermis (a), grey matter (b), bone structures (i.e., outer table, inner-table, bone, teeth (c), blood vessels (d), diploe (e), and eye-region, i.e., vitreous humour, aqueous humour, retina/choroid/sclera, lens, cornea, optical nerve (f). SAR values were normalized to obtain a whole-head SAR equal to 0.46 W/kg. Max SAR in scale equal to 1 W/kg

Fig. 7

The pie-chart shows the percentage of RF-power absorbed by each anatomical structure. The absorbed RF power (i.e., RF energy per unit time) was higher for ASEs with high volume and conductivity, such as epidermis/dermis, grey matter, white matter, and muscle, and lower for ASEs with low conductivity, such as bone or outer/inner table