PROCEDIMIENTO DE EXPORTACIÓN VÍA AÉREA
DECLARACIÓN ÚNICA DE ADUANA 3.2.2.1 ASPECTOS GENERALES
Diffusion in a living system is an intrinsic property of tissues that is independent of magnetic resonance relaxation properties (e.g. longitudinal and transverse relaxation) (57, 58). The effects of diffusion on the attenuation of precessing nuclear spins were firstly reported by Erwin Hahn in 1950 (59). The precessing nuclear moments contained in liquid molecules, mostly of low viscosity, are not only attenuated by the influence of longitudinal time and transverse time but also de-phased by the self-diffusion of the molecules when placed in an inhomogeneous magnetic field (58).
To obtain information about molecular diffusion in an imaging experiment, a diffusion- encoding gradient is applied as an additional readout gradient. The addition of the diffusion gradient leads to compression of the MR signals in the time domain and produces a poorer signal-to-noise ratio (SNR) (58). The common imaging technique used in diffusion MRI is Echo Planar Imaging (EPI) which can be the application of multiple sequences of gradient-echo or spin-echo sequences.
For a fixed diffusion weighting and a single diffusivity, it can be shown that the signal in a diffusion-weighted experiment is given by equation (2.21).
𝐼 = 𝐼0𝑒−𝑇𝐸/𝑇2𝑒−𝑏𝐷 (2.21) where 𝑇2 is the transverse relaxation time, 𝐼0 is the signal intensity in the absence of any 𝑇2 or diffusion weighting, 𝑇𝐸 is the echo time and 𝑏 is the ‘b-factor’ or ‘b-value’.
The approximation of the diffusivity of DW-MRI is independent of the T2. This diffusivity, 𝐷 is quantitatively derived as follows.
𝐼1 = 𝐼0𝑒−𝑇𝐸/𝑇2𝑒−𝑏1𝐷 (2.22)
𝐼2 = 𝐼0𝑒−𝑇𝐸/𝑇2𝑒−𝑏2𝐷 (2.23)
(2.24)
where 𝐼1 and 𝐼2 are signals in a diffusion weighted experiment at b-values of 𝑏1 and 𝑏2 respectively.
Derivation of Apparent Diffusion Coefficient 2.2.2.1
During the DWI procedure, a target image frame is divided into small and equal imaging voxels. In each imaging voxel, a diffusion-weighted intensity varies according to the rate of attenuation of precessing nuclear spins. Spin attenuation varies depending on tissue properties, for example diffusion attenuation in CSF is much greater than that observed in white matter because the diffusion of water molecules is relatively unhindered in the CSF than that in the white matter.
After DWI acquisition, a reconstruction map namely Apparent Diffusion Coefficient (ADC) can be calculated by using DWI at two b-values as shown in the equation (2.24). An ADC map measures the magnitude of diffusion of molecules in mm2/s and can be rewritten as equation (2.25). 1 2 1 2 ln ) ( 1 I I b b D
𝐴𝐷𝐶 = − (ln𝐼1
𝐼0) /(𝑏1− 𝑏0) (2.25)
where 𝐼1 and 𝐼0 is the signal intensity with gradient having 𝑏1 s/mm2 and without the diffusion weighting having 𝑏0 = 0 s/mm2 respectively.
Both DWI and ADC maps provide measurement of molecular diffusion. However, a DWI and an ADC image exhibit dissimilar image intensity on restricted and unrestricted space as shown in Figure 2.12.
(a) DWI at b = 0 s/mm2 (b) DWI at b-value = 1000 s/mm2 (c) ADC
Figure 2.12: Signal intensity of restricted and unrestricted space based on DWI and ADC. An unrestricted area like CSF is bright on DWI at b = 0 s/mm2 and an ADC map, but dark on DWI at b-value = 1000 s/mm2 (case EP904). It is vice versa for a restricted area.
On areas with restricted diffusion, DWI appears bright, whereas ADC appears dark because accumulated nuclear spins in the area results in high signal on DWI and limited diffusion activity gives low signal intensity on ADC. On areas with unrestricted diffusion, DWI appears dark, whereas ADC appears bright because a small number of nuclear spins in the area result in low signal on DWI and unhindered diffusion gives high signal on ADC. The comparison of signal intensity obtained from DWI and ADC in restricted and unrestricted areas is tabulated in Table 2.1.
Table 2.1: Comparison of signal intensity of DWI and ADC images.
Image Signal intensity in restricted area Signal intensity in unrestricted area DWI Bright (Spins are more accumulated
and yield higher signal.)
Dark (Less spins are caught and yield lower signal.)
ADC Dark (Molecules cannot diffuse randomly; give low signal.)
Bright (Molecules can move more freely; give high signal.)
The Optimal b-Value 2.2.2.2
The b-value is a factor in diffusion weighted sequences and summarises the influence of the gradients on a DWI. The b-value can be estimated from the Stejskal-Tanner equation (2.26). The b-value is increased by either increasing the gradient strength (𝐺) or the temporal separation of the gradients (δ).
𝑏 = (𝛾2𝐺2𝛿2) (∆ −𝛿
3) (2.26)
where 𝛾 is the gyromagnetic ratio and ∆ is the time interval between the leading edges of the gradient lobes.
The higher the b-value, the stronger the diffusion weighting, which may show lesions more vividly but offers poorer SNR, as shown in Figure 2.13 and 2.14 because of longer TE, increased T2 and increased susceptibility to magnetic field gradient (60, 61). Susceptibility to magnetic field gradient accelerates the dephasing between protons, resulting in signal decay through T2* or severe image distortion. The optimal b-value was suggested by Bito et al. (62) to be 1.1/ADC which is considered as a rule of thumb. However, the gradient power is restricted as shown in the Stejskal-Tanner equation (2.26). To obtain the optimal b-value, the diffusion-encoding gradient must be increased, but this will result in poorer SNR as mentioned above. The optimal b-value in practice is
recommended to be in the range of 900 to 1200 s/mm2 (61). In the clinical setting, the acquisition of diffusion MRI is commonly done at b = 0 as a reference, and up to 1000 s/mm2 (generally found at 1000, 800 and 500 s/mm2). A b-value of zero corresponds to an EPI T2-weighted image.
Generally an ADC map is acquired based on two b-values, which can be less accurate to reflect the pathology information than an ADC map derived from three b-values (e.g. b=0, b=500 and b=1000 s/mm2) because the lower SNR of b=1000 s/mm2 images presenting a higher standard deviation can be partially compensated by the median value of b=500 s/mm2 (60).
(a) b-value = 0 (b) b-value = 800 s/mm2
Figure 2.13: The difference of imaging features and deterioration of SNR-based DWI at b- value = 0 and 800 s/mm2 (case MB719).
(a) b-value = 0 (b) b-value = 500 s/mm2 (c) b-value = 1000 s/mm2
Figure 2.14: The difference of imaging features and deterioration of SNR-based DWI at b- value = 0, 500 and 1000 s/mm2 (case MB950).
Rotational Variance 2.2.2.3
ADC in human white matter appears to depend on the direction of the applied diffusion- encoding gradient (63, 64). In the same region, if a subject moves when a particular diffusion-encoding gradient is applied, ADC values will be altered. This directional dependence or rotational variance of ADC could lead to complicated interpretation of the actual diffusion tissue in the particular region and could result in incorrect analysis. This issue can be addressed by using the rotationally invariant model of diffusion tensor imaging as described in the following section.