SENSE

Apart from having high SNR, one other distinct advantages of phased array coil is its ability to perform parallel imaging to accelerate MRI scan time. Parallel imaging such as the pioneering SMASH and SENSE achieve reduced MRI scan time by decreasing the number of sampling position in the phase encoding step of the K-space. However by doing so, aliasing occurred. It is then by mean of using sensitivity profile from individual surface coil, missing phase encoding information can be approximately recovered and full unaliased field of view image can be reconstructed. Parallel imaging can at lest reduce MRI scan time by two fold and this greatly reduce patient discomforts.

Foremost importance in SENSE is obtaining the sensitivity profile. Either through experimental mean, if you have excess to a MRI system but can be cumbersome or through numerical method which required tedious calculation and not to mention if dielectric properties is consider, the solution will get even more complicated and time consuming. A quick fix is to use FEKO to obtain an approximate sensitivity profile. In this section, we will demonstrate how easily you can obtain sensitivity profile through FEKO and run your own SENSE reconstruction simulation. The example shown here is for a 2 Tesla (85 Mhz) head coil simulation with reduction factor of 2. The sensitivity profile is calculated by FEKO while SENSE simulation is written in MatLab.

To begin, 4 surface coils resonating at 85 Mhz placed around a 4 layered dielectric sphere with each layer having approximated conductivity, permittivity and permeability that roughly represent a human head [8] is shown in Fig 1, this can be set using the GF card.

Fig 1

Assuming minimum mutual coupling exist between coils, each surface coil will then have sensitivity profile which is the magnetic field strength [9] in the near field calculated by FEKO as shown in Fig 2(a)-(d). Here the number of magnetic field data calculated is 128 x 128.

Fig 2(a)

Fig 2(b)

Fig 2(c)

Fig 2(d)

According to reciprocity theorem, multiplying the magnetic field of Fig 2(a)-(d) with a 128 x 128 pixel brain image of Fig 3, Fig 4(a)-(d) simulate the image received by individual surface coil.

Fig 3

Fig 4(a)

Fig 4(b)

Fig 4(c)

Fig 4(d)

Fig 5(a)-(d) is the 128x128 synthetic K-space which is obtained by performing 2D inverse Fourier transform of Fig 4(a)-(b). With a reduction factor of 2, every even number of the phase encoding part of the K-space in Fig 5(a)-(d) is not acquired, which we will then have a reduced 64x128 K-space data illustrated in Fig 6(a)-(d). This will caused aliased image as shown in Fig 7(a)-(d).

Fig 5(a)

Fig 5(b)

Fig 5(c)

Fig 5(d)

Fig 6(a)

Fig 6(b)

Fig 6(c)

Fig 6(d)

Fig 7(a)

Fig 7(b)

Fig 7(c)

Fig 7(d)

Using the sensitivity profile of Fig 2(a)-(d), and solving the unfolding matrix [3], a full 128x128 brain image is reconstructed from the aliased images as depicted in Fig 8.

Fig 8