Phased Array Coil

Phased array coil generally consisted of a number of mutually decoupled surface coils that simultaneously receive MR signals. Although an individual surface coil gives the highest possible SNR for a volume comparable to the diameter of the coil, a single surface coil can only effectively image a limited region whose dimensions are also comparable to the diameter of the coil. Thus, by combining these non-interacting surface coils forming a phased array coil, an image of a larger area can be obtained with uniform sensitivity and higher SNR as compared to standard MR volume coil.

The most important aspect in building a phased array coil is to minimize the mutual coupling that exists between neighbouring surface coils. Mutual coupling causes splitting of the resonance frequency and disrupt phased array transmit and receive pattern. Several decoupling techniques such as overlapping of coils [1], insertion of capacitors between coils [5-6] and network decoupling interface [7] are some of the methods used to reduce mutual coupling between coil elements.

In this section, we will demonstrate how a 2 Tesla (85 Mhz) receive only 3 element phased array coil using the overlapping method is constructed with the help of FEKO and tested in a Brukers 2T MRI system. The size of each surface coil is 120mm by 120mm using 10mm wide copper tape.

Overlapping method involves careful geometric overlapping of coils, so that mutual decoupling can be achieved. Before the actual prototype 3 element phased array coil is built, the design is run in FEKO to obtain the overlapping distant. Illustrated in Fig 1(a) shows 3 surface coils resonating at 85Mhz placed 5mm apart from each other, which causes the splitting of the resonance frequency in the S11 response as shown in Fig1(b). Fig 2(a) depict how the 3 surface coils are overlapped and the optimum overlapping distant between coils from FEKO simulation is 92mm. Fig 2(b) is the S11 response  showing  the splitting of the resonance frequency has merged back which indicates mutual coupling is minimized.

Fig 1(a)

Fig 1(b)

Fig 2(a)

Fig 2(b)

(Click picture to see enlarged)

From FEKO simulation results, the prototype 3 element phased array coil is constructed as closely to the design in FEKO simulation with overlapping distant between coils of 92mm as shown in Fig 3. Fig 4(a)-(c) are images of saline bags used as phantoms acquired by the prototype 3 element phased array coil using Bruker 2T MRI system. From these 3 images, it can be observed that mutual coupling between coils is minimized when correct overlapping distant is used and each surface coil effectively images only a small region comparable to their dimension.

Fig 3

Fig 4(a)

Fig 4(b)

Fig 4(c)