MRI的表面線圈-1270954-楊英健總結(jié).ppt

1、橫向磁場的射頻表面線圈(敏感度和空間選擇性),1270954 楊英健,表面線圈的發(fā)展現(xiàn)狀,在一個(gè)標(biāo)準(zhǔn)的磁共振成像(MRI)系統(tǒng)中，核磁共振(NMR)信號的檢測是基于一個(gè)或多個(gè)射頻(RF)線圈的法拉第感應(yīng)的現(xiàn)象。,RF線圈用于傳輸射頻脈沖和檢測存在靜態(tài)主磁場B0的人體內(nèi)發(fā)出的MR信號。射頻線圈產(chǎn)生的磁場B1和主磁場B0垂直。,RF射頻線圈緊密接觸樣本用于產(chǎn)生駐波模式 。 駐波模式：頻率和振幅均相同、振動(dòng)方向一致、傳播方向相反的兩列波疊加后形成的波。,最近，以高場強(qiáng)的MRI為背景，在長時(shí)間的刺激下，通過移動(dòng)射頻波發(fā)射磁場，由正交驅(qū)動(dòng)的RF天線接收MR信號，NMR實(shí)驗(yàn)表明一個(gè)主要優(yōu)勢是可以使磁場更加

2、均勻覆蓋樣本。,大多數(shù)的MRI和MR光譜(MRS)研究已經(jīng)應(yīng)用了各種各樣的射頻表面線圈，包括雙重調(diào)諧線圈設(shè)計(jì)。近幾年，專門設(shè)計(jì)的由很多表面射頻線圈組成的射頻相控陣列已經(jīng)用來提高信噪比(SNR)，或者通過并行采集技術(shù)才減少獲取時(shí)間。 將電路頻率調(diào)節(jié)到諧振狀態(tài)的行為或過程；特指通過改變電感、電容來實(shí)現(xiàn)頻率的改變，以使接收設(shè)備（如收音機(jī)）的頻率與所收的信號發(fā)生共振的一種頻率調(diào)節(jié)。,在傳統(tǒng)的法拉第RF線圈的MRI系統(tǒng)中，NMR檢測的靈敏度與由線圈產(chǎn)生的RF磁場B1的幅值成比例。由于這種原因，在很多的臨床MRI掃描儀中，經(jīng)常用到獨(dú)立的體積較大的僅用于發(fā)射的RF線圈和一個(gè)僅用于接收的RF表面線圈來進(jìn)行試驗(yàn)

3、優(yōu)化。大體積的發(fā)射線圈確保在整個(gè)樣本上產(chǎn)生良好的均勻射頻磁場，同時(shí)接收射頻表面線圈給與被至于線圈表面較近的組織提供最大的敏感度。,另一種線圈設(shè)計(jì)由橫向磁場射頻表面線圈組成，橫向磁場射頻表面線圈由很多線性的有傳導(dǎo)能力的elements組成，置于我們感興趣區(qū)域的中心，通過圓形（或者矩形）的返回導(dǎo)電路徑相連接。在線性elements中流動(dòng)的電流在射頻線圈的中心區(qū)域產(chǎn)生一個(gè)B1磁場，這個(gè)磁場實(shí)質(zhì)上是一個(gè)橫向的射頻磁場。,最簡單和使用最多的射頻表面線圈的設(shè)計(jì)是由圓環(huán)或者矩形環(huán)組成，來提供B1磁場，這個(gè)磁場在線圈的中心區(qū)域產(chǎn)生并垂直于線圈平面(軸向的射頻磁場)。 這種射頻線圈的設(shè)計(jì)尤其適合帶有水平磁場B0

4、的MRI系統(tǒng)，很多的MRSMRI已經(jīng)被臨床前和臨床應(yīng)用證實(shí)了。,在線性elements中流動(dòng)的電流在射頻線圈的中心區(qū)域產(chǎn)生一個(gè)B1磁場，這個(gè)磁場實(shí)質(zhì)上是一個(gè)橫向的射頻磁場。,Fig.1 Schematic design and prototypes of CL (A, C) RF coils tuned to 64 MHz and used for MRI testing with phantoms.,所謂的“蝶形線圈”提出兩個(gè)或者更多的elements在給定的角度和中心位置下彼此交叉。蝶形線圈提供的B1磁場分布的最大值是在線性elements的交叉處，并且磁場的幅值沿著遠(yuǎn)離正交于線圈平面的方

5、向減小。最近，沿著蝶形線圈軸線的固有SNR的獨(dú)立性進(jìn)行了詳細(xì)的研究。,所謂的“8型”（FO8）線圈提出了至少兩個(gè)中心線性電流，沿著平行方向傳導(dǎo)，兩個(gè)線圈在彼此給定的距離放置。FO8線圈所提供的B1磁場分布的最大值沿著相應(yīng)的線性elements，呈帶狀分布。FO8線圈最開始被提出是用于垂直磁場的的MRI系統(tǒng)中。有2篇文獻(xiàn)已經(jīng)表明與標(biāo)準(zhǔn)的軸向磁場圓形圓環(huán)（CL）線圈進(jìn)行比較，F(xiàn)O8線圈顯示出在沿著線圈Z軸在一個(gè)特定的位置產(chǎn)生了最大的B1磁場強(qiáng)度。,Fig1.Schematic design and prototypes of FO8 (B, D) RF coils tuned to 64 MHz

6、and used for MRI testing with phantoms.The diameter is 2R = 100 mm and the separation between the linear parallel elements of the FO8 coil is 2s = 10 mm. We assume that the main B0 field is directed along the y-axis.,在前人的研究中，已經(jīng)表明了由多個(gè)線性elements構(gòu)成的FO8線圈也可以在平行于線圈表面的X-Y平面產(chǎn)生顯著的在空間上具有選擇行的B1，盡管標(biāo)準(zhǔn)的CL線圈在相同的R

7、OI中心可產(chǎn)生恒定的射頻場的幅值。,來自于已經(jīng)定義好的樣本的ROI MRI/MRS信號，對于空間的選擇和加強(qiáng)，F(xiàn)O8射頻線圈的具體特征相當(dāng)有趣。例如，我們已經(jīng)表明，用2個(gè)elements的FO8線圈原型的調(diào)諧頻率為64Hz，這樣在MRI掃描儀內(nèi)，當(dāng)線圈朝向特定的方向時(shí)，避免信號的損失是有可能的。,而且，最近的研究表明對于那些在活的有機(jī)體內(nèi)MRS中的應(yīng)用，信號是來自樣本特定的解刨區(qū)域，距離表面有一定深度的地方，F(xiàn)O8線圈的射頻磁場的空間分布需要空間的選擇和信號的增強(qiáng)具有一定的優(yōu)勢。,在最近的一篇論文中，對于MRS應(yīng)用，我們已經(jīng)比較了CL線圈和FO8線圈。MRS數(shù)據(jù)的獲取來自成人的小腿，研究表明為

8、了得到一個(gè)良好的SNR，F(xiàn)O8線圈的直徑應(yīng)該選擇為2R=10cm，兩個(gè)線性電流elements的距離是2s=1cm。 PRESS 光譜集中在線圈表面的12mm，在大約20mm的區(qū)域以內(nèi)，獲得了比同樣直徑的標(biāo)準(zhǔn)CL更高的PRESS SNR。 在相對狹窄的區(qū)域并且非常接近表面的情況下，小型的CL線圈表現(xiàn)出了一個(gè)更高的B1磁場幅值，并且在更深處的位置幅值迅速減少。,應(yīng)用準(zhǔn)靜態(tài)的比奧-薩法爾定律來模擬B1磁場的分布,比奧-薩法爾定律,用準(zhǔn)靜態(tài)的比奧-薩法爾定律數(shù)值積分來模擬RF磁場的B1。,用這種方法，我們來研究RF磁場的三維分布（FO8線圈的不同的幾何參數(shù)）。,Fig2.The simulated

9、RF magnetic field components (B1x, B1y, B1z) and the modulus of the field, normalized to unit current, of the CL (A) and FO8 (B) coils, evaluated in the xy plane at z = 6 mm. For both coils the diameter was 10 cm and the FO8 coil had 2s = 0.6 cm.,Fig. 2A, the B1x and B1y field components of the CL c

10、oil show a linear variation in the central ROI, while the B1z component is quite flat in the same central ROI. As expected, see Fig. 2A, in the same xy plane the |B1| field distribution of the CL coil is flat in the central ROI and increases towards the position of the coil circular current path. Fi

11、g. 2B shows that in the xy plane the B1x field component of the FO8 coil exhibit a maximum value along a narrow strip, directed along the y direction, corresponding to the position of the two linear current elements. In Fig. 2B is also shown that the B1y component is quite flat and the B1z component

12、 increases with a steep linear variation in the same central ROI. As shown in Fig. 2B the |B1| field of the FO8 coil, in the central ROI, exhibit a maximum value along a narrow strip corresponding to the linear current elements.,圖3,The simulated RF magnetic field components (B1x, B1y, B1z) and the m

13、odulus of the field, normalized to unit current, of the CL (A) and FO8 (B) coils,evaluated in the xy plane at z = 30 mm. For both coils the diameter was 10 cm and the FO8 coil had 2s = 0.6 cm.,Fig. 4. Simulated (AC) and measured (D) RF |B1| field distributions corresponding to a CL coil (2R = 10 cm)

14、 and FO8 coils (2R = 10 cm) with 2s = 0.6 and 1 cm. The data in the xy plane were evaluated at z = 6 mm.,As shown in Fig. 3A and B, as we move at a large distance (z = 30 mm) from the coil plane, the field components of both the CL and FO8 coils decreases of about one order of magnitude. Moreover, f

15、rom Fig. 3 it can be seen that the |B1| field distribution of both coils are quite similar, with a broad maximum in the same central ROI. Figs. 2 and 3 provide the comparison of the spatial distribution of each RFfield component and the field modulus in the transverse plane at two distinct locations

16、. These plots are useful for qualitative and quantitative comparison of the two RF coil configurations. For example, the information about the RF field components (B1x, B1y, B1z) is useful in the design process when selecting the RF coil orientation with respect to the main field B0 or when consider

17、ing the operation mode as transmit/receive or receive-only.,工作臺的校正,我們可以用工作臺和MRI方法對射頻磁場B1分布進(jìn)行測量。在工作臺上對線圈進(jìn)行測試，用擾動(dòng)球面的方法來繪制磁場的空間分布和測量RF磁場的最大幅值。 RF線圈的敏感度 其中Pc在共振頻率下，通過網(wǎng)絡(luò)分析器耦合在RF線圈上的電源。網(wǎng)絡(luò)分析器通過匹配電路連接到RF線圈，作為RF源。這個(gè)參數(shù)用來比較這兩種線圈B1磁場的敏感度和磁場沿Z軸的空間分布。,The tuning/matching chip capacitor values and the measured RF

18、parameters (S11, Q) of the CL and FO8 RF surface coil prototypes tuned to 25.85 MHz.,a. All coil prototypes were tuned to 25.85 MHz. b .S11 = 10 log(Pr/P0), where P0 and Pr are the incident and reflected power to the coil, respectively. S11 was measured with a network analyser (HP8753A). c .Q = f0/

19、f, where f0 and f are the resonant frequency and the 3 dB bandwidth, respectively. d .The phantom was a plastic cylinder (diameter 10 cm) positioned at about 2 mm from the RF coil surface and containing 100 ml of physiological saline solution.,Theoretical and experimental values of the RF coil sensi

20、tivity (), the position of maximum RF field along the z-axis (zmax), the RF field selectivity along the z axis( z), and the normalised in-plane spatial homogeneity (x/2R, y/2R) of the CL and FO8 RF coils (2R = 6, 10, 18 cm; 2s = 0.6, 1, 2 cm). The CL and FO8 prototypes were tuned to 25.85 MHz.,a.All

21、 coil prototypes were tuned to 25.85 MHz. b.The theoretical RF coil sensitivity, , was obtained as |B1|/Pc1/2, where |B1| is the modulus of the calculated RF field amplitude at position zmax and Pc(=I2R/2) is the power provided by a unit current into the RF coil when matched to a resistance of 50. c

22、.The measured RF coil sensitivity, , was obtained as |B1|/ Pc1/2, where |B1| is the modulus of the measured field amplitude at position zmax and Pc is the power coupled to the RF coil by the network analyzer at the resonance frequency and with impedance matching better than- 20 dB.,Fig. 5. Normalize

23、d RF field distribution, |B1|FO8(z)/|B1|CL(0), of FO8 coils as a function of the axial z-position calculated for a range of: (A) 2R values and constant 2s = 1 cm; and (B) 2s values and constant 2R = 10 cm. zmax,Fig. 5B shows the simulated |B1| field distributions along the z-axis of medium size FO8

24、coils (2R = 10 cm) obtained by setting 2s = 1, 2, 3 cm. For comparison, the z-axis |B1| field distribution of the CL coil with 2R = 10 cm is also shown. The |B1| field amplitude was normalized to |B1|CL(0), which is the maximum field value of the CL coil (2R = 10 cm) obtained at z = 0. The CL coil s

25、hows a |B1| field distribution with a maximum at z = 0 and a gradual decrease as the distance from the centre is increased. At z about 4 cm the |B1| field of the CL coil reaches its half amplitude.,As shown in Fig. 5B, the |B1| field distribution of the FO8 coils is zero at z = 0 and it shows a maxi

26、mum at zmax = 0.5 cm for 2s = 1 cm, and the position maximum value moves far from the coil plane as 2s increases. We observe that for a given FO8 coil diameter there is always a 2s range that ensures a higher |B1| amplitude (with respect to the CL coil of same diameter) within a region along the z-a

27、xis. For example, the results of Fig. 5B show that to obtain a |B1| amplitude gain within the region 0 z 2.5 cm, the element separation must fulfil the condition 2s 2 cm.,Fig. 6. Normalized theoretical RF field sensitivity, S = |B1|FO8(zmax)/|B1|CL(0), of FO8 coils versus the 2R parameter for severa

28、l 2s values. The RF field sensitivity versus 2R is linear for all 2s values reported and the logarithmic scale was chosen for easy of representation of the data.,As shown in Fig. 6, to be able to obtain an effective sensitivity gain (S 1) it is necessary to choose the appropriate 2R and 2s values. F

29、or example, it is evident that for large diameters (2R 20 cm) there is a wide range of 2s values (0.6 2s 5 cm) giving a significant sensitivity gain. Conversely,for a coil diameter less than 20 cm, the sensitivity gain is obtained only within a restricted range of 2s values. In the central ROI, as 2

30、R increases, the field contribution of the circular current return path of the FO8 coil become negligible with respect to the field contribution given by the linear current elements; for a given 2s value it is always feasible to select a FO8 coil diameter producing a sensitivity gain larger than 1.,

31、Fig. 7. Normalized theoretical zmax position, corresponding to the maximum |B1| value of the FO8 coil, versus the 2R parameter for a range of 2s values.,As shown in Fig. 7, the zmax data show a moderate increase as the coil diameter increases,depending on the selected 2s value. For example, with 2s

32、= 0.6 cm the zmax value is practically constant to about 0.3 cm; while for 2s = 5 cm the zmax value varies between about 1.7 and 2.3 cm, within the range of 2R values studied. For a fixed 2R value, however, the position zmax shows a significant variation as a function of the 2s values. For example, at 2R = 10 cm the zmax is between 0.3 and 1.7 cm as 2s ranges from 0.6 to 5 cm.,Fig. 8. Normalized theoretical RF field selectivity, z, of the FO8 coilver