活體之心肌血氧對比影像---利用具T2磁化準備之穩定態磁振造影

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行政院國家科學委員會專題研究計畫成果報告

活體之心肌血氧對比影像：利用具 T2 磁化準備之穩定態磁振造影

計畫類別：個別型計畫

計畫編號： NSC93-2218-E-011-093-

執行期間： 93 年 12 月 01 日至 94 年 07 月 31 日執行單位：國立臺灣科技大學電機工程系

計畫主持人：黃騰毅共同主持人：劉益瑞

計畫參與人員：吳宣諭、莊子聿

報告類型：精簡報告

處理方式：本計畫涉及專利或其他智慧財產權，1 年後可公開查詢

中華民國 94 年 10 月 26 日

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活體之心肌血氧對比影像：利用具 T2 磁化準備之穩定態磁振造影 In-vivo Myocardial Blood Oxygen-level Dependent(BOLD) imaging by the novel

MR sequence:T2-prepared TrueFISP 計畫編號：NSC 93-2218-E-011 -093 執行期限：93 年 12 月 1 日至 94 年 7 月 31 日

主持人：黃騰毅台灣科技大學電機工程系共同主持人：劉益瑞逢甲大學自動控制系一、中文摘要

我們的研究在發展一個快速且對物體移動不敏感的影像技術，適合用於偵測心肌血氧對比影像。這個影像技術由 T2 磁化對比準備及暫態 TrueFISP 所組成，可用於絕對的心肌 T2 參數定量。實作的過程包括研發組合是射頻脈衝來減少 B0/B1 磁場不均勻所造成的 T2 參數量度誤差、自動化的在掃描過程中改變 T2 磁化準備、以及暫態 TrueFISP 影像法去加速影像的擷取(一個心跳之內完成)。在人體實驗裏我們證實了我們能得到心肌 T2 參數的絕對定量圖，而沒有明顯的影像假影。結合了這些優點，我們認為這項技術可成為可靠且敏感性高的心肌血氧對比影像序列。

關鍵詞：血氧對比影像，心肌

Abstract

A fast and motion-insensitive technique, which is suitable for myocardial BOLD imaging, is presented in this work. This method consisted of T2-magnetization preparation and transient-state TrueFISP (T2-TrueFISP) to obtain absolute T2 map on the myocardium. The implementation included the composite RF pulses to minimize the B0 and/or B1 effect on the T2 preparation, the alternation of T2 preparation during scan for the T2 mapping, and transient-state TrueFISP to speed up the readout to acquire the prepared magnetization within one heart beat. In-vivo experiments showed that absolute myocardial T2 mapping can be obtained without prominent artifacts. Combining the advantages, the measurements of myocardial T2 demonstrated in this study can be a robust and sensitive tool for the myocardial BOLD contrast evaluation.

Keywords: myocardial T2 map, BOLD imaging, transient-state, balanced steady-state free precession

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二、緣由與目的

Myocardial blood oxygenation level-dependent (BOLD) MRI is a promising tool to non-invasively assess the hemodynamic significance of coronary artery disease(1-3). The method is based on the detection of local T2 or T2* changes in the myocardium in response to a vasodilatory challenge (4-6). Administration of vasodilatory drugs such as dipyridamole or adenosine increases coronary blood flow in the healthy myocardium, and produces changes in tissue blood volume and blood oxygenation, leading to local changes in myocardial T2 and T2* relaxations. Myocardial BOLD contrast has been successfully demonstrated in animal models (7-9), healthy humans (6,10) and patients with ischemic coronary artery disease (1-3) or with heart failure (11).

Current techniques, based on T2* BOLD contrast, are all affected by magnetic field inhomogeneities making regional evaluation difficult in some of the myocardial walls, especially along the heart-lung interface (12,13).

Mapping T2 BOLD contrast instead of T2* contrast offers a promising alternative approach (14). Although the T2 changes from BOLD contrast are smaller in the myocardium than the T2*

changes (15), the insensitivity to field inhomogeneities makes T2 mapping a better strategy for the detection of regional differences in the heart. A method providing high T2 contrast, high SNR and good motion insensitivity in the heart is thus highly desirable.

In this project, we proposed to investigate the possibility of obtaining robust in-vivo myocardial BOLD imaging by overcoming various technical difficulties. Using the novel transient-state TrueFISP imaging technique plus T2 magnetization preparation, we looked into the alternations in T2-relaxation changes according to different T2 preparation and the overall image quality, especially at heart-lung interface. We also verified the measured T2 relaxation times by phantom studies with our method and standard spin-echo.

三、結果與討論

Sequence implementation

Transient-state TrueFISP with T2 preparation (abbreviated T2-TrueFISP hereafter) was successfully implemented on a 1.5T whole-body MR system (Magnetom Sonata, Siemens Medical Solution, Erlangen, Germany) for measurements of myocardial T2 relaxation. As shown in Fig.1(a), the T2-TrueFISP scheme employed in our work consisted of 3 blocks: 1) T2 magnetization preparation block (T2-prep), 2) transient state stabilization (STAB), and 3) transient state TrueFISP readout, whose functions will be described later. Absolute T2 mapping was achieved by altering TEs in the T2-prep block plus single exponential fitting on a pixel-by-pixel basis. In addition, the T2-TrueFISP sequence was combined with free-breathing scanning techniques, PACE and SBI. The accuracy of the T2 maps obtained using our method was validated using an MnCl2 phantom against standard CPMG multi-echo measurement values, as well as on healthy subjects against measurements using multi-echo spin-echo EPI. The sequence parameters and protocol were optimized for myocardial BOLD study in vivo, including the T2-prep echo time (TE ), transient state TrueFISP echo trains, and the methods to control

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T2 Preparation

S T A B

Transient State TrueFISP readout

Spoiler (a)

(b)

90 180 180 180 180 90 x y y -y -y -x

τ 2τ 2τ 2τ τ

(c) Spoiler

90 90 x -x

Fig.1 (a) The 3 blocks of T2-TrueFISP, including T2 magnetization preparation, transient state stabilization (STAB), and transient state TrueFISP readout (b) T2-prep at the shortest possible echo time (c) T2-prep at the longer echo time controlled by the refocusing time τ.

Phantom experiments

Fig.2(a) shows the images obtained from the MnCl2 phantom study. T2-weighted images with different TE (22ms, 44ms, 66ms, 88ms) in the upper row and the lower row were acquired by MESE and T2-TrueFISP, respectively. The concentration of each bottle was depicted in Fig.2(b).

Notice that the image contrast acquired by T2-TrueFISP was fairly similar to the images acquired from MESE. In T2-TrueFISP, minor distortions in signal intensity can be found on the boundary of the bottles. Some image ghosts were also found particularly in the background regions.

T2-prep TrueFISP

22ms 44ms 66ms 88ms

Multi-echo Spin-echo

(a)

0.3mM

0.4mM

0.5mM Water

0.1mM MnCl2

0.2mM

(b)

Fig.2 (a) the images obtained from MnCl₂ phantom study. T2-weighted images with different TE (22ms, 44ms, 66ms, 88ms) in upper and lower row were acquired by MESE and T2- TrueFISP (flip angle=50), respectively. The approximate concentration of each bottle was depicted by (b). Note the image contrasts obtained by both method are fairly similar. Subtle artifacts are found in T2-TrueFISP images, which may be due to the strong susceptibility at the boundary of bottles.

Fig.3 shows the relationship between the T2 values obtained from T2-TrueFISP measurements and those obtained from MESE measurements. No prominent difference in T2 was found using T2-TrueFISP with different flip angles. The T2 values measured from T2-TrueFISP were also highly correlated (R² > 0.999 for the data acquired by all flip angles) with the T2 values measured by MESE.

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0 50 100 150 200 250 300 0

50 100 150 200 250 300 350

T2 measurement of MnCl

2 phantom by different method

T2 (ms) measured from MESE

T2 (ms) measured from T2-TrueFISP

20ô 30ô 40ô 50ô 60ô 70ô

Fig.3 The relation between the T2 values obtained from T2-TrueFISP measurements with different flip angles and the T2 values obtained from MESE measurements. The good agreement between the two sequences showed the prepared T2 contrast was accurately acquired by the transient-state TrueFISP readout. The data points of the bottle containing pure water were removed because the short TE range (22-88 ms) used in our study did not allow accurate estimations of long T2 (~

sec).

In vivo T2 measurements

Figs.4(a,b) and Figs.4(c,d) show the images acquired from one of the subjects (male, 29 yrs) using T2-TrueFISP with hard pulses and composite pulses, respectively. Note that the banding artifacts, indicated by white arrows, can be clearly observed in the image acquired using hard pulses (upper row) but not in the images acquired using composite pulses (lower row). Figs.4(c) and (d) are acquired using different values of TET2-prep (c: 2.59ms, d: 55ms). Notice the signal intensity of myocardium is changed according to the echo time of the T2 preparation. Neither motion artifacts nor flow artifacts were found.

Hard Pulse

Composite Pulse

2.59ms 55ms

(a) (b)

(c) (d)

Fig.4 the T2-TrueFISP images acquired from one of the subjects (male, 29yrs), using hard pulses with different TET2-preps(a:2.59ms,b:55ms) and composite pulses with different TET2-preps (c:2.59ms,d:55 ms ) , respectively. Note that the banding artifacts, indicated by white arrows, could be clearly seen in the image acquired using hard pulses but not in the images acquired using composite pulses.

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performed within one breath-hold to obtain a total of 4 acquisitions plus one dummy scan. From the T2-TrueFISP protocol, 4 images (TET2-prep: 2.59ms, 55ms, 2.59ms, 55ms, matrix size: 128x128) were obtained. Two example images with TET2-prep equal to 2.59ms and 55ms are shown in Fig.5(a) and Fig5(b), respectively. Fig.5(c) shows the T2 map calculated from the 4 images.

From the ssMESE-EPI protocol, a total of sixteen images (4 averages with 4 TEs) were obtained.

The two images with TE equal to 34ms and 68ms are shown in Fig.5(e) and Fig.5(f), respectively.

Fig.5(g) shows the T2 map calculated from the sixteen images. Figs.5(d) and (h) are the zoomed versions from Figs.5(c) and (g), respectively, for better visualization. Notice that the image distortions indicated by the white arrows in the ssMESE-EPI images (Fig.5(f)) are absent in the T2-TrueFISP images (Fig.5(b)). The border between blood pool and myocardium can be clearly identified in the T2-TrueFISP images as compared with ssMESE-EPI, due to higher spatial resolution and less flow artifacts. The mean T2 values across all ROIs of the nine subjects scanned with both sequences are 54.37±5.80ms and 51.16±4.07ms from T2-TrueFISP and ssMESE-EPI, respectively.

0ms 20 40 60 80 100

Short TE Long TE T2 map Zoomed T2-map T2-

TrueFISP

ssMESE- EPI

(a)

(e)

(b)

(f)

(c)

(g)

(d)

(h)

0ms 20 40 60 80 100

Short TE Long TE T2 map Zoomed T2-map T2-

TrueFISP

ssMESE- EPI

(a)

(e)

(b)

(f)

(c)

(g)

(d)

(h)

Fig.5 T2-weighted images and its according T2 map acquired from one subject (male,34yrs) (a) T2-TrueFISP image with TET2-prep :2.59ms (b) TET2-prep :55ms. (c) T2 map (d) zoomed T2 map.

(e)ssMESE-EPI image with TE:34ms (f)TE:68ms (g) T2 map (h) zoomed T2 map. Notice the image distortions indicated by the white arrow in (f) are not shown in the T2-TrueFISP images (b). The border between blood pool , myocardium and papillary muscle can be clearly identified in the T2-TrueFISP images due to their higher resolution and less flow artifact

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四、結論

We conclude that by combining the T2-TrueFISP sequence and the PACE method, the highly reproducible measurements of myocardial T2 as demonstrated in this study suggest that our proposed technique is a robust and sensitive tool for myocardial BOLD contrast evaluation.

The result of this study has been published in the meeting of international society of magnetic resonance in medicine, Miami, U.S.A.

五、參考文獻

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