ECG Triggering and Gating for Ultrasonic Small Animal Cardiac Flow Imaging

由於老鼠與人類在基因和構造上的相似性，老鼠被廣泛的用做心血管疾病的研究模型。而超音波具有非侵入式和即時的成像特性，適合對心臟構造和血液功能做連續和長時間的觀測。但一般的醫用超音波不適用於快速搏動(約300-600bpm)和尺寸微小(小於10mm)的老鼠心臟，必須應用空間解析度達100μm以下的高頻超音波進行成像。現行的商用高頻超音波系統仍以單一探頭做機械式掃描為主，限制了成像的每秒影格數(frame rate)在30-60fps或更低。本研究應用心電圖訊號來觸發心臟超音波影像的擷取，再進行回溯式的影像重建，可等效的提升每秒影格數至數百至數千張，以對快速搏動的老鼠心臟進行高時間解析度的造影。 在實驗室之前的研究中，對心搏率300-400bpm的麻醉中老鼠，以發射中心頻率25MHz, 脈波間隔頻率(PRF)2kHz的高頻超音波，進行心電圖觸發的心臟B-mode影像重建，等效每秒影格數可達2000fps. 而本研究希望在心電圖觸發的B-mode成像基礎上，發展血液流速成像(Color Doppler mode). 首先以低頻探頭(3.5MHz), 脈波間隔頻率1kHz對血流仿體環境進行模擬心電圖訊號觸發的成像，重建出符合血流輸出波形變化的流速圖。接著進一步將此功能實現在實驗室開發的單一探頭高頻系統上。以高頻探頭(30MHz), 脈波間隔頻率5-10kHz再次進行仿體實驗。並對照PW Doppler取得的流速對時間變化，比較出重建的color Doppler影像在時序上是與PW Doppler相吻合的，但速度的量值普遍較PW Doppler低。 仿體實驗證實了，心電圖訊號觸發的回溯式血流成像，在血流速度呈現穩定的週期性變化下，可重建出等效每秒影格數達5000-10000fps的一系列color Doppler影像。而活體中的心電圖變異、紊亂的心臟血流等變因造成的影響，還需要實驗來探討。Ultrasound has been used to image murine models noninvasively and longitudinally in real-time for cardiovascular disorder studies. To improve spatial resolution to 100μm or higher, high frequency ultrasound (>20MHz) has to be used. Because of the frame rate limitation due to the fact that mechanical scanning on single element transducers need to be applied, the temporal resolution up to hundreds fps is hard to achieve. ECG gating strategies which synchronize the image acquisition with the cardiac cycle, can retrospectively reconstruct a series of images at equivalent high frame rate and thus is of interest in this study. In previous research, B-mode in vivo images were retrospectively reconstructed, with the center frequency of 25MHz and the pulse repetition frequency at 2kHz, and the frame rate is equivalently raised to 2000frames/second. The current study is extended from ECG gated/triggered B-mode imaging to Color Doppler imaging. Firstly, phantom experiments under the center frequency of 3.5MHz and the pulse repetition frequency at 1kHz were performed. The series of Color Doppler images corresponding to an output flow cycle were reconstructed with emulated ECG output that gates data acquisition. Phantom experiments were conducted again on a single element transducer high frequency system, under the frequency of 30MHz and the pulse repetition frequency at 5kHz~10kHz. The reconstructed Color Doppler images were compared to PW Doppler velocity profile quantitatively. The two are temporally matched and the spatial information is provided only by the former, but the velocity in the former is lower than that in the latter. ECG gating/triggering Color Doppler is suitable for retrospective flow imaging, taking the frame rate up to 5000-10000fps equivalently. In vivo tests with ECG and flow turbulence are on-going