Arteryel Spin Etiketleme ile Manyetik Rezonans Görüntülemede Echo- Planar Kodlama Hatalarının Manyetik Alan İzlemesi ve Gradyan Dürtü Fonksiyonu ile Düzeltimi

Arteryel spin etiketleme (ASE) tabanlı perfüzyon manyetik rezonans görüntüleme tekniği çok düşük sinyal-gürültü oranına sahiptir. Dolaysıyla bu yöntem k-uzayı verisinin hızlı bir şekilde toplanmasını gerektirir. Bu genellikle veri-okuma sekansı olarak gradyan-echo EPI kullanılarak sağlanır. Bu hızlı k-uzayı geçişi, gradyanların sistem kapasitesinin limitlerinde çalışmasını ve veri toplama sırasında çok hızlı yön değiştirmelerini gerektirir ve bu gereklilik gradyan alanlarında sapmalara sebep olur. Gradyan alanlarının zamana göre değişimlerinde meydana gelen bu sapmalar görüntü oluşturmada önemli hatalar ortaya çıkmasına neden olur. Bu çalışmada, gradyan alanlarının oluşumları dinamik bir manyetik alan kamerası ile doğrudan ölçülerek ASE görüntüleri asıl gradyan alanları kullanılarak yeniden oluşturulmuştur. Bununla beraber, gradyan sisteminin doğrusal ve zamanla-değişmez bir sistem olduğu varsayılarak gradyan dürtü fonksiyonu (GDF) tanımlanabilir. Tanımlanan GDF, bir defaya mahsus bir kalibrasyon yöntemi olarak asıl gradyan şekillerinin kestirimlerinin yapılmasında ve bu kestirimler yardımı ile görüntü oluşturulmada kullanılabilir. Ek olarak ASE görüntüleri, GDF ile kestirimi yapılmış gradyan alanları kullanılarak yeniden oluşturulmuştur. Gradyan alanı sapmalarının nicel perfüzyon manyetik rezonans görüntülerine olan etkileri araştırılmıştır

Correcting the Echo-Planar Image Encoding Imperfections in Arterial Spin Labeling MRI using Magnetic Field Monitoring and Gradient Impulse Responses

Arterial spin labeling (ASL) based perfusion MRI has inherently very low signal-to-noise ratio (SNR). Therefore, these methods require rapid k-space coverage. This is often achieved by using a readout sequence such as gradient-echo-EPI. However, such a fast k-space traversal typically needs the gradients to be operated at the maximum of the hardware specks and to be alternated rapidly during the execution which makes acquisition highly sensitive to gradient field deviations. The resulting deviations in the gradient field time courses cause substantial artifacts on the reconstructed images. In this work, we directly recorded the gradient waveform fidelities using a dynamic field camera technology and reconstructed the acquired ASL data with monitored gradient waveforms. Furthermore, by assuming that the gradient chain is a linear time-invariant (LTI) system that enables to define the gradient impulse response function (GIRF) of the system as a one-time calibration procedure, the actual gradient waveforms can be predicted. We therefore additionally performed the reconstruction based on such GIRF-predicted k-trajectories. The ultimate effects of gradient field deviations on to the absolute perfusion maps were explored

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