Quantitative Ultrashort Echo Time Magnetic Resonance Imaging Methods

MRI has been widely used in clinical examinations for its rich contrast in soft tissues and non-invasive nature.Ultrashort TE(UTE)technique overcomes the echo time limitation from clinical routine sequences,and is able to image tissues with short(1ms<T2<10ms)and even ultrashort T2(<1ms),including bone,Achilles tendon,ligament,cartilage,and myelin sheath.The quantitative T1 and T2 relaxation times of above tissues have been applied for disease diagnoses.However,the clinical applications of traditional UTE method have been limited by it low scan efficiency,long scan time,and limited single quantification output.In this thesis,we aimed to improve the quantification efficiency and robustness of UTE imaging through optimization of signal excitation,acquisition and quantification model.First,the excitation pulse was redesigned and optimized using parallel transmission MRI.We proposed to implement a UTE method using a self-refocused gradient extended spiral-in RF pulse and spiral-out acquisition,based on 2D spatial selective parallel transmission pulse design method,which doubles the speed of traditional 2D UTE method.A squeezed trajectory design method was proposed to reduce the peak RF voltage and integrated power,which could improve the patient safety in MRI scanning.The squeezed trajectory design method was applied in reduced FOV imaging to either accelerate scan under the same image resolution or increase image resolution under the same scan time.Second,the acquisition and quantification model in UTE method was optimized to simultaneously quantify multiple tissue parameters.We proposed a UTE-MRF method in combination with MRF scheme.MRF encodes tissue relaxation times into signal waveform via continuous changing scan parameters in data sampling,such that it can simultaneously quantify multiple relaxation times in one acquisition.However,MRF could not image fast decay spins due to its TE limitation.In the proposed UTE-MRF,TE was shortened to 0.05 ms.To increase the sensitivity of UTE-MRF to short and ultrashort T2 tissues,a sinusoidal TE variation pattern was introduced into MRF signal encoding.Frequency offset map was directly derived through amplitude demodulation using the phase images from MRF image series.UTE-MRF simultaneously produced T1,T2,PD,B0 and bone enhanced image in a single scan.Finally,2D UTE-MRF method was further extended with dual echo readout and into 3D acquisition for brain imaging.TE was found to have an impact in the quantification of brain tissue relaxation times using the dual echo UTE-MRF method,which could be related to short T2 and ultrashort T2 components in the brain.A whole brain coverage with isotropic resolution of 0.75×0.75×0.75mm3 was achieved in 3D UTE-MRF.Dual echo UTE-MRF and 3D UTE-MRF methods showed robust quantification result in areas with high susceptibility,e.g.,frontal lobe,nasal cavity and skull basis.In this work,the above mentioned methods were proposed to solve the problems of long scan time,single parameter measurement,and limited measurement range of tissue relaxometries in quantitative UTE imaging.These methods could be further extended to clinical applications,high field magnetic resonance imaging,and multimodality PET-MRI systems.