The susceptibility effects in NMR (nuclear magnetic resonance) produce some artifacts in conventional spin echo imaging as well as gradient echo imaging which is recently popular. In this thesis, the nature of susceptibility effects is exploited for both spin echo imaging and gradient echo imaging, and subsequently a correction method of the artifacts such as geometrical displacement and signal loss is presented. In gradient echo imaging techniques recently under investigation in various forms of SSFP (steady state free precession), FLASH (fast low angle shot), GRASS (gradient recalled acquisition steady states mode) and FISP (fast imaging steady state free precession), the signal loss phenomenon is specially severe because the phase dispersion of spins due to field inhomogeneity is not refocused with the gradient refocusing inherent in the method. Therefore, these techniques based on gradient echo imaging have been difficult to use in clinical situation.
As a reduction of the susceptibility artifact in gradient echo imaging, a technique using a tailored RF pulse is proposed. It is applied to the case of high local magnetic field inhomogeneity due to the susecptibility. On the basis of slice selection, the signal loss and void phenomena due to susceptibility in a voxel are studied and a correction method is also discussed. The description of the tailored RF pulse for this purpose and its application are given and experimental results obtained using a human volunteer are presented.
Furthermore the bloods containing deoxyhemoglobin such as veins are known as paramagnetic substances. These paramagnetic substances surrounding normal tissues which are known as diamagnetic substances produce the magnetic field inhomogeneity around their vicinities. Consequently, the signal from these vicinities where field inhomogeneities are created is reduced or void out due to the phase incoherency of spins. This signal loss phenomenon apparently appears in gradient echo imaging, and is known as susceptibility effect in bloods.
An angiography technique using the susceptibility effect of bloods is proposed. Magnetic property of blood containing deoxyhemoglobin is known as paramagnetic and it produces susceptibility effect. By use of specially tailored RF pulse, the signals from normal tissues are suppressed while the signals from the blood interfaces where strong susceptibility-induced field is created are enhanced. The design strategies of the tailored RF pulse and its operation onto the regions of normal tissues and the blood interfaces will be discussed. Experimental results obtained using both a phantom and a human volunteer are also presented.
In general, an image contrast due to susceptibility effect is developed around an object with high magnetic susceptibility material. This is due to that the different susceptibilities within object produce field gradient around the object so high that the phase in voxel becomes highly non-uniform, i.e., the phases of spins within a voxel are incoherent. In this thesis we propose a general scheme of manipulating the susceptibility effect and eventually develop the technique for the contrast enhancement using tailored RF pulses. As an example, an imaging of blood containing deoxyhemoglobin which is paramagnetic relative to surrounding tissues is performed. Experimental results and a theoretical study of the method on the contrast enhancement and the artifact correction will be reported.
As an application of the techniques with tailored RF pulses, a novel flow suppression technique is proposed. The pulsatile nature of blood flow makes zipper-like artefacts along the coding direction in 2D Fourier transform image. Spatial presaturation, one of the correction methods, is known to be effective in eliminating flow artefacts when the Fourier spin echo acquisition is employed. However, this method requires additional RF pulse and spoiling gradient for presaturation. In this paper a new flow saturation technique which does not require additional saturation-RF and gradient is proposed. The proposed technique is equivalent to the exsting saturation technique but the elimination of the flow component is achieved by a pair of tailored 90˚-180˚ RF pulses in the spin echo sequence. By use of two tailored RF pulses with opposite polarity, a linear phase is generated for those moving materials and consequently all spins of moving materials become dephased thereby no signal. Computer simulations and experimental results obtained using both a phantom and a human volunteer are also presented.
In appendix, some advances of chemical shift imaging techniques are described. Usually, in high field inhomogeneity environments, e.g., in the case of that the field inhomogeneity of image region is much severer than chemical shift difference between water and fat, three point measurements were required to separate the water and fat correctly. As and advanced technique, the water and fat separation with only two measurements, is proposed and thereby saving the experimental time required for field inhomogeneity correction.
As another advance for chemical shift imaging, the automatic correction of the water-fat overlapping effect due to phase wrap around is also proposed. This technique is then applied to dual-peak spectroscopic imaging with large magnetic field inhomogeneity and one set of chemical shifts imaging is obtained under such circumstance with single scan. The pulse sequence being implemented with the proposed technique is given and some experimental results obtained with phantoms and human volunteers are presented.
All the experiments in this thesis are performed with a 2.0 T KAIS whole-body NMR system.
본 논문은 핵자기 공명 영상에서 자화율에 의한 효과를 분석 하고, 이를 바탕으로 자화율에 의한 영상 훼손을 감소시킨 영상법 및 이것을 발전시킨 혈액속의 강자성 물질을 이용한 새로운 정맥 영상술을 제안 하였다. 또한, 여기서 제안한 펄스 합성 방법을 이용하여 혈류에 의한 영상 훼손을 감소시키는 새로운 영상술을 개발하였다.
먼저, 자화율에 의해서 영상 훼손을 감소시키는 영상법이 새롭게 제안 하였는데, 자화율에 의한 효과는 영상의 화소내 위상분포를 변화시키므로 결국 영상 신호의 감소로 나타나게 된다. 여기에서 화소 내의 위상분포는 이 논문에서 새롭게 제안한 RF 펄스로 조절할 수 있다. 즉, 선택된 영상 단면 내에서 원하는 위상분포가 결정되면 컴퓨터에 의해서 RF펄스가 합성 된다. 즉 디자인된 펄스에 의해 생성되는 위상 분포는 자화율에 의해 생기는 위상 분포를 보상 시켜줌으로서 영상훼손을 감소시킨다. 이 방법의 장점은 기존의 영상 기법을 그대로 쓰면서 새로 디자인된 RF 펄스를 써서 기존의 방법들로는 풀수 없었던 자화율에 의한 영상 훼손을 쉽게 감소시키는 장점을 갖고 있다.
한편, 사람 혈액속에 헤모글로빈은 산소와의 결합상태에 따라서 산소와 많이 결합되있는 동맥의 혈액은 반자성을, 산소와 비교적 결합이 덜한 정맥의 혈액은 상자성의 자기 특성을 갖는다. 상자성 물질은 보통 핵자기 공명 영상에서 위에서 언급한 자화율에 의한 효과로 나타난다. 이런 효과를 이용하면 새로운 정맥 혈관 영상술이 가능하다. 본 논문에서는 영상의 위상분포를 조절하는 합성된 RF 펄스를 사용하여 혈관이외의 부분에서는 신호가 나오지 않게 하고 자화율의 효과가 있는 혈관에서만 신호가 나오게 하여 결국 혈관만의 영상을 얻는 새로운 영상 기법을 제안 하였다. 이 영상 기법을 인체의 머리 부분에 적용하여 정맥만의 영상을 얻었다.
마지막으로 여기서 제안된 RF 펄스의 개념을 이용하여 보통의 스핀에코 영상법에서 문제가 되어 왔던 유속에 의한 영상훼손을 제거하였다. 기존에 사용되어진 RF 펄스대신에 선택된 화소 내에 포물선 위상 분포를 갖도록 RF 펄스를 합성 하여 가해주면 90˚ RF 펄스에 의해 생성된 포물선 위상과 180˚ RF 펄스에 의해 생긴 포물선 위상의 차에 의해서 단면 내에 흐르는 물질의 화소내의 스핀들은 서로 상쇄되는 일차원적인 위상이 생성되게 된다. 결국 흐르는 물질은 이 상쇄시키는 일차원적인 위상에 의해서 신호를 만들지 못하고 오직 움직이지 않는 물질에서만 신호가 나오게 된다. 이 방법은 기존의 흐름 감소 영상법이 특별한 펄스를 부가시켜 사용해야만 하는 단점이 있는데 반하여 기존의 펄스 시이컨스(sequence)를 그대로 사용한다는 장점이 있고 쉽게 기존의 펄스 시이컨스에 적용될 수 있다.
이상의 새로운 방법들은 컴퓨터 모의 실험및 인체 실험을 통하여 그 효용성을 입증하였고 앞으로 실제 임상실험에 그 응용을 기대할 수 있을 것이다.
