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Optimizing T2 Quantitative Estimation of Knee Cartilage with Magnetic Resonance Imaging Applying Dictionary-based Methods
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Resumo(s)
Relaxation time T2 is very useful for early detection of osteoarthritis in the knee articular cartilage using magnetic resonance imaging, as this condition is characterized by the loss of collagen integrity and increase of water content that leads to an increase of the T2 value. In clinical settings, T2 mapping is usually performed with a multi-spin-echo sequence followed by a mono-exponential fitting. However, the dictionary-based method has enabled more accurate T2 estimates by matching the multi-spin-echo data to a dictionary of pre-computed echo modulation curve. The focus of this dissertation is to compare different multi-spin-echo protocols for T2 mapping, in a phantom, by varying the flip angle of the refocusing radiofrequency pulse, as this affects the total acquisition time in order to meet the restrictions on the specific absorption rate, to find the most suitable protocol for in vivo acquisitions in the knee articular cartilage. A series of acquisitions were performed with the different multi-spin-echo sequence protocols on the ISMRM/ NIST phantom with a 3.0 T scanner, varying only the refocusing pulse flip-angle from [90, 120:5:145, 180]º, creating T2 maps through a mono-exponential fitting and echo modulation curve match for each protocol. The results obtained for both methods confirm a higher accuracy of T2 estimates with echo modula tion curve match compared to mono-exponential fitting. The comparison of the estimates obtained for the different protocols with the dictionary-based method showed reasonable values, however the 125º was considered the most suitable flip angle protocol for the in vivo acquisitions. The intra-scanner repro ducibility of the method for the selected protocol showed reliable results, exhibiting similar estimates for the three repetitions. This protocol was subsequently used in preliminary in vivo tests, with application of the dictionary method in three asymptomatic healthy subjects, obtaining promising values, of 32.3±8.8 ms; 36.9±10.5 ms; 36.7±9.5 ms, compared to the literature, which presents T2 values for a healthy knee cartilage at around 33.0 ms. The impact that physical stress has on the estimates of T2 in cartilage was also studied, revealing a decrease in the relaxation time after direct impact on the articular cartilage (T2 at rest vs stress, 36.7±9.5 ms vs 34.6±10.1 ms and 36.0±9.1 ms vs 34.9±10.1 ms, for the right and left knees, respectively). The overall results of this work presented a practical multi-spin-echo protocol for clinical use of knee cartilage imaging to create T2 maps estimated with the dictionary-based method.
Descrição
Tese de mestrado integrado, Engenharia Biomédica e Biofísica (Engenharia Clínica e Instrumentação Médica) Universidade de Lisboa, Faculdade de Ciências, 2022
Palavras-chave
Mapas T2 Imagem por Ressonância Magnética Método do Dicionário Osteoartrite Sequência Multi-Spin-Echo Teses de mestrado - 2022