磁共振影像图像质量控制概要.ppt

* Activity one Go to a film station. View the images. Record you observations (changes in SNR, Spatial Resolution, Contrast, and Time) in the Quick Look book. Activity two Image B represents the standard protocol at your institution. The next patient to be scanned needs a slice thickness of image A so you cut the slice thickness to 3 mm. How much SNR change resulted from image B to image A? The radiologist requires the SNR in image B. 2. Describe the scan parameter changes you will make to return the SNR to image B status. * The increase in SNR is significant when the bandwidth is narrowed. Reducing the bandwidth in half (16kHz to 8kHz) is equivalent to doubling the NEX (2 to 4). The benefit is no increase in time. There are many negatives though. 1. As the RBW narrows the chemical shift increases. Chemical shift is a mismapping of the signal on the matrix in the frequency direction. The signal mismapping occurs at fat/water interfaces and is due to the variation in precessional frequency between fat and water. A detailed discussion is in Application Guide Volume 2. The chemical shift artifact is more severe as field strength increases which means that for high field users there is a significant tradeoff when buying signal by reducing RBW. 2. Another way to think about RBW is the frequency at which we’re sampling the signal. Clearly when the signal is sampled at a higher frequency, we wouldn’t have to listen as long as we would with a lower RBW in order to acquire 256 samples. Therefore, the readout gradient is on for a longer time as RBW narrows, and thus the TE is pushed out. 3. Pushing the TE out can lead to increased motion and potentially less slices. （2）採用呼吸門控技術，調整相位編碼與運動週期同 步，消除呼吸運動偽影； （3）縮短檢查時間，如選梯度回波成像、減少信號采 集次數或改變矩陣等； （4）用預飽和技術，去除呼吸時腹壁運動引起的偽影； （5）屏氣，減少呼吸運動； （6）腹帶加壓，以限制呼吸幅度。 2、自主性運動偽影 人體自主運動吞咽、眼球轉動等，可在圖像上產生各種不同形狀的偽影，造成圖像模糊、品質下降。 （1） 改變掃描參數，儘量縮短掃描時間，減少產生偽影的幾率，如梯度回波技術的應用、減少信號採集次數、減少矩陣等； 控制自主性運動偽影的措施： （2）儘量使病人體位舒適，可用海綿塊或