Department of Radiology, Seattle Children's Hospital, 4800 Sandpoint Way, Room R4488, Seattle, WA 98105, USA.
Pediatr Radiol. 2011 Oct;41(10):1259-65. doi: 10.1007/s00247-011-2082-7. Epub 2011 May 24.
MRI is being increasingly used to evaluate tissue relaxation in the setting of iron overload. Diagnostic accuracy is strongly dependent upon the acquisition and analysis methods employed. Typically, a multi-echo train of relaxation data is acquired, the resulting curve is fit using a non-linear (exponential) function, and the derived relaxation time is converted to iron concentration by a calibration formula derived from paired MRI-biopsy samples. A theoretically valid processing alternative is to fit a straight line to the relaxation data after logarithmic transformation (log-linear). This log-linear method is more computationally efficient, allowing a full relaxation map to be generated in near real time. This method is present on all scanner platforms and has been published for use in assessing iron concentration. These factors imply methodological validity.
To use in vivo and simulation data to show that log-linear fitting can generate highly erroneous relaxation results in iron-loaded tissues.
After IRB approval, exponential and linear fitting were compared in a cohort of 20 patients being evaluated for hepatic iron overload. Simulation analyses were performed to characterize the main factors impacting derived results.
In human subjects, log-linear analyses demonstrated gross deviation from exponential results at a moderate relaxation shortening (T2* ~5 ms). Simulation analyses demonstrated that the discrepancy was caused by noise effects and additional signal components violating mono-exponential function shape.
Log-linear processing results in increasingly erroneous estimation of T2* with iron-loading. Therefore, this method should not be employed for measurement of relaxation behavior in clinical samples.
MRI 正越来越多地用于评估铁过载情况下的组织弛豫。诊断准确性强烈依赖于所采用的采集和分析方法。通常，采集弛豫数据的多回波序列，使用非线性（指数）函数拟合得到的曲线，并通过从配对 MRI-活检样本中得出的校准公式将得出的弛豫时间转换为铁浓度。一种理论上有效的处理方法是对数转换（对数线性）后拟合弛豫数据的直线。这种对数线性方法计算效率更高，可以在接近实时的情况下生成完整的弛豫图。这种方法在所有扫描仪平台上都可用，并已发布用于评估铁浓度。这些因素暗示了方法学的有效性。
使用体内和模拟数据证明对数线性拟合可能导致铁负荷组织中的弛豫结果出现严重误差。
在经过 IRB 批准后，在 20 名接受肝脏铁过载评估的患者队列中比较了指数和线性拟合。进行了模拟分析以描述影响得出结果的主要因素。
在人体研究中，对数线性分析在中等弛豫缩短（T2*~5ms）时表现出与指数结果的明显偏差。模拟分析表明，这种差异是由噪声效应和违反单指数函数形状的其他信号分量引起的。
随着铁负荷的增加，对数线性处理会导致 T2*的估计误差越来越大。因此，这种方法不应用于测量临床样本中的弛豫行为。
