Gore John C, Yankeelov Thomas E, Peterson Todd E, Avison Malcolm J
Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee 37232-2310, USA.
J Nucl Med. 2009 Jun;50(6):999-1007. doi: 10.2967/jnumed.108.059576. Epub 2009 May 14.
The limitations on the sensitivity for detecting small changes in MRI, CT, and ultrasound pulse-echo images are used to estimate the practical requirements for molecular imaging and targeted contrast enhancement for these modalities. These types of imaging are highly unlikely to approach the sensitivity for detecting molecular processes of radionuclear methods, and the prospects for achieving sufficient concentrations of appropriate agents in vivo are poor for several types of applications such as small-molecule targeting of specific receptors. However, using relatively large carrier systems such as particles and liposomes, sufficient concentrations of paramagnetic agents may be delivered to achieve MR-signal changes adequate for detection. The use of higher-resolution MR images will aid the prospects for molecular imaging in small animals. Theoretic considerations also predict that a similar approach, using rather large particles or carriers of materials with a high atomic number, may also be successful for CT, especially with additional developments such as the use of monochromatic x-rays. The prospects of molecular imaging by x-ray imaging may not be as bleak as has been predicted. For ultrasound detection, gas-filled bubbles can provide a sufficient backscattered sound intensity to be detectable at concentrations and sizes not much different from agents designed for these other modalities.
磁共振成像（MRI）、计算机断层扫描（CT）和超声脉冲回波图像检测微小变化的灵敏度限制，被用于估计这些成像方式在分子成像和靶向对比增强方面的实际需求。这类成像极不可能达到放射性核素方法检测分子过程的灵敏度，而且对于几种应用（如小分子靶向特定受体）而言，在体内实现足够浓度的合适造影剂的前景不佳。然而，使用相对较大的载体系统（如颗粒和脂质体），可以递送足够浓度的顺磁性造影剂，以实现足以被检测到的磁共振信号变化。使用更高分辨率的磁共振图像将有助于小动物分子成像的前景。理论上的考虑还预测，采用类似方法，使用相当大的颗粒或具有高原子序数材料的载体，对CT成像也可能成功，特别是随着诸如使用单色X射线等进一步发展。X射线成像进行分子成像的前景可能并不像所预测的那么黯淡。对于超声检测，充气气泡可以提供足够的反向散射声强，在与为其他成像方式设计的造影剂浓度和尺寸相差不大的情况下即可被检测到。
