Techniques in Pediatric Liver Imaging

The results of pediatric liver CT and MR imaging depend mainly on good sedation and thus an interdisciplinary approach to evaluation is necessary. Significant changes in the examination method are needed in both helical CT and MRI when dealing with pediatric patients.

The CT protocol typically requires image acquisition during the arterial and portal-venous phases and also, whenever necessary, the equilibrium phase. The slice thickness is typically 5 mm or less with a pitch ranging from 1 to 1.5. The contrast medium is usually administered in quantities of 2 ml/kg, either by means of a mechanical injection method or by manual injection, depending on the caliber of the vein used for access. The arterial phase usually begins 10-15 sec after injecting the bolus of contrast medium, while the portal-venous phase begins immediately after the conclusion of the arterial phase, generally after a delay of 20-40

sec. In the event of a suspected malignant neoplasm, the chest must also be examined to check for the presence of lung metastases.

MRI examinations typically involve acquisition of T1- and T2-weighted spin echo (SE) or turbo spin echo (TSE) sequences in axial and coronal orientation, and T2-weighted fast spin echo (FSE) or, preferably, single-shot sequences in the axial plane. If necessary, gradient echo (GRE) images can also be acquired to examine vascular structures. In addition, T1-weighted fat-suppressed images should be acquired to evaluate the fat content of lesions. This can be important for the evaluation of teratoma in children. Generally, the slice thickness should be 5 mm or less with an interslice gap of less than 10%.

In order to optimize imaging results, different techniques of respiratory gating can be applied in children. Either an external trigger signal from a breathing belt or navigator techniques can be used to overcome motion artifacts from breathing. Alternatively, motion insensitive single shot T2-weighted HASTE sequences or motion insensitive T1-weighted spoiled GRE single shot sequences can be applied for imaging the pediatric abdomen and liver. For further details and limitations of these techniques, please refer to Chapter 1.

After the administration of a gadolinium (Gd)-based contrast agent, GRE images can be acquired at different time points. Typically, these are acquired in the axial orientation and usually with fat suppression. The arterial phase image of liver perfusion in pediatric patients should be acquired approximately 10-15 sec after the start of contrast agent injection. The portal venous phase follows at approximately 20-30 sec post-injection. Generally the acquisition time for the entire liver should be below 15 sec. Since there are no restraints on MRI regarding radiation exposure, the T1-weighted sequence should be repeated continuously 4 or 5 times to reliably achieve all phases of liver perfusion. In addition to dynamic imaging of the liver, steady state imaging should be performed in the equilibrium phase after contrast agent injection. Usually, this is performed using T1-weighted and T1-weighted fat-suppressed imaging sequences [89].

Contrast enhanced MR angiography (CE-MRA) can very precisely evaluate the arterial and venous vascular anatomy of the liver in pediatric patients, but this method is usually reserved for cases requiring detailed preoperative vascular mapping or for cases in which chemo-emboliza-tion is required. To achieve maximum results, MRA studies should be performed under general anesthesia and controlled ventilation to allow for a sufficient breath-hold interval to acquire a high resolution contrast-enhanced 3D dataset.

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