Imaging Protocol and Strategy

Every examination begins with the acquisition of localizing images obtained in the axial, coronal and sagittal planes using SSFP sequences or GRE sequences with ECG gating. Alternatively, fast gradient-echo sequences without ECG-gating can be used. The aim is to obtain a detailed description of anatomic structures (e.g. heart or major vessels) in the region of interest in order to further characterize the underlying anomaly.

ECG-gated CINE images may be acquired for evaluation of the heart and the great vessels. These fast gradient echo images may demonstrate flow and functionality of the vessel anatomy and subsequent pathological processes. The anatomic course of a vessel and flow direction can be demonstrated

Fig. 3a-d. This case demonstrates a patient with coarctation distal to the left subclavian artery. Evaluation of the aortic arch and supraaortic branching vessels is clearly possible on early arterial phase imaging (a) anterior-posterior view; (b) posterior-anterior view (arrows). However, no assessment of the descending aorta distally to the stenosis is possible at this time. On images acquired directly after this first phase the thoracic arterial and venous vessels are apparent (c) anterior-posterior view. Note that at this time-point the descending aorta distal to the stenosis is also depicted and is available for further evaluation (d) (arrows). This case demonstrates that several acquisitions after contrast agent administration may provide additional information on arterial and venous vessel status as well as on the direction of blood flow (Gd-BOPTA, 0.1 mmol/kg)

immediately. In cases of vascular stenosis, for example in patients with aortic coarctation, a post-stenotic jet phenomenon can often be observed.

T1w spin-echo sequences can be used for the evaluation of wall structures. Due to their excellent anatomic resolution small endoluminal webs and baffles can be demonstrated. In cases of en-dovascular thrombus formation T1w fat suppressed images can be acquired to increase the signal intensity of the blood clot and thereby better emphasize the thrombotic material.

For investigation of vessels after endovascular stenting ECG-gated double- or triple-inversion recovery fast spin-echo sequences can be used to clearly elaborate the post-surgical vascular anatomy. Since these images are acquired in the diastolic phase during a solitary breath-hold, and demonstrate very low susceptibility artefacts in the presence of metallic implants, the image quality is only slightly reduced and vascular structures can be evaluated satisfactorily.

Notwithstanding the value of non-contrast sequences, it is contrast-enhanced 3D MR angiogra-phy (CE-MRA) in conjunction with the use of extracellular gadolinium contrast agents that plays the key role in the diagnosis of vascular malformations. CE-MRA permits differentiation between arterial, venous and lymphoid vessels, depicts the blood supply of tumoral lesions, and may highlight the nature of a pathological process. With the 3D acquisition technique an entire volume of interest can be covered with one slab, allowing precise characterisation of the vessel shape and course. Furthermore, the detection of collateral vessels, the extension of a stenosis or the 3D relationship to other anatomic landmarks such as the heart or other great vessels can be revealed with one acquisition. Multiplanar reconstruction as a postprocessing option can be helpful for a detailed evaluation of complicated vessel anatomy.

In order to evaluate the vascular architecture of the thorax or even the whole body in small children, contrast agent application during ongoing image acquisition may demonstrate the arterial and venous anatomy in one extended breath-hold (Fig. 3). The administration of a test bolus or automatic bolus triggering is rarely helpful in small children firstly because the amount of contrast agent required for a test bolus often approximates the total contrast agent dose required, and secondly because variable congenital shunting or post surgical variations sometimes render the order of enhancement of the pulmonary, and systemic arterial and venous structures unpredictable. With ac quisition times of 6 to 8 seconds several perfusion phases of the volume of interest can be detected in one single breath-hold to better characterize arterial perfusion, venous return, arterio-venous shunting or pooling of the contrast agent in the case of tumor tissue. Additionally, VIBE (Volume Interpolated Breath-hold Examination) sequences may be acquired after contrast agent application in order to depict intramural pathologies and their correlation to surrounding anatomic structures.

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