Determination of the pressure gradient in children with coarctation of the aorta by low-field magnetic resonance imaging

During the past few years magnetic resonance imaging (MRI) has gained increasing importance in the noninvasive examination of congenital heart defects. Practically all existing examinations have been carried out with a magnetic field strength exceeding 1 (T high-field MRI). Flow quantification is considered to be an advanced MRI application and, in the past, has been available for high-field systems only. Therefore until recently, functional examinations such as MRI tomographic flow quantification were reported exclusively for high-field MRI units. From December 1998 to December 1999, nine patients (five girls and four boys, mean age 130 plus minus 3.6 months, range 62-185 months) were investigated by means of MRI after a previous surgical repair or interventional balloon dilatation of a coarctation of the aorta (mean postinterventional time period 23 plus minus 0.4 months). The examination was carried out without sedation in an open low-field unit (Siemens Magnetom Open 0.2 T). Cardiac-triggered spin-echo sequences were used with a 3-mm to 7-mm slice thickness in an axial and a double oblique plane. The measurement of the immediate poststenotic flow velocity was done by flow-sensitive sequences developed for the study (phase-sensitive flow measurement sequences: TE, 6-12 msec; TR, 50 msec; flip angle, 60 degrees; Vmax, 200-1200 cm/sec; two acquisitions). All patients were examined on the same day with comparative echocardiographic procedures. In all cases, an excellent anatomical evaluation of the aortic arch was possible. The diameters of the residual stenosis were measured by MRI and correlated well r = 0.95; p ? 0.001; mean difference 0.44 +/- 2.47 mm) with echocardiographic results. No wall damage was observed in any of the cases studied. The pressure gradient of the stenosis calculated from the flow sequence was between 17 and 50 mmHg and corresponded well (r = 0.93; p = 0.001; mean difference 0.67 +/- 11 mmHg) with the results obtained from echocardiography. The study demonstrates that examination of the aortic arch is possible in a low-field MRI system, with its significant advantages (lower patient discomfort and more cost-effective examination). In addition, a quantitative flow measurement in low-field MRI was realized for the first time. Low-field MRI therefore seems to be a good, noninvasive method for examining patients with a poor echocardiographic representation of the aortic arch.