Pre-acquisition delay

Figure 3.39. First-order (frequency dependent) phase errors arise from a dephasing of magnetisation vectors during the pre-acquisition delay which follows the excitation pulse. When data collection begins, vectors with different frequencies have developed a significant phase difference which varies across the spectrum.

The simple one-pulse experiment is shown below, the delay dl is called the relaxation delay and it ensures that in a multi-scan experiment the spin system has returned to equilibrium before the next pulse (see Check it 5.2.1.5). The delay dlO is not normally part of the ordinary sequence scheme and represents the pre-acquisition delay (Bruker nomenclature del, de2). This delay is automatically inserted to enable time for switching between transmit and receive mode to minimize pulse breakthrough. For further aspects the reader is referred to section S.2.3.4 Check its 3.2.3.4 and 3.2.3.6). 

Pre-saturation In this technique prior to data acquisition, a highly selective low-power rf pulse irradiates the solvent signals for 0.5 to 2 s to saturate them. No irradiation should occur during the data acquisition. This method relies on the phenomenon that nuclei which have equal populations in the ground and excited states are unable to relax and do not contribute to the FID after pulse irradiation. This is an effective pulse sequence of NOESY-type pre-saturation that consists of three 900 pulses RD - 900 - tx - 900 - tm - 90° - FID, where RD is the relaxation delay and t and tm are the presaturation times. 

Pre- and post-contrast series are combined. Time delay should be adapted to pathology arterial time should be acquired in the context of trauma or tumor. Very delayed acquisition can sometimes be necessary to look for stagnant iodine within the parenchyma in acute pyelonephritis (Ishikawa et al. 1985 Dalla Palma et al. 1995) or a peri-renal leak in cases of renal fracture. Two-dimensional reformatting is especially useful in evaluating renal fractures or retroperitoneal tumors (Fig. 1.1.9). Maximal intensity projection images in the coronal and sagittal planes advantageously replace IVU at any time of the examination. 

In non-hepatopathic patients, with no history of neoplasia, our protocol includes a pre-contrast scan followed by late arterial and portal venous phases. The acquisition of a late arterial phase is justified by the relatively high percentage of hypervascular benign lesions (see focal nodular hyperplasia) which might be easily missed only on pre-contrast and portal venous phase (Fig. 2.9). A delayed scan is used only in cases of suspected haemangiomas if arterial and portal venous patterns are doubtful and there is the need to confirm complete and delayed enhancement. 

Survey IH spectra were run for product identification using a sweep width of 8000 Hz (20 ppm), a pulse width of at least 12 microseconds (30 degrees), an acquisition time of 2-4 seconds and a pulse delay of 2-4 seconds. Corresponding 13C spectra were acquired using a sweep width of at least 25000 Hz (250 ppm), an acquisition time of 1.6 seconds, and a pulse delay of 2-3 seconds. Spectra were accumulated until the desired signal-to-noise ratio was achieved. Pre-saturation of the large H2O peak was often used (standard Varian software) to minimize this resonance in the spectrum. 

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