In recent times, magnetic resonance imaging (MRI) has tion on the excitation within the bandwidth of the image.
Thus, only at the fundamental frequency do the spins receive seen the development of so-called ''tagging'' pulse sequences which have been primarily conceived for studying the full power needed to flip to the desired angle, and at all other harmonics, the flip angles are modulated by the sinc organ motion. The common underlying principle of all tagging techniques is to encode, via radiofrequency modulation, function. This sinc modulation in frequency space confines the excitation within a predefined area in the imaging plane. an endogenous marker on a sample of interest and observe the evolution of such markers over time. The contrast be-Numerous other functions can be similarly imposed on the DANTE excitation in space with variations in power distri-tween the tags and the surroundings exists only so long as the spins in the tagged regions can retain the memory of bution between the excitation harmonics. For example, a series of sinc RF pulses may be considered more appropriate their excitation, i.e., on the order of their longitudinal-recovery time T 1 . These techniques register motion in all direc-if a ''square'' modulation with uniform excitation at each harmonic is desired. We chose the sinc modulation scheme tions by visual cues (spatial distortion of tags) and have no special hardware demands. Hence, they have been exten-to deliver the maximum power at the fundamental frequency and to demonstrate clearly the effect of the modulation im-sively used to generate valuable information on bulk motion of specific parts of an organ (1-3), large-scale motion of posed. Moreover, this scheme of excitation may help the reader to perceive the L-DANTE pulse sequence as an inter-spins within an organ, as in angiography (4), and bulk turbulence (5). However, when localized displacements occur in mediate between a DANTE and a single-frequency CW-type excitation. Within a given interpulse spacing, as the pulse isolated regions within an imaging plane, it may be optimal to tag only those specific regions of interest, leaving the rest widths become smaller, a DANTE scheme of excitation is approached while larger pulse widths approach a CW-type of the imaging plane unperturbed.
We intended to modify the existing tagging pulse se-single excitation.
The SPAMM excitation scheme, in its simplest form (6), quences to adapt them for such studies of localized motion. The advantage gained by using such modified pulse seachieves a sinusoidal variation in intensity to generate alterquences, over their corresponding parent pulse sequences, nate bright and dark tags. A potential disadvantage of the would vary. If tagging is achieved with spatial modulation technique, as mentioned above, lies in its inability to preof magnetization or SPAMM (6, 7) per say, the magnetizaserve anatomical details otherwise obtainable from the stantion in the entire imaging plane is modulated, and hence dard spin-echo part of the pulse sequence. The localized anatomical information becomes difficult to decipher. A seversion of the same pulse sequence (L-SPAMM) essentially lective SPAMM tagging sequence would avoid this problem. uses a frequency-selective SPAMM premodulation scheme. In the case of DANTE (delays alternating with nutations The excitation pulses in the SPAMM premodulation are for tailored excitation) (8, 9) tagging, a selective DANTE made band-selective (either sinc or Gaussian) and are used sequence would need lower RF power to tag only preselected in the presence of a readout gradient to select an isolated areas, since excitation at multiple harmonics would be elimiarea (or a band of spins) over which the modulations are nated. We describe in this note, via simulations and experiapplied. The choice of location and width of this area, for ments, variants of SPAMM and DANTE pulse sequences a given RF duration and type, is determined, respectively, that may be used to localize tags within an imaging plane. by the fundamental frequency of the spin-selective pulses
The localized DANTE (L-DANTE) pulse sequence is and the magnitude of the selective gradient used. Within the identical to the DANTE tagging sequence (8, 9), except excitation band, the SPAMM modulation is achieved by the for the fact that it uses longer RF pulses, on the order of usual SPAMM encoding gradient. The L-DANTE and the milliseconds (ms) of duration. This imposes a sinc modula-L-SPAMM pulse sequences are shown in Fig. 1 and Fig. 2 respectively. Bloch-equation-based simulation (10) was used to inves-* To whom correspondence should be addressed.