In all current parallel imaging techniques, aliasing artifacts resulting from an undersampled acquisition are removed by means of a specialized image reconstruction algorithm. In this study a new approach termed "controlled aliasing in parallel imaging results in higher acceleration" (CAIPIRINHA) is presented. This technique modifies the appearance of aliasing artifacts during the acquisition to improve the subsequent parallel image reconstruction procedure. This new parallel multislice technique is more efficient compared to other multi-slice parallel imaging concepts that use only a pure postprocessing approach. In this new approach, multiple slices of arbitrary thickness and distance are excited simultaneously with the use of multi-band radiofrequency (RF) pulses similar to Hadamard pulses. These data are then undersampled, yielding superimposed slices that appear shifted with respect to each other. The shift of the aliased slices is controlled by modulating the phase of the individual slices in the multi-band excitation pulse from echo to echo. We show that the reconstruction quality of the aliased slices is better using this shift. This may potentially allow one to use higher acceleration factors than are used in techniques without this excitation scheme. Additionally, slices that have essentially the same coil sensitivity profiles can be separated with this technique. Magn Reson Med 53:684 -691, 2005.

The CAIPIRINHA (Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration) concept in parallel imaging has recently been introduced, which modifies the appearance of aliasing artifacts during data acquisition in order to improve the subsequent parallel imaging reconstruction procedure. This concept has been successfully applied to simultaneous multislice imaging (MS CAIPIRINHA). In this work, we demonstrate that the concept of CAIPIRINHA can also be transferred to 3D imaging, where data reduction can be performed in two spatial dimensions simultaneously. In MS CAIPIRINHA, aliasing is controlled by providing individual slices with different phase cycles by means of alternating multi-band radio frequency (RF) pulses. In contrast to MS CAIPIRINHA, 2D CAIPIRINHA does not require special RF pulses. Instead, aliasing in 2D parallel imaging can be controlled by modifying the phase encoding sampling strategy. This is done by shifting sampling positions from their normal positions in the undersampled 2D phase encoding scheme. Using this modified sampling strategy, coil sensitivity variations can be exploited more efficiently in multiple dimensions, resulting in a more robust parallel imaging reconstruction. Magn Reson Med 55:549 -556, 2006.

A novel procedure is proposed to extract T 1 , T 2 , and relative spin density from the signal time course sampled with a series of TrueFISP images after spin inversion. Generally, the recovery of the magnetization during continuous TrueFISP imaging can be described in good approximation by a three parameter monoexponential function S(t) ‫؍‬ S stst (1-INV exp(-t/T* 1 ). This apparent relaxation time T* 1 ≤ T 1 depends on the flip angle as well as on both T 1 and T 2 . Here, it is shown that the ratio T 1 /T 2 can be directly extracted from the inversion factor INV, which describes the relation of the signal value extrapolated to t ‫؍‬ 0 and the steady-state signal. The balanced SSFP MR imaging technique (1), also named balanced FFE and FIESTA, here referred to as TrueFISP (2), was proposed more than a decade ago and has generated much renewed interest during recent years due to technical advances in gradient and receiver performance. It provides the capability of extremely rapid imaging while preserving a high SNR efficiency. With a fixed flip angle ␣, the steady-state signal is an increasing function of the ratio T 2 /T 1 , so that high signal is generally obtained from fluid compartments with a long T 2 . The resulting image contrast renders the technique beneficial for several different applications, e.g., evaluation of cardiac function (3) or coronary angiography (4,5).In practice, magnetization reaches its steady-state condition after a certain transition period. A smooth signal time course towards steady state can be achieved by preparation with an RF pulse of flip angle -␣/2, preceding the imaging sequence at a time TR/2 before the first ␣ pulse (6). Whereas new elaborate pulse schemes have also been described (7), the ␣/2-based technique is robust and allows the implementation of additional magnetization preparation experiments immediately before a TrueFISP readout. In combination with the ␣/2 prepulse, an inversion recovery TrueFISP sequence has been proposed for magnetization prepared steady-state imaging (6). This approach has recently gained renewed interest as a promising tool for fast T 1 quantification (8). The intensities of a TrueFISP image series acquired after spin inversion and ␣/2 preparation were reported to follow the free longitudinal relaxation curve very closely, even at comparatively high flip angles of 50°.In a subsequent study from our group using numerical simulations and phantom experiments, it was also found that the recovery time course under a train of TrueFISP pulses can be described by monoexponential behavior (9). However, apparent relaxation times T* 1 were measured which strongly depended on the flip angle, T 1 and T 2 . It was demonstrated that this property can be used to quantify both T 1 and T 2 by fitting T* 1 curves measured with different flip angles to theoretical response curves. This numerically described behavior was confirmed by the results of a recent publication wherein an elegant simplifying calculation was presented, yielding a compact mathematical descripti...

Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single point- In MR experiments, the transversal magnetization decays with T * 2 . Standard MR sequences offer echo times (TEs) in the range of a few milliseconds for spin-echo sequences and down to 1 ms for gradient-echo sequences. Signals arising from tissues with a very short T 2 , well below 1 ms, are therefore not visible using standard sequences, as the signal has already decayed by the time of acquisition. In the image, these tissues appear dark, similar to air cavities or noise. A short T 2 can usually be found in tissue with strong couplings of solid materials like teeth, ligaments, tendons, and bones in the human body.Many regions of the human body have already been investigated with ultrashort echo time sequences. Clinical applications of sequences with ultrashort TE are used in orthopedics, dental imaging, and many other special applications. Studies not only of the knee (1), Achilles

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