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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Current parallel imaging techniques for accelerated imaging require a fully encoded reference data set to estimate the spatial coil sensitivity information needed for reconstruction. In dynamic parallel imaging a time-interleaved acquisition scheme can be used, which eliminates the need for separately acquiring additional reference data, since the signal from directly adjacent time frames can be merged to build a set of fully encoded full-resolution reference data for coil calibration. In this work, we demonstrate that a time-interleaved sampling scheme, in combination with autocalibrated GRAPPA (referred to as TGRAPPA), allows one to easily update the coil weights for the GRAPPA algorithm dynamically, thereby improving the acquisition efficiency. This method may update coil sensitivity estimates frame by frame, thereby tracking changes in relative coil sensitivities that may occur during the data acquisition. Image acquisition time is one of the most important considerations for clinical magnetic resonance imaging. Recently, several partially parallel acquisition (PPA) strategies (1-7) have been described to speed up the acquisition time by decreasing the number of phase encoding steps by a reduction factor R. Normally, this undersampling is performed by increasing the distance between adjacent acquired k-space lines while maintaining the maximum kvalues. All PPA reconstruction algorithms require extra coil sensitivity information from an array of multiple radiofrequency receiver coils to remove the aliasing artifacts that result from undersampled k-space. Naturally, this sensitivity information is acquired in an additional reference experiment, thereby degrading the efficiency of the actual PPA experiment. In dynamic parallel imaging, a timeinterleaved acquisition scheme similar to UNFOLD (8) and TSENSE (9) may be exploited in order to obtain this sensitivity information directly from the actual accelerated dynamic imaging experiment, thereby realizing the full image acceleration. To this end, directly adjacent time frames can be merged to build a fully encoded, full-resolution reference data set, which can be used as autocalibration signals (ACS) for an improved GRAPPA (7) reconstruction. With every acquired time frame in the series a new set of ACS lines can be generated. This allows one to update the coil coefficients for the GRAPPA algorithm dynamically, thereby automatically tracking changes in relative coil sensitivities over time efficiently. In particular, this method is beneficial whenever coil sensitivity maps, as required for the SENSE algorithm, are difficult to obtain. This is the case in, for example, inhomogeneous regions with low signal-to-noise ratio (SNR) (e.g., the lung). In this work, TGRAPPA reconstructions of accelerated (reduction factor 2 to 4) real-time (nongated), free breathing cardiac studies are presented. METHODSAll experiments were performed on a Sonata 1.5-T clinical whole body scanner (Siemens Medical Solutions, Erlangen, Germany) equipped with eight independent receiver channe...

Recently a self-calibrating SMASH technique, AUTO-SMASH, was described. This technique is based on PPA with RF coil arrays using auto-calibration signals. In AUTO-SMASH, important coil sensitivity information required for successful SMASH reconstruction is obtained during the actual scan using the correlation between undersampled SMASH signal data and additionally sampled calibration signals with appropriate offsets in k-space. However, AUTO-SMASH is susceptible to noise in the acquired data and to imperfect spatial harmonic generation in the underlying coil array. In this work, a new modified type of internal sensitivity calibration, VD-AUTO-SMASH, is proposed. This method uses a VD k-space sampling approach and shows the ability to improve the image quality without significantly increasing the total scan time. This new k-space adapted calibration approach is based on a k-space-dependent density function. In this scheme, fully sampled low-spatial frequency data are acquired up to a given cutoff-spatial frequency. Key words: SMASH; AUTO-SMASH; simultaneous acquisition; PPA; RF coil array; MR image reconstructionThe traditional method for reducing MRI acquisition time has been the use of faster gradient hardware in conjunction with shorter data acquisition periods. Recently developed partially parallel acquisition (PPA) techniques (1-7) allow an elegant and significant reduction in imaging time by using the spatial information inherent in a multiple receiver coil array. In these rapid MRI techniques multiple phase-encoded data are derived in parallel from a single phase-encoded NMR signal.The simultaneous acquisition of spatial harmonics (SMASH) technique (3) was the first practical PPA method. A factor of two to four savings in scan time has been demonstrated in vivo using SMASH with commercially available radiofrequency (RF) coil arrays, and up to an eightfold increasing in imaging speed has been achieved in phantoms using specialized RF hardware (8). When applied to single-shot imaging, SMASH enables single-shot images with increased spatial resolution without increasing imaging time and without requiring increased gradient performance or increased RF power deposition (9). One notable limitation of the original SMASH imaging technique was its demand on the measurement of component coil sensitivities for spatial harmonic generation. This can be a cumbersome, inaccurate, and time-consuming procedure, which in the worst-case scenario can eliminate the time advantage of the SMASH technique, and thereby limits potential applications of faster imaging with SMASH.To address these limitations, an internal calibration technique for SMASH imaging, AUTO-SMASH (4), in which coil sensitivity information can be detected during the actual scan by an auto-calibration mechanism, was developed. AUTO-SMASH has the major advantage that the component coil-weights, necessary for SMASH reconstruction, can be determined for each individual scan independently and without a significant increase in imaging time. The advantages of this ...

In this work a theoretical description for practical quantitative estimation of the noise enhancement in generalized autocalibrating partially parallel acquisitions (GRAPPA) reconstructions, equivalent to the geometry (g)-factor in sensitivity encoding for fast MRI (SENSE) reconstructions, is described. The GRAPPA g-factor is derived directly from the GRAPPA reconstruction weights. The procedure presented here also allows the calculation of quantitative g-factor maps for both the uncombined and combined accelerated GRAPPA images. This enables, for example, a fast comparison between the performances of various GRAPPA reconstruction kernels or SENSE reconstructions. The applicability of this approach is validated on phantom studies and demonstrated using in vivo images for 1D and 2D parallel imaging. Magn Reson Med 62: 739 -746, 2009.