MR angiography using steady-state free precession.

Contrast-enhanced MR angiography (CE-MRA) using steady-state free precession (SSFP) pulse sequences is described. Using SSFP, vascular structures can be visualized with high signal-to-noise ratio (SNR) at a substantial (delay) time after the initial arterial pass of contrast media. The peak blood SSFP signal was diminished by <20% 30 min after the initial administration of 0.2 mmol/kg of Gd-chelate. The proposed method allows a second opportunity to study arterial or venous structures with high image SNR and high spatial resolution. A mask subtraction scheme using spin echo SSFP-S(-) acquisition is also described to reduce stationary background signal from the delayed SSFP angiography images.     

Visualization of external carotid artery and its branches: Non–contrast‐enhanced MR angiography using balanced steady‐state free‐precession sequence and a time‐spatial labeling inversion pulse

Purpose

To evaluate visibility of the external carotid artery (ECA) and its branches using three‐dimensional (3D) balanced steady‐state free‐precession (SSFP) MR angiography with a time‐spatial labeling inversion pulse (Time‐SLIP), and to provide an optimal value of the inversion time (TI).

Materials and Methods

Peripheral‐pulse‐wave‐gated 3D balanced SSFP images were obtained in 20 healthy volunteers. Images with a Time‐SLIP using four different TIs (600, 900, 1200, and 1500 ms) and without a Time‐SLIP, referred to as sequence A to E, were acquired for each subject and compared for visibility scores of ECA system and relative signal intensity (SI) of ECA.

Results

Average Friedman rank for overall visibility was 1.63, 3.01, 3.59, 3.58, and 3.20 for sequence A to E, respectively. Sequence C and D yielded significantly higher visibility than sequence A, B, and E. The mean relative SI value was 0.97, 0.87, 0.81, 0.76, and 0.67 for sequence A to E, respectively.

Conclusion

Balanced SSFP MR angiography with a Time‐SLIP is superior to that without a Time‐SLIP, showing excellent visualization of ECA system in approximately 3 min in average with sufficient background suppression including veins and salivary ducts. A TI of 1200 ms was considered to be optimal for this purpose. J. Magn. Reson. Imaging 2009;30:678–683. © 2009 Wiley‐Liss, Inc.     

Non‐contrast‐enhanced MR angiography for selective visualization of the hepatic vein and inferior vena cava with true steady‐state free‐precession sequence and time‐spatial labeling inversion pulses: Preliminary results

Purpose

To selectively visualize the hepatic vein and inferior vena cava (IVC) using three‐dimensional (3D) true steady‐state free‐precession (SSFP) MR angiography with time‐spatial labeling inversion pulse (T‐SLIP), and to optimize the acquisition protocol.

Materials and Methods

Respiratory‐gated 3D true SSFP scans were conducted in 23 subjects in combination with two different T‐SLIPs (one placed in the thorax to suppress the arterial signal and the other in the abdomen to suppress the portal venous signal). One of the most important factors was the inversion time (TI) of abdominal T‐SLIP, and the image quality was evaluated at four different TIs of 800, 1200, 1600, and 2000 msec in terms of relative signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), and mean visualization scores.

Results

No significant difference was observed in SNR and CNR between each TI. However, IVC visualization scores were better at TIs of 1600 and 2000 msec, and overall image quality was better at TIs of 1200 and 1600 msec. Therefore, the TI of 1600 msec was considered to provide the optimal balance between IVC visualization and signal suppression of the portal vein in our protocol.

Conclusion

True SSFP scan with T‐SLIPs enabled selective visualization of the hepatic vein and IVC without an exogenous contrast agent. J. Magn. Reson. Imaging 2009;29:474–479. © 2009 Wiley‐Liss, Inc.     

Non-contrast-enhanced MR angiography of the thoracic aorta using cardiac and navigator-gated magnetization-prepared three-dimensional steady-state free precession

PURPOSE: To assess the usefulness of non-contrast-enhanced MR angiography using cardiac and navigator-gated magnetization-prepared three-dimensional (3D) steady-state free precession (SSFP) imaging for the diagnosis of diseases of the thoracic aorta. MATERIALS AND METHODS: Twenty-two patients with diseases of the thoracic aorta were examined using a 1.5 Tesla unit. Non-contrast-enhanced MR angiography was done using parasagittal 3D SSFP combined with cardiac-gating and k-space weighted navigator-gating techniques, using T2-prepared and fat-suppression pulses. Imaging quality and the diagnostic capability of this technique were compared with the imaging quality of 2D SSFP or contrast-enhanced 3D MR angiography and with final diagnoses. RESULTS: Non-contrast-enhanced 3D MR angiography provided signal-to-noise and contrast-to-noise ratios of the thoracic aorta comparable to non-contrast-enhanced 2D or contrast-enhanced 3D MR angiography (P > 0.17). This imaging technique gave accurate diagnoses in 19 of the 22 patients. CONCLUSION: Non-contrast-enhanced MR angiography using cardiac and navigator-gated magnetization-prepared 3D SSFP technique was useful for the diagnosis of diseases of the thoracic aorta.     

Three-dimensional breathhold SSFP coronary MRA: a comparison between 1.5T and 3.0T

PURPOSE: To assess the feasibility of three-dimensional breathhold coronary magnetic resonance angiography (MRA) at 3.0T using the steady-state free precession (SSFP) sequence, and quantify the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) gains of coronary MRA from 1.5T to 3.0T using whole-body and phased-array cardiac coils as the signal receiver. MATERIALS AND METHODS: Eight healthy volunteers were scanned on 1.5T and 3.0T whole-body systems using the SSFP sequence. Numerical simulations were performed for the SSFP sequence to optimize the flip angle and predict signal enhancement from 1.5T to 3.0T. Coronary artery images were acquired with the whole-body coil in transmit-receive mode or transmit-only with phased-array cardiac coil receivers. RESULTS: In vivo studies of the same volunteer group at both field strengths showed increases of 87% in SNR and 83% in CNR from 1.5T to 3.0T using a whole-body coil as the signal receiver. The corresponding increases using phased-array receivers were 53% in SNR and 92% in CNR. However, image quality at 3.0T was more variable than 1.5T, with increased susceptibility artifacts and local brightening as the result of increased B(0) and B(1) inhomogeneities. CONCLUSION: Coronary MRA at 3.0T using a three-dimensional breathhold SSFP sequence is feasible. Improved SNR at 3.0T warrants the use of coronary MRA with faster acquisition and/or improved spatial resolution. Further investigations are required to improve the consistency of image quality and signal uniformity at 3.0T.     

Non‐contrast‐enhanced flow‐independent peripheral MR angiography with balanced SSFP

Flow‐independent angiography is a non‐contrast‐enhanced technique that can generate vessel contrast even with reduced blood flow in the lower extremities. A method is presented for producing these angiograms with magnetization‐prepared balanced steady‐state free precession (bSSFP). Because bSSFP yields bright fat signal, robust fat suppression is essential for detailed depiction of the vasculature. Therefore, several strategies have been investigated to improve the reliability of fat suppression within short scan times. Phase‐sensitive SSFP can efficiently suppress fat; however, partial volume effects due to fat and water occupying the same voxel can lead to the loss of blood signal. In contrast, alternating repetition time (ATR) SSFP minimizes this loss; however, the level of suppression is compromised by field inhomogeneity. Finally, a new double‐acquisition ATR‐SSFP technique reduces this sensitivity to off‐resonance. In vivo results indicate that the two ATR‐based techniques provide more reliable contrast when partial volume effects are significant. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Performance of steady-state free precession for imaging carotid artery disease

PURPOSE: To evaluate steady-state free precession (SSFP) for diagnosing carotid artery disease. MATERIALS AND METHODS: Following bilateral x-ray angiography, seven patients with suspected carotid artery disease were imaged with SSFP, black blood fast spin echo (BB FSE), and time-of-flight MR angiography (TOF MRA). The techniques were compared for characterizing the vessel lumen. Flow phantom experiments were also performed, using speeds of 0 to 40 cm/second, to further evaluate the merits of each MR technique. RESULTS: In the patient studies, of the 14 arteries available, a correct grading of stenosis was possible with SSFP in 9 of 14, FSE in 12 of 14, and TOF in 13 of 14, assuming x-ray angiography as the gold standard. The SSFP technique was the least reliable and had severe artifacts in 5 of 14 arteries, making these images nondiagnostic. The flow phantom demonstrated that although the SSFP technique performs well under slow or no flow, it breaks down at higher flow levels. CONCLUSION: The continuous SSFP sequence used here was not reliable for imaging carotid artery disease owing to artifact in many cases. Nevertheless, the high speed of this SSFP technique does allow it to serve as a rapid scouting method prior to a more detailed evaluation with other MRI methods.     

Fat-suppressed steady-state free precession imaging using phase detection.

Fully refocused steady-state free precession (SSFP) is a rapid, efficient imaging sequence that can provide diagnostically useful image contrast. In SSFP, the signal is refocused midway between excitation pulses, much like in a spin-echo experiment. However, in SSFP, the phase of the refocused spins alternates for each resonant frequency interval equal to the reciprocal of the sequence repetition time (TR). Appropriate selection of the TR results in a 180 degrees phase difference between lipid and water signals. This phase difference can be used for fat-water separation in SSFP without any increase in scan time. The technique is shown to produce excellent non-contrast-enhanced, flow-independent angiograms of the peripheral vasculature.     

Free-breathing renal magnetic resonance angiography with steady-state free-precession and slab-selective spin inversion combined with radial k-space sampling and water-selective excitation.

The impact of radial k-space sampling and water-selective excitation on a novel navigator-gated cardiac-triggered slab-selective inversion prepared 3D steady-state free-precession (SSFP) renal MR angiography (MRA) sequence was investigated. Renal MRA was performed on a 1.5-T MR system using three inversion prepared SSFP approaches: Cartesian (TR/TE: 5.7/2.8 ms, FA: 85 degrees), radial (TR/TE: 5.5/2.7 ms, FA: 85 degrees) SSFP, and radial SSFP combined with water-selective excitation (TR/TE: 9.9/4.9 ms, FA: 85 degrees). Radial data acquisition lead to significantly reduced motion artifacts (P < 0.05). SNR and CNR were best using Cartesian SSFP (P < 0.05). Vessel sharpness and vessel length were comparable in all sequences. The addition of a water-selective excitation could not improve image quality. In conclusion, radial k-space sampling reduces motion artifacts significantly in slab-selective inversion prepared renal MRA, while SNR and CNR are decreased. The addition of water-selective excitation could not improve the lower CNR in radial scanning.     

Noncontrast SSFP pulmonary vein magnetic resonance angiography: impact of off-resonance and flow

Purpose

To investigate pulmonary vein (PV) off‐resonance and blood flow as causes of signal void artifacts in noncontrast steady‐state‐free‐precession (SSFP) PV magnetic resonance angiography (MRA).

Materials and Methods

PV blood off‐resonance was measured on 11 healthy adult subjects and 10 atrial fibrillation (AF) patients. Noncontrast PV MRA was performed using a 3D slab‐selective SSFP sequence at 1.5T on seven healthy subjects with signal profile shifts of 0–125 Hz. The time‐resolved blood flow velocity of the PVs was measured on five healthy subjects. The impact of flow was studied on six healthy subjects, on whom SSFP PV MRA was acquired twice with the electrocardiogram (ECG) trigger delay corresponding to low and high flow, respectively.

Results

The PV off‐resonances were 97 ± 27 Hz, 65 ± 20 Hz, 74 ± 25 Hz, and 52 ± 17 Hz for right inferior, left inferior, right superior, and left superior PVs, respectively, on healthy subjects, and 74 ± 20 Hz, 38 ± 9 Hz, 51 ± 20 Hz, and 28 ± 11 Hz on AF patients (P<0.01 for all). The off‐resonance caused severe signal voids in the PVs. Signal acquired during mid‐diastole with high PV flow caused additional signal voids in the left atrium, which was reduced by setting the ECG trigger delay to late‐diastole.

Conclusion

PV off‐resonance and flow causes signal void artifacts in noncontrast 3D slab‐selective SSFP PV MRA. J. Magn. Reson. Imaging 2010;32:1255–1261. © 2010 Wiley‐Liss, Inc.     

Rapid dark-blood carotid vessel-wall imaging with random bipolar gradients in a radial SSFP acquisition

PURPOSE: To investigate and evaluate a new rapid dark-blood vessel-wall imaging method using random bipolar gradients with a radial steady-state free precession (SSFP) acquisition in carotid applications. MATERIALS AND METHODS: The carotid artery bifurcations of four asymptomatic volunteers (28-37 years old, mean age = 31 years) were included in this study. Dark-blood contrast was achieved through the use of random bipolar gradients applied prior to the signal acquisition of each radial projection in a balanced SSFP acquisition. The resulting phase variation for moving spins established significant destructive interference in the low-frequency region of k-space. This phase variation resulted in a net nulling of the signal from flowing spins, while the bipolar gradients had a minimal effect on the static spins. The net effect was that the regular SSFP signal amplitude (SA) in stationary tissues was preserved while dark-blood contrast was achieved for moving spins. In this implementation, application of the random bipolar gradient pulses along all three spatial directions nulled the signal from both in-plane and through-plane flow in phantom and in vivo studies. RESULTS: In vivo imaging trials confirmed that dark-blood contrast can be achieved with the radial random bipolar SSFP method, thereby substantially reversing the vessel-to-lumen contrast-to-noise ratio (CNR) of a conventional rectilinear SSFP "bright-blood" acquisition from bright blood to dark blood with only a modest increase in TR (approximately 4 msec) to accommodate the additional bipolar gradients. CONCLUSION: Overall, this sequence offers a simple and effective dark-blood contrast mechanism for high-SNR SSFP acquisitions in vessel wall imaging within a short acquisition time.     

Double average parallel steady-state free precession imaging: optimized eddy current and transient oscillation compensation.

Balanced steady-state free precession (SSFP) imaging is sensitive to off-resonance effects, which can lead to considerable artifacts during a transient phase following magnetization preparation or steady-state interruption. In addition, nonlinear k-space encoding is required if contrast-relevant k-space regions need to be acquired at specific delays following magnetization preparation or for transient artifact reduction in cardiac-gated k-space segmented CINE imaging. Such trajectories are problematic for balanced SSFP imaging due to nonconstant eddy current effects and resulting disruption of the steady state.In this work, a novel acquisition strategy for balanced SSFP imaging is presented that utilizes scan time reduction by parallel imaging for optimized "double average" eddy current compensation and artifact reduction during the transient phase following steady-state storage and magnetization preparation. Double average parallel SSFP imaging was applied to k-space segmented CINE SSFP tagging as well as nongated centrically encoded SSFP imaging. Phantom and human studies exhibit substantial reduction in steady-state storage and eddy current artifacts while maintaining spatial resolution, signal-to-noise ratio, and similar total scan time of a standard SSFP acquisition. The proposed technique can easily be extended to other acquisition schemes that would benefit from nonlinear reordering schemes and/or rely on interruption of the balanced SSFP steady state.     

MR imaging of flow using the steady state selective saturation method

The steady state selective saturation method is a fast and efficient technique to study flow with magnetic resonance imaging. The pulse sequence used generates no signal from stationary matter and simultaneously optimizes signal flowing into a predetermined volume. The method can be used to generate three-dimensional angiography in less than 5 min. Other modifications allow the quantitative measurement of flow velocities and distinction of veins and arteries according to the direction of flow.     

4D radial coronary artery imaging within a single breath-hold: cine angiography with phase-sensitive fat suppression (CAPS).

Coronary artery data acquisition with steady-state free precession (SSFP) is typically performed in a single frame in mid-diastole with a spectrally selective pulse to suppress epicardial fat signal. Data are acquired while the signal approaches steady state, which may lead to artifacts from the SSFP transient response. To avoid sensitivity to cardiac motion, an accurate trigger delay and data acquisition window must be determined. Cine data acquisition is an alternative approach for resolving these limitations. However, it is challenging to use conventional fat saturation with cine imaging because it interrupts the steady-state condition. The purpose of this study was to develop a 4D coronary artery imaging technique, termed "cine angiography with phase-sensitive fat suppression" (CAPS), that would result in high temporal and spatial resolution simultaneously. A 3D radial stacked k-space was acquired over the entire cardiac cycle and then interleaved with a sliding window. Sensitivity-encoded (SENSE) reconstruction with rescaling was developed to reduce streak artifact and noise. Phase-sensitive SSFP was employed for fat suppression using phase detection. Experimental studies were performed on volunteers. The proposed technique provides high-resolution coronary artery imaging for all cardiac phases, and allows multiple images at mid-diastole to be averaged, thus enhancing the signal-to-noise ratio (SNR) and vessel delineation.     

Breath-hold 3D steady-state free precession coronary MRA compared with conventional X-ray coronary angiography

PURPOSE: To evaluate the use of breath-hold three-dimensional (3D) steady-state free precession (SSFP) coronary magnetic resonance angiography (MRA) in patients with coronary artery disease (CAD) in comparison with conventional coronary x-ray angiography (XRA). MATERIALS AND METHODS: Twenty-eight patients with suspected CAD were examined with the use of a breath-hold 3D-SSFP-MRA sequence and conventional XRA. To assess the accuracy of MRA, two clinicians who were blinded to patient information independently reviewed the MRA and XRA data, which were presented in a randomized order. To identify discrepancies between MRA and XRA, and assess features of coronary lesions on MRA, two additional clinicians examined MRA and XRA data that were presented side by side, divided into proximal, mid, and distal segments, and compared them segment by segment. RESULTS: The sensitivity and specificity for diagnosing significant coronary stenoses (> 50% diameter narrowing) were 64% and 94%, respectively. At sites of coronary lesions identified on XRA, bright signals and enlarged vessel profiles, in addition to the characteristic narrow lumen, were frequently observed on MRA. CONCLUSION: Breath-hold SSFP coronary MRA has good specificity but inconclusive sensitivity in diagnosing significant coronary stenoses, and provides important image features for depicting coronary lesions.     

Optimization of signal behavior in the transition to driven equilibrium in steady-state free precession sequences.

A new technique to avoid the initial signal fluctuations in steady-state free precession (SSFP)-sequences, such as trueFISP, FIESTA, and refocused FFE, is presented. The "transition into driven equilibrium" (TIDE) sequence uses modified flip angles over the initialization phase of a SSFP experiment, which not only avoids image artifacts but also improves the signal-to-noise ratio (SNR) and contrast behavior compared to conventional approaches. TIDE is demonstrated to be robust against variations of T(1) and T(2), and leads to a monotonous signal evolution for off-resonance spins. The basic principles can also be applied repetitively to optimize continuous 3D acquisitions.     

Projection imaging of the right coronary artery with an intravenous injection of contrast agent.

Contrast-enhanced (CE) MR angiography of the right coronary artery (RCA) was performed using 2D thick-slice projection imaging with a small (8 mL) intravenous injection of contrast agent in six volunteers. With a tight contrast bolus injection, the RCA was enhanced for a few seconds after the contrast bolus was washed out of the right ventricle. This allowed data to be acquired when only the RCA was enhanced. Using 2D thick-slice magnetization prepared steady-state free precession (SSFP) imaging, background signal was suppressed and a complete data set was acquired in three heartbeats. A mean vessel length of 7.1 +/- 0.9 cm was depicted with a signal-to-noise ratio of 11.8 +/- 0.7 and contrast-to-noise ratio of 6.1 +/- 0.6. Thick-slice 2D projection CE SSFP is a promising method to depict the RCA.     

Quiescent‐interval single‐shot unenhanced magnetic resonance angiography of peripheral vascular disease: Technical considerations and clinical feasibility

We performed technical optimization followed by a pilot clinical study of quiescent‐interval single‐shot MR angiography for peripheral vascular disease. Quiescent‐interval single‐shot MR angiography acquires data using a modified electrocardiographic (ECG)‐triggered, fat suppressed, two‐dimensional, balanced steady‐state, free precession pulse sequence incorporating slice‐selective saturation and a quiescent interval for maximal enhancement of inflowing blood. Following optimization at 1.5 T, a pilot study was performed in patients with peripheral vascular disease, using contrast‐enhanced MR angiography as the reference standard. The optimized sequence used a quiescent interval of 228 ms, α/2 catalyzation of the steady‐state magnetization, and center‐to‐out partial Fourier acquisition with parallel acceleration factor of 2. Spatial resolution was 2‐3mm along the slice direction and 0.7‐1mm in‐plane before interpolation. Excluding stented arterial segments, the sensitivity, specificity, and positive and negative predictive values of quiescent‐interval single‐shot MR angiography for arterial narrowing greater than 50% or occlusion were 92.2%, 94.9%, 83.9%, and 97.7%, respectively. Quiescent‐interval single‐shot MR angiography provided robust depiction of normal peripheral arterial anatomy and peripheral vascular disease in less than 10 min, without the need to tailor the technique for individual patients. Moreover, the technique provides consistent image quality in the pelvic region despite the presence of respiratory and bowel motion. Magn Reson Med 63:951–958, 2010. © 2010 Wiley‐Liss, Inc.     

Fast phase-contrast velocity measurement in the steady state.

A new method of encoding flow velocity as image phase in a refocused steady-state free precession (SSFP) sequence, called steady-state phase contrast (SSPC), can be used to generate velocity images rapidly while retaining high signal. Magnitude images with refocused-SSFP contrast are simultaneously acquired. This technique is compared with the standard method of RF-spoiled phase contrast (PC), and is found to have more than double the phase-signal to phase-noise ratio (PNR) when compared with standard PC at reasonable repetition intervals (TRs). As TR decreases, this advantage increases exponentially, facilitating rapid scans with high PNR efficiency. Rapid switching between the two necessary steady states can be accomplished by the insertion of a single TR interval with no flow-encoding gradient. The technique is implemented in a 2DFT sequence and validated in a phantom study. Preliminary results indicate that further TR reduction may be necessary for high-quality cardiac images; however, images in more stationary structures, such as the descending aorta and carotid bifurcation, exhibit good signal-to-noise ratio (SNR) and PNR. Comparisons with standard-PC images verify the PNR advantage predicted by theory.