Navigator-gated Technique Research Articles

Two-Dimensional Spoiled Gradient-Recalled Echo Magnetic Resonance Imaging of the Liver Using Respiratory Navigator-Gating Techniques.

We assessed the feasibility of T1-weighted 2-dimensional spoiled gradient-recalled (2D SPGR) acquisition in steady-state imaging of the liver with various respiratory navigator gating techniques. A total of 12 healthy volunteers underwent in-phase and out-of-phase 2D SPGR imaging of the liver during breath-holding and free-breathing. Four techniques for respiratory navigation, 2 conventional navigator techniques and 2 self-navigator techniques, were used for free-breathing imaging. Good navigator waveforms were obtained in conventional navigation, whereas fluctuations were evident in self navigation. All of the 4 navigator-based methods provided better images in terms of background signals and visual image quality compared with images obtained with no respiratory control. However, differences remained in comparison with breath-holding. Superiority of self-navigation to conventional navigation was not shown. Navigator-gating techniques improved 2D SPGR images of the liver acquired during free-breathing, suggesting feasibility and beneficial effects, although navigator-based images were still inferior to breath-hold images.

Evaluation of a respiratory navigator-gating technique in Gd-EOB-DTPA-enhanced magnetic resonance imaging for the assessment of liver tumors

ObjectivesWe investigated the clinical usefulness of respiratory navigator-gating technique for the assessment of liver tumors in Gd-EOB-DTPA-enhanced magnetic resonance (MR) imaging. MethodsEighty patients who underwent Gd-EOB-DTPA-enhanced MR imaging to evaluate known or suspected liver tumors were enrolled. Three-dimensional spoiled gradient-recalled echo images of the liver were acquired in the hepatobiliary phase by the following three methods: breath-hold imaging, navigator-gated low-resolution imaging, and navigator-gated high-resolution imaging. Navigator-gated imaging was performed during free breathing. Spatial resolution was identical between breath-hold imaging and gated low-resolution imaging. Signal intensities in the liver, muscle, and spleen were measured in 20 patients. Image quality was visually evaluated in all 80 patients. The detection rate and lesion conspicuity were assessed for 71 malignant liver lesions identified in 29 patients. ResultsThe liver-to-muscle and liver-to-spleen signal ratios were significantly lower for gated images compared to breath-hold images. Images of acceptable quality were obtained in most patients by all three methods, and the overall image quality of axial images did not differ significantly among the imaging methods, although superior reformatted coronal images were obtained by gated high-resolution imaging. The detection rates of malignant liver lesions were similar among the three imaging methods, although lesion conspicuity was significantly better for breath-hold imaging compared to gated imaging. ConclusionsNavigator-gated imaging provided image qualities and detection rates of malignant liver lesions comparable to breath-hold imaging in Gd-EOB-DTPA-enhanced MR imaging; however, no additional benefits of high-resolution imaging were proven for lesion evaluation.

Optimal techniques for magnetic resonance imaging of the liver using a respiratory navigator-gated three-dimensional spoiled gradient-recalled echo sequence

PurposeTo optimize the navigator-gating technique for the acquisition of high-quality three-dimensional spoiled gradient-recalled echo (3D SPGR) images of the liver during free breathing. Materials and methodsTen healthy volunteers underwent 3D SPGR magnetic resonance imaging of the liver using a conventional navigator-gated 3D SPGR (cNAV-3D-SPGR) sequence or an enhanced navigator-gated 3D SPGR (eNAV-3D-SPGR) sequence. No exogenous contrast agent was used. A 20-ms wait period was inserted between the 3D SPGR acquisition component and navigator component of the eNAV-3D-SPGR sequence to allow T1 recovery. Visual evaluation and calculation of the signal-to-noise ratio were performed to compare image quality between the imaging techniques. ResultThe eNAV-3D-SPGR sequence provided better noise properties than the cNAV-3D-SPGR sequence visually and quantitatively. Navigator gating with an acceptance window of 2mm effectively inhibited respiratory motion artifacts. The widening of the window to 6mm shortened the acquisition time but increased motion artifacts, resulting in degradation of overall image quality. Neither slice tracking nor incorporation of short breath holding successfully compensated for the widening of the window. ConclusionThe eNAV-3D-SPGR sequence with an acceptance window of 2mm provides high-quality 3D SPGR images of the liver.

Free-Breathing Single Navigator Gated Cine Cardiac Magnetic Resonance at 3 T

Cardiac magnetic resonance imaging (CMRI) is an important tool to assess cardiac function. However, one of the limitations of CMRI is the need for frequent breath-holding (BH) steps. This may be inconvenient to some patients and limit the use of this modality in patients unable to cooperate because of cognitive reasons or physically incapable of performing the required BH steps. The purpose of this study is to overcome the intrinsic timing and computation limitations of dual-navigator cine imaging and demonstrate the feasibility of free-breathing (FB) cine cardiac left ventricular function with a single-respiratory-navigator gating at 3 T. Eight participants underwent cine CMRI with both the conventional 2-dimensional cine BH and FB navigator-gated techniques. Scan parameters were identical, except in the FB technique, in which a respiratory navigator and only 2 signal averages were used. Images were scored for quality. Left ventricular end-systolic volume and end-diastolic volume were calculated. The differences in the end-systolic volume and end-diastolic volume assessed by the BH and FB were not statistically significant with P = 0.9 and 0.2, respectively. There was a good agreement between LV volumes with the limits of agreement (± 2 SD = ± 22.36 mL). Image quality score was not significantly different (P = 0.76). Free-breathing cine imaging utilizing a single-respiratory-navigator gating technique is comparable to conventional BH technique in both qualitative and quantitative imaging measures. Therefore, the FB cine technique can be used as an alternative for children and patients who are unable to hold their breath.

Reproducibility of coronary vessel wall imaging techniques

Background Coronary wall thickness measurements must be highly reproducible to be useful in longitudinal studies. While 2D turbo-spin-echo (TSE) and spiral techniques are commonly used, 3D techniques reduce partial-volume effects and allow greater vessel coverage. Recently, 3D coronary wall imaging was demonstrated with 100% respiratory efficiency (RE) using beat-to-beat respiratory motion correction (B2B-RMC) [1] which uses motion information determined from low resolution 3D images of the fat around the vessel as a surrogate for vessel motion. Here we assess the reproducibility of this technique together with that of navigator-gated 2D TSE and spiral acquisitions. Methods Cross-sectional right coronary artery wall images were obtained in 10 healthy subjects on a Siemens 1.5T Avanto scanner using dark-blood prepared B2B-RMC 3D spiral imaging, navigator-gated 2D TSE imaging and navigator-gated 2D spiral imaging. Acquisition order was randomized and subjects were imaged on two separate occasions to assess inter-study reproducibility using the intra-class correlation-coefficient (ICC) and Bland Altman analysis. All acquisitions used 0.7x0.7mm inplane resolution. B2B-RMC 3D spiral acquisitions acquired 8x3.0mm slices (16x1.5mm reconstructed) and 2D techniques acquired 1x6mm slice. Navigator-gated techniques used a 5mm window while B2B-RMC excluded data acquired at only very extreme respiratory positions, retrospectively correcting the rest. Durations, assuming 100% RE, were 600 cardiac cycles (CC) for B2B-RMC, 75CC for 2D spiral and 202-576CC for 2D

Improved renal perfusion measurement with a dual navigator‐gated Q2TIPS fair technique

A dual navigator-gated, flow-sensitive alternating inversion recovery (FAIR) true fast imaging with steady precession (True-FISP) sequence has been developed for accurate quantification of renal perfusion. FAIR methods typically overestimate renal perfusion when respiratory motion causes the inversion slice to move away from the imaging slice, which then incorporates unlabeled spins from static tissue. To overcome this issue, the dual navigator scheme was introduced to track inversion and imaging slices, and thus to ensure the same position for both slices. Accuracy was further improved by a well-defined bolus length, which was achieved by a modification version of Q2TIPS (quantitative imaging of perfusion using a single subtraction, second version with interleaved thin-slice TI(1) periodic saturation): a series of saturation pulses was applied to both sides of the imaging slice at a certain time after the inversion. The dual navigator-gated technique was tested in eight volunteers. The measured renal cortex perfusion rates were between 191 and 378 mL/100 g/min in the renal cortex with a mean of 376 mL/100 g/min. The proposed technique may prove most beneficial for noncontrast-based renal perfusion quantification in young children and patients who may have difficulty holding their breath for prolonged periods or are sedated/anesthetized.

A fully automatic and highly efficient navigator gating technique for high‐resolution free‐breathing acquisitions: Continuously adaptive windowing strategy

A fully automatic and highly efficient free-breathing navigator gated technique, continuously adaptive windowing strategy (CLAWS), is presented. Using a novel and dynamic acquisition strategy that ensures all potential navigator acceptance windows are possible, CLAWS acquires an image with the highest possible efficiency regardless of variations in the respiratory pattern. Unnecessary prolongation of scan durations due to respiratory drift or navigator acceptance window adjustments are avoided. As CLAWS requires no setting of the acceptance window, nor monitoring of the navigator traces during the scan, operator dependence is minimized and ease of use improved. CLAWS was compared against a standard accept/reject algorithm (ARA) and an end-expiratory following ARA (EE-ARA) in 20 healthy subjects and 10 patients (ARA only). The respiratory efficiency was compared against the retrospectively determined best possible respiratory efficiency for each acquisition. On average, the difference between CLAWS scan times and best possible scan times was 0.6% (± 1.3%). For the ARA and EE-ARA techniques, mean differences were 14.4% (± 20.9%) and 32.6 ± 10.9%, respectively. Had the CLAWS algorithm been used with the ARA and EE-ARA traces, mean differences would have been 0.2% (± 1.1%) and 0.5% (± 1.7%), respectively. Image quality was the same for all techniques: respiratory gating, motion artifacts, navigator, and coronary artery imaging.

Free‐breathing high‐spatial‐resolution delayed contrast‐enhanced three‐dimensional viability MR imaging of the myocardium at 3.0T: A feasibility study

To assess the feasibility of free-breathing high-spatial-resolution delayed contrast-enhanced three-dimensional (3D) viability magnetic resonance imaging (MRI) at 3.0 T for the detection of myocardial damages. Twenty-five patients with myocardial diseases, including myocardial infarction and cardiomyopathies, were enrolled after informed consent was given. Free-breathing 3D viability MRI with high spatial resolution (1.5 x 1.25 x 2.5 mm) at 3.0 T, for which cardiac and navigator gating techniques were employed, was compared with breath-hold two-dimensional (2D) viability imaging (1.77 x 1.18 x 10 mm) for assessment of contrast-to-noise ratio (CNR) and myocardial damage. Free-breathing 3D viability imaging was achieved successfully in 21 of the 25 patients. This imaging technique depicted 84.6% of hyperenhancing myocardium with a higher CNR between hyperenhancing myocardium and blood and with excellent agreement for the transmural extension of myocardial damage (k = 0.91). In particular, the 3D viability images delineated the myocardial infarction and linear hyperenhancing myocardium, comparable to the 2D viability images. Free-breathing high-spatial-resolution delayed contrast-enhanced 3D viability MRI using 3.0 T was feasible for the evaluation of hyperenhancing myocardium, as seen with myocardial infarction and cardiomyopathies.

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

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. 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. 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. 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.

Contrast-enhanced dynamic MR nephrography using the TurboFLASH navigator-gating technique in children

The purpose of this work was to test the feasibility of an MR examination protocol for the comprehensive assessment of renal morphology, excretion, and split renal function using a navigator-gated TurboFLASH sequence. A navigator-gated T1-weighted single-slice TurboFLASH sequence suitable for dynamic MR urography and nephrography was implemented. A protocol was developed, allowing for assessment of urinary excretion and split renal function by recording the renal clearance of a gadolinium (Gd) diethylenetriamine pentacetic acid (DTPA) bolus.

Contrast-enhanced dynamic MR nephrography using the TurboFLASH navigator-gating technique in children

The purpose of this work was to test the feasibility of an MR examination protocol for the comprehensive assessment of renal morphology, excretion and split renal function using a navigator-gated TurboFLASH sequence. A navigator-gated T1-weighted single-slice TurboFLASH sequence suitable for dynamic MR urography and nephrography was implemented. A protocol was developed allowing for assessment of urinary excretion and split renal function by recording the renal clearance of a gadolinium (Gd) diethylene-triamine-pentacetic-acid (DTPA) bolus. Ten patients aged between 14 months and 14 years (mean age 4.8+/-4.6 years) were evaluated with the following indications: pelvicalyceal dilatation (n=4), follow-up after pyeloplasty (n=1), duplex systems (n=3), large renal cyst (n=1), and renal insufficiency (n=1). Dynamic MR urography and MR split renal function were compared to MAG3 scintigraphy. Evaluation of morphology, excretion and function required 50-60 minutes examination time, plus 10 minutes for post-processing. The TurboFLASH sequence yielded image acquisition at nearly identical diaphragm positions allowing for accurate region-of-interest evaluation within the renal parenchyma and the urinary passage. Static and dynamic MR urography showed the morphology of the urinary tract and excretion with sufficient diagnostic imaging quality, and the results were in diagnostic compliance with scintigraphy. MRI and scintigraphy yielded similar results for split renal function with a correlation coefficient of R=0.968 determined by linear regression. Our conclusions were that the method is robust, easy to perform on a clinical 1.5 T MRI system, rapid to evaluate and post-process and, therefore, easy to incorporate into clinical routine. Compared to scintigraphy, the higher spatial resolution of the MR examination provides additional important information improving the management of the pediatric patients without the application of radioactive tracers.

Simultaneous multiple volume (SMV) acquisition algorithm for real-time navigator gating.

Navigator gating techniques can effectively reduce motion effects in MRI by accepting data only when the object is in a small range of positions at the cost of significantly prolonging scan time. A simultaneous multiple volume (SMV) algorithm is reported here that can substantially increase the scan efficiency while maintaining the effectiveness of motion suppression. This is achieved by acquiring different image volumes at different motion states. Initial experiments demonstrate that SMV can significantly increase the scan efficiency of navigator MRI.

Coronary artery disease: assessment with a comprehensive MR imaging protocol--initial results.

A comprehensive magnetic resonance (MR) imaging protocol for assessment of coronary artery disease (CAD) is presented. The protocol includes multiphase gradient-echo cine MR imaging for assessment of cardiac function, first-pass myocardial perfusion imaging at rest and during adenosine stress, MR imaging with delayed contrast enhancement for assessment of myocardial viability, and coronary MR angiography with a three-dimensional respiratory navigator-gated technique. In 10 patients, the protocol was completed in 61.5 minutes +/- 5.5 and yielded high image quality and diagnostic accuracy. This protocol may provide an integrated noninvasive screening tool for patients with CAD.

Magnetic Resonance Angiography

Magnetic Resonance Angiography

3D MR angiography of pulmonary arteries using real-time navigator gating and magnetization preparation.

An ECG-triggered magnetization-prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real-time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil-beam navigator echoes were acquired immediately before and after the readout train and processed in real-time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept-or-reject-reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real-time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.