Miguel Trelles

SEMAC-VAT and MSVAT-SPACE sequence strategies for metal artifact reduction in 1.5T magnetic resonance imaging.

Objectives:To evaluate the ability of four magnetic resonance imaging (MRI) techniques to correct for metallic artifacts. These techniques consisted of 3 2D techniques and one 3D technique. In 2D imaging the techniques View Angle Tilting (VAT), Slice Encoding for Metal Artifact Correction (SEMAC) and a technique that employed a combination of the first two (SEMAC-VAT) were evaluated. In 3D imaging the technique Multiple Slab acquisition with VAT based on a SPACE sequence was evaluated (MSVAT-SPACE). Materials and Methods:Agarose phantoms and tissue phantoms with two commonly used metal implants (stainless steel and titanium) as well as two volunteers with metal implants were imaged at 1.5T. All phantoms and volunteers were imaged using VAT, SEMAC, SEMAC-VAT and MSVAT-SPACE techniques, as well as 2D and 3D conventional imaging techniques. Each technique was optimized for different image contrast mechanisms. Artifact reduction was quantitatively assessed in the agarose phantoms by volumetric measurement. Image quality was qualitatively assessed by blinded reads employing two readers. Each reader independently viewed the tissue phantom images and in vivo human images. Statistical analysis was performed using a Friedman test, Wilcoxon test and weighted Cohens kappa test. Results:T1-weighted, T2-weighted, PD-weighted and STIR image contrasts were successfully implemented with the evaluated artifact reduction sequences in both the phantom experiments and in vivo images. For all evaluated image contrasts and both metal implants, a reduction in the volume of metal artifacts was seen when compared with 2D conventional acquisitions. The 2D metal artifact volumes on average were reduced by 49% ± 16%, 56% ± 15% and 63% ± 15% for VAT, SEMAC and SEMAC-VAT acquisitions respectively. When Friedman and Wilcoxon tests were applied the difference in metal artifact volume was found to be statistically significant when VAT, SEMAC and SEMAC-VAT were compared with the 2D conventional techniques. In 3D imaging on average MSVAT-SPACE reduced metal artifact volume compared with the 3D conventional imaging technique by 72% ± 23% for all evaluated image contrasts and both metal implants. The metal artifact volume differences were statistically significant when MSVAT-SPACE was compared with the 3D conventional technique. The blinded reads demonstrated that SEMAC-VAT and MSVAT-SPACE had distinctly superior quality compared with conventional acquisitions. Quality was measured in terms of artifact size, distortions, image quality and visualization of bone marrow and soft tissues adjacent to metal implants. This was the case for both tissue phantom images and human images with good interobserver agreement. Conclusions:SEMAC-VAT (2D) and MSVAT-SPACE (3D) demonstrated a consistent, marked reduction of metal artifacts for different metal implants and offered flexible image contrasts (T1, T2, PD and STIR) with high image quality. These techniques likely will improve the evaluation of postoperative patients with metal implants.

CTA for Screening of Complicated Atherosclerotic Carotid Plaque—American Heart Association Type VI Lesions as Defined by MRI

BACKGROUND AND PURPOSE: High-resolution carotid MR imaging can accurately identify complicated American Heart Association lesion type VI plaques, which are characterized by thrombus, hemorrhage, or a ruptured fibrous cap. The purpose of this study is to evaluate whether CTA can be used as screening tool to predict the presence or absence of American Heart Association lesion type VI plaques as defined by high-resolution MR imaging. METHODS: Fifty-one patients with suspected ischemic stroke or TIA with carotid CTA and carotid MR imaging performed within 14 days of the event/admission from April 2008 to December 2010 were reviewed. Vessels with stents or occlusion were excluded (n = 2). Each carotid artery was assigned an American Heart Association lesion type classification by MR imaging. The maximum wall thickness, maximum soft plaque component thickness, maximum calcified component thickness, and its attenuation (if the soft plaque component thickness was >2 mm) were obtained from the CTA. RESULTS: The maximum soft plaque component thickness proved the best discriminating factor to predict a complicated plaque by MR imaging, with a receiver operating characteristic area under the curve of 0.89. The optimal sensitivity and specificity for detection of complicated plaque by MR imaging was achieved with a soft plaque component thickness threshold of 4.4 mm (sensitivity, 0.65; specificity, 0.94; positive predictive value, 0.75; and negative predictive value, 0.9). No complicated plaque had a soft tissue plaque thickness <2.2 mm (negative predictive value, 1) and no simple (noncomplicated) plaque had a thickness >5.6 mm (positive predictive value, 1). CONCLUSIONS: Maximum soft plaque component thickness as measured by carotid CTA is a reliable indicator of a complicated plaque, with a threshold of 2.2 mm representing little to no probability of a complicated American Heart Association lesion type VI plaque.

Computed tomography angiography vs 3 T black-blood cardiovascular magnetic resonance for identification of symptomatic carotid plaques

BackgroundThe purpose of this prospective study was to perform a head-to-head comparison of the two methods most frequently used for evaluation of carotid plaque characteristics: Multi-detector Computed Tomography Angiography (MDCTA) and black-blood 3 T-cardiovascular magnetic resonance (bb-CMR) with respect to their ability to identify symptomatic carotid plaques.Methods22 stroke unit patients with unilateral symptomatic carotid disease and >50% stenosis by duplex ultrasound underwent MDCTA and bb-CMR (TOF, pre- and post-contrast fsT1w-, and fsT2w- sequences) within 15 days of symptom onset. Both symptomatic and contralateral asymptomatic sides were evaluated. By bb-CMR, plaque morphology, composition and prevalence of complicated AHA type VI lesions (AHA-LT6) were evaluated. By MDCTA, plaque type (non-calcified, mixed, calcified), plaque density in HU and presence of ulceration and/or thrombus were evaluated. Sensitivity (SE), specificity (SP), positive and negative predictive value (PPV, NPV) were calculated using a 2-by-2-table.ResultsTo distinguish between symptomatic and asymptomatic plaques AHA-LT6 was the best CMR variable and presence / absence of plaque ulceration was the best CT variable, resulting in a SE, SP, PPV and NPV of 80%, 80%, 80% and 80% for AHA-LT6 as assessed by bb-CMR and 40%, 95%, 89% and 61% for plaque ulceration as assessed by MDCTA. The combined SE, SP, PPV and NPV of bb-CMR and MDCTA was 85%, 75%, 77% and 83%, respectively.ConclusionsBb-CMR is superior to MDCTA at identifying symptomatic carotid plaques, while MDCTA offers high specificity at the cost of low sensitivity. Results were only slightly improved over bb-CMR alone when combining both techniques.

MR angiography of carotid artery aneurysms in a porcine model at 3 Tesla: comparison of two different macrocyclic gadolinium chelates and of dynamic and conventional techniques.

To evaluate the differences in image quality of two macrocyclic gadolinium‐based contrast agents, gadobutrol and gadoterate meglumine, using time‐resolved, contrast‐enhanced MR angiography (CE‐MRA) in a porcine carotid artery aneurysm model and to compare image quality between dynamic and conventional, single acquisition CE‐MRA.

Contrast dose, temporal footprint, and spatial resolution tradeoffs in dynamic contrast-enhanced MRA performed in a porcine model of a carotid aneurysm.

Objective To evaluate the tradeoffs between temporal and spatial resolution and contrast dosing in dynamic contrast-enhanced magnetic resonance angiography (CE-MRA). Methods Bilateral carotid artery aneurysms were created in a swine model. Dynamic CE-MRA using 1 mol/L gadobutrol was performed at 3 T, with high temporal (high-temp), middle temporal (mid-temp), and low temporal (low-temp) resolutions. High temporal CE-MRA was performed twice using 1 mL and 2 mL gadobutrol (2 mL/s). Middle temporal and low-temp sequences were performed once with 2 mL gadobutrol (2 mL/s). The signal-to-noise ratio (SNR) was quantitatively assessed. Blinded reads were used to qualitatively evaluate contrast dose and image quality. Results The mean SNRs of high-temp, mid-temp, and low-temp resolutions were 56.7, 47.5, and 48.1. There was no significant difference between the 3 sequences with 2 mL gadobutrol. The mean SNR of the high-temp resolution with 2 mL was significantly higher than that with 1 mL (56.7 vs 39.9). In qualitative analysis, the 3 temporal sequences with 2 mL gadobutrol showed no significant differences regarding overall image quality and diagnostic value. High temporal resolution with 2 mL consistently showed the superiority of image quality than that with 1 mL. Conclusions High temporal dynamic CE-MRA with 2 mL (0.04 mmol/kg body weight) gadobutrol can produce consistently superior image quality over that with 1 mL (0.02 mmol/kg body weight). For a given contrast dose, the tradeoffs between temporal and spatial resolution will not result in significant differences in image quality in TWIST (time-resolved angiography with interleaved stochastic trajectories).

Diffusion Tensor Imaging

We have developed a method for the simultaneous estimation of local diffusion and the global fiber tracts based upon the information entropy flow that computes the maximum entropy trajectories between locations and depends upon the global structure of the multi-dimensional and multi-modal diffusion field. Computation of the entropy spectrum pathways requires only solving a simple eigenvector problem for the probability distribution for which efficient numerical routines exist, and a straight forward integration of the probability conservation through ray tracing of the convective modes guided by a global structure of the entropy spectrum coupled with a small scale local diffusion. The intervoxel diffusion is sampled by multi b-shell multi q-angle DWI data expanded in spherical waves. This novel approach to fiber tracking incorporates global information about multiple fiber crossings in every individual voxel and ranks it in the most scientifically rigorous way. This method has potential significance for a wide range of applications, including studies of

Spin Echo Imaging

The Spin Echo (SE) sequence was introduced in 1950 by Hahn long time before the MRI era began. In a common SE sequence the excitation is always performed with a 90° pulse turning the longitudinal magnetization into transversal magnetization. Afterwards, the transversal magnetization decays with the time constant T2* due to a dephasing process of spins induced by magnetic field inhomogeneities. After half of the desired echo time (TE) a 180° excitation pulse is played out resulting in a rephasing process of the dephased spins by the static magnetic field inhomogeneities. Due to this rephasing process the signal reaches its maximum after TE, the so called Spin echo. Therefore, the effect of static magnetic field inhomogeneities is suppressed and the amplitude of the spin echo is given by the relaxation time T2 – the effect of the non-static magnetic field inhomogeneities induced by the stochastic spin-spin-interaction (T2) can not be reversed. The benefit of the SE sequence is therefore its insensitivity to magnetic field inhomogeneities. Drawback is given by the long scan time due the waiting interval until the next spin echo experiment can be performed (TR), i.e. longitudinal magnetization due to the T1 relaxation process has recovered to be excited again. The SE sequence is schematically presented in Fig. 1 including all additional magnetic field gradients for the spatial encoding.

46 Contrast-Enhanced MRA: Basics; Renal, Abdomen

Fig. 46.1A demonstrates the abdominal aorta and common iliac arteries, with moderate to severe stenosis noted at the origin of the left renal artery (arrow). Figure 46.1B demonstrates extensive atherosclerotic disease involving the aorta, with severe stenosis at the origin of the left renal artery. Both studies were performed at 1.5 T. The study presented in Fig. 46.2 (reprinted with permission from U. Kramer, Invest Radiol 2007;42:747) illustrates the feasibility of high spatial resolution contrastenhanced MRA at 3 T, providing a further improvement in evaluation of the renal artery and its branches. Early branching is demonstrated, involving both renal arteries, in this potential, living, related kidney donor. The voxel size was 1 mm3, the contrast dose 0.1 mmol/kg injected at 2 mL/sec and the scan time 16 sec (with a parallel imaging factor of 3 employed). An additional advantage of this type of acquisition is the ability to reconstruct high-resolution images in any desired plane, given the high spatial resolution and isotropic voxel dimensions. Such reformatted images are similarly advantageous for the evaluation of renal artery stenosis. In this application, 3 T offers substantial advantages compared with 1.5 T, largely due to the inherent increase in SNR. Improved suppression of background tissue, due to the prolongation of T1 at 3 T, aids as well by further increasing CNR. Contrast-enhanced MRA (CE-MRA) has become the exam of choice for evaluation of the abdominal aorta and renal arteries. The standard imaging sequence is a fast 3D