Three tesla magnetic resonance angiography with ultrashort echo time describes the arteries near the cerebral aneurysm with clip and the peripheral cerebral arteries

Ultra-short echo time (UTE) MR imaging: A brief review on technical considerations and clinical applications

BACKGROUND

 With the availability of MRI sequences with ultrashort echo times (UTE sequences), a signal can be gained from tissue, which was formerly only indirectly accessible. While already extensively employed in various research settings, the widespread transition of UTE imaging to clinical practice is just starting.

METHODS

 Based on a systematic literature search as well as knowledge gained through annual participation in conferences dedicated to advances in MRI, this review aims to give a brief overview of technical considerations and challenges of UTE imaging and summarizes the major areas of application of UTE imaging.

RESULTS

 UTE is already employed in clinical practice for structural lung imaging as well as the characterization of tissue composition and its alterations in selected musculoskeletal, cardiovascular, or neurodegenerative diseases. In specific contexts it can replace CT examinations with ionizing radiation and is especially attractive for pediatric patients and longitudinal monitoring of disease progression and treatment.

CONCLUSION

 UTE imaging provides an interesting and very valuable tool for various clinical purposes and promises a multitude of new insights into tissue properties. While some challenges remain, ongoing adoption in the clinical routine can be expected, as UTE approaches provide a new contrast and capture a signal in tissue formerly invisible on MR imaging.

KEY POINTS

  · UTE imaging gains relevance in clinical settings. · UTE imaging is employed for the characterization of tissue composition and its alterations in selected musculoskeletal, cardiovascular, or neurodegenerative diseases. · UTE imaging is employed in the clinical routine for structural lung imaging. · UTE imaging promises a multitude of new insights into tissue properties.

Making the invisible visible-ultrashort echo time magnetic resonance imaging: Technical developments and applications

Magnetic resonance imaging (MRI) uses a large magnetic field and radio waves to generate images of tissues in the body. Conventional MRI techniques have been developed to image and quantify tissues and fluids with long transverse relaxation times (T2s), such as muscle, cartilage, liver, white matter, gray matter, spinal cord, and cerebrospinal fluid. However, the body also contains many tissues and tissue components such as the osteochondral junction, menisci, ligaments, tendons, bone, lung parenchyma, and myelin, which have short or ultrashort T2s. After radio frequency excitation, their transverse magnetizations typically decay to zero or near zero before the receiving mode is enabled for spatial encoding with conventional MR imaging. As a result, these tissues appear dark, and their MR properties are inaccessible. However, when ultrashort echo times (UTEs) are used, signals can be detected from these tissues before they decay to zero. This review summarizes recent technical developments in UTE MRI of tissues with short and ultrashort T2 relaxation times. A series of UTE MRI techniques for high-resolution morphological and quantitative imaging of these short-T2 tissues are discussed. Applications of UTE imaging in the musculoskeletal, nervous, respiratory, gastrointestinal, and cardiovascular systems of the body are included.

Follow-up imaging of clipped intracranial aneurysms with 3-T MRI: comparison between 3D time-of-flight MR angiography and pointwise encoding time reduction with radial acquisition subtraction-based MR angiography

OBJECTIVE

Metallic susceptibility artifact due to implanted clips is a major limitation of using 3D time-of-flight magnetic resonance angiography (TOF-MRA) for follow-up imaging of clipped aneurysms (CAs). The purpose of this study was to compare pointwise encoding time reduction with radial acquisition (PETRA) subtraction-based MRA with TOF-MRA in terms of imaging quality and visibility of clip-adjacent arteries for use in follow-up imaging of CAs.

METHODS

Sixty-two patients with 73 CAs were included retrospectively in this comparative study. All patients underwent PETRA-MRA after TOF-MRA performed simultaneously with 3-T MRI between September 2019 and March 2020. Two neuroradiologists independently compared images obtained with both MRA modalities to evaluate overall image quality using a 4-point scale and visibility of the parent artery and branching vessels near the clips using a 3-point scale. Subgroup analysis was performed according to the number of clips (less-clipped [1-2 clips] vs more-clipped [≥ 3 clips] aneurysms). The ability to detect aneurysm recurrence was also assessed.

RESULTS

Compared with TOF-MRA, PETRA-MRA showed acceptable image quality (score of 3.97 ± 0.18 for TOF-MRA vs 3.73 ± 0.53 for PETRA-MRA) and had greater visibility of the adjacent vessels near the CAs (score of 1.25 ± 0.59 for TOF-MRA vs 2.27 ± 0.75 for PETRA-MRA, p < 0.0001). PETRA-MRA had greater visibility of vessels adjacent to less-clipped aneurysms (score of 2.39 ± 0.75 for less-clipped aneurysms vs 2.09 ± 0.72 for more-clipped aneurysms, p = 0.014). Of 73 CAs, aneurysm recurrence in 4 cases was detected using PETRA-MRA.

CONCLUSIONS

This study demonstrated that PETRA-MRA is superior to TOF-MRA for visualizing adjacent vessels near clips and can be an advantageous alternative to TOF-MRA for follow-up imaging of CAs.

1.5 Tesla Non-ultrashort but Short Echo Time Magnetic Resonance Angiography Describes the Arteries Near a Clipped Cerebral Aneurysm

Cerebral aneurysm and mother artery assessment after clipping is essential to evaluate aneurysm remnant, regrowth, and clip slippage. Usually, cerebral angiography and contrast-enhanced computed tomography angiography (CTA) are used for the evaluation, but they have the side effect of contrast medium and are time-consuming. Time-of-flight magnetic resonance angiography (TOF-MRA) is a non-invasive and fast modality, but clip-induced artifacts limit the signal near the metal clip. Recent ultrashort echo time (UTE)-MRA reduces metal artifacts but its availability is still low worldwide. Therefore, we developed a modified TOF-MRA sequence, named short TE-MRA, using Optima MR 360 1.5T Advance (GE Healthcare Life Sciences, Buckinghamshire, UK). It could describe the artery near the clip using general MRA equipment without recent UTE-MRA technology. We present a subarachnoid hemorrhage patient who underwent short TE-MRA about a year after clipping for the aneurysms at the bilateral internal carotid arteries. Short TE-MRA described the left internal carotid, middle cerebral, and anterior cerebral arteries. The right middle and anterior cerebral arteries were described, but the right internal carotid artery was not. Normal TOF-MRA could not describe them. Without recent UTE-MRA technology, short TE-MRA might be an alternative method for evaluating the artery near the clip. Short TE-MRA can be performed by general MRA equipment with a little time, so it may be helpful until UTE-MRA is widely used. Further research is needed on whether short TE-MRA can describe the aneurysm remnant, regrowth, and clip slippage up to the clinically useful level.

The feasibility of non-contrast-enhanced zero echo time magnetic resonance angiography for characterization of intracranial atherosclerotic disease

Background

Accurate and non-invasive assessment of intracranial atherosclerotic disease (ICAD) is important because of its effect on treatment planning. The aim of this study is to investigate if zero echo time (zTE) magnetic resonance angiography (zTE-MRA) is feasible in the characterization of ICAD.

Methods

A total of 175 patients with ICAD were recruited. ZTE-MRA and time-of-flight (TOF)-MRA sequences were conducted for all participants using a 3T clinical MR system. Forty-one patients also underwent digital subtraction angiography (DSA), and were confirmed to have intracranial arterial stenosis (ICAS). Weighted kappa (κ) statistics were used to assess the inter-observer agreement and diagnostic consistency of both zTE- and TOF-MRA, using DSA as a reference. The Wilcoxon signed-rank test was used to evaluate differences in image quality between zTE- and TOF-MRA images. The nonparametric test of multiple paired samples was used to compare the results of vascular stenosis diagnosis between zTE-, TOF-MRA and DSA.

Results

Supported by high inter-observer agreement (weighted κ=0.78), zTE-MRA generated significantly higher scores than TOF-MRA for susceptibility artifact signal (mean: 3.03±0.98 vs. 2.72±1.09; P=0.017) and flow signal in parent artery (mean: 3.63±0.49 vs. 3.07±0.82; P<0.001). Additionally, zTE-MRA showed more robust diagnostic performance than TOF-MRA for patients with ICAD and degree of vascular stenosis (P<0.05), and was highly consistent with reference DSA images (weighted κ=0.80).

Conclusions

ZTE-MRA has potential for use as a routine clinical method for patients with ICAD.

The dataset on the clipped cerebral aneurysm and their radiological findings in three-dimensional computed tomography, time-of-flight magnetic resonance angiography (TOF-MRA), and Pointwise Encoding Time Reduction with Radial Acquisition (PETRA)-MRA

These data present the 141 intracranial arterial branches' visibilities near the 72 cerebral aneurysms in postoperative 58 patients treated with titanium or cobalt-chromium-nickel-molybdenum (CCNM) alloy clips. The visibilities were evaluated using time-of-flight magnetic resonance angiography (TOF-MRA), pointwise encoding time reduction with radial acquisition (PETRA)-MRA, which uses MRA with ultrashort echo time (UTE-MRA) and subtraction technique between saturated and non-saturated images, and three-dimensional computed tomography angiography (3DCTA). We retrospectively acquired the data from the medical records of Suwa Red Cross Hospital. Each method's appearance was compared, and associations between visibility on PETRA-MRA, arterial diameter, clip numbers, clip shapes, clip materials, and amounts of hematoma were summarized. Our article on PETRA-MRA's usefulness for proximal and branched arteries evaluation after cerebral aneurysm clipping [1] was based on these data. This dataset would be useful for reference value for other neurosurgeons or radiologists for further analysis on PETRA-MRA and another UTE-MRA like SILENT-MRA after cerebral aneurysm clipping.

Usefulness of 3 Tesla Ultrashort Echo Time Magnetic Resonance Angiography (UTE-MRA, SILENT-MRA) for Evaluation of the Mother Vessel after Cerebral Aneurysm Clipping: Case Series of 19 Patients

It is important to assess the cerebral arteries near the clip after cerebral aneurysm clipping. Contrast-enhanced computed tomography angiography has side effects of contrast medium and radiation exposure. Time-of-flight magnetic resonance angiography (TOF-MRA) is a fast and non-invasive method, but clip-induced artifact limits the assessment around the clip. Recently, 3 tesla MRA with ultrashort echo time called SILENT MRA (GE Healthcare Life Sciences, UK) has been reported to have the potential to overcome these disadvantages. We herein present consecutive 19 cerebral aneurysm patients treated by clipping and evaluated using SILENT MRA. The 19 patients (15 women and 4 men) underwent TOF-MRA and SILENT MRA during the same scan session. Two neurosurgeons independently assessed the visibility of the mother vessel at the clipping site in TOF-MRA and SILENT MRA. We also investigated the factors related to visibility in SILENT MRA. All patients’ mother vessels were not described in TOF-MRA, and that of 16 patients (84%) were described in SILENT MRA. Overall agreement was 100% in the two neurosurgeons, and the fixed marginal kappa = 1.00 (95% CI: 0.36–1.00). Univariate analysis revealed that larger aneurysm dome and long clip blade length contributed to the visibility of the mother vessel in SILENT MRA. (p = 0.023, 0.007, each). In conclusion, SILENT MRA can be applied for the assessment of the arteries and aneurysm neck remnants near the clip. Using clips with long blade and ligation with its tip would be related to the visibility of the mother vessels in SILENT MRA.

Usefulness of Pointwise Encoding Time Reduction with Radial Acquisition and Subtraction-Based Magnetic Resonance Angiography after Cerebral Aneurysm Clipping

Objective

Time-of-flight magnetic resonance angiography (MRA) is limited by clip-induced artifacts after cerebral aneurysmal clipping. Recently, ultrashort echo time was shown to reduce metal artifacts. We assessed the pointwise encoding time reduction with radial acquisition (PETRA) sequence in subtraction-based MRA as an ultrashort echo time method during follow-up for clipping surgery.

Methods

We retrospectively evaluated 114 branches of 63 aneurysms in 56 patients treated with titanium clips using MRA and 3-dimensional computed tomography angiography. The appearance using each method was compared, and the associations between visibility on PETRA-MRA, clip number and shape, and amount of hematoma were examined. Furthermore, the visibility of the aneurysm remnants and 2 clipping cases with cobalt-chromium-nickel-molybdenum clips were evaluated.

Results

No branches were visible using time-of-flight-MRA, but 79 of 114 branches (69.3%) were visible on PETRA-MRA. PETRA-MRA was effective for follow-up imaging in 33 of 63 aneurysms (52.4%). The median vessel diameters were 1.67 mm (interquartile range, 1.24–2.62 mm) and 0.96 mm (interquartile range, 0.59–1.53 mm) in the visible and invisible groups, respectively. Only the vessel diameter correlated significantly (P < 0.001) with the visibility on PETRA-MRA. A receiver operating characteristic curve for the association between the vessel diameter and visibility on PETRA-MRA showed a cutoff value of 1.26 mm for vessel diameter. Cobalt-chromium-nickel-molybdenum clips produced a strong artifact, even on PETRA-MRA. All 4 residual aneurysms were visible on PETRA-MRA.

Conclusions

PETRA-MRA can be useful for follow-up aneurysm imaging when the diameter of vessels adjacent to the clip exceeds 1.26 mm. However, its usefulness is limited to titanium clips.