Fast magnetic resonance imaging techniques

Radial sequences and compressed sensing in pediatric body magnetic resonance imaging

Magnetic resonance imaging (MRI) is often an ideal imaging modality for children of any age for any anatomy and for many pathologies. MRI sequences can be prescribed to produce high-resolution images of anatomical structures, characterize tissue composition, and detect physiological states and organ function. Shortening imaging sequences in any manner possible has been a topic of research and development in MRI since its emergence. Selection of imaging sequence parameters influences more than just the appearance and signal qualities of the imaged tissues; these details along with spatial encoding and data readout steps determine the time it takes to acquire an image. As each piece of image data is acquired and encoded with spatial and temporal information it is stored in k-space. As k-space is filled, either completely or partially, a diagnostic image or physiological data can be reconstructed. Shortening the length of time required for the readout step by efficiently filling k-space using compressed sensing and radial techniques is the subject of this manuscript.

Incidental Findings on Pediatric Abdominal Magnetic Resonance Angiography

RATIONALE AND OBJECTIVES

Abdominal magnetic resonance angiography (MRA) has gained favor in pediatric patients owing to its lack of ionizing radiation and noninvasive nature. Reports exist regarding incidental findings on body MRA in adult patients. However, the incidental findings in pediatric abdominal MRA have not been previously reported. Our study aims to determine the frequencies, characteristics, and categories of incidental findings in pediatric patients undergoing abdominal MRA.

MATERIALS AND METHODS

Retrospective study was performed in 78 consecutive contrast-enhanced abdominal MRA of patients between ages 0 and 20 years over a 7-year time period. The presence of incidental vascular and extravascular findings was noted. Reports were categorized in consensus by two radiologists as no incidental finding (group A), normal or normal variants or nonsignificant incidental common findings (group B), or abnormal incidental findings (group C). Group C was reviewed to determine whether additional management was performed.

RESULTS

A total of 40 boys and 38 girls (51%:49%) were reported, with a mean age of 12.3 years (standard deviation ±5.6 years, range 7 days to 20 years). Three most common indications for MRA were renal artery stenosis (24.4%), vasculitis (21.8%), and suspected intra-abdominal venous thrombosis (14.1%). We identified a total of 92 incidental findings in 50 of 78 patients; 60 findings in 29 patients in group B, and 32 findings in 21 patients in group C. Atelectasis at the lung bases was the most common incidental finding in group B (14 of 78 patients). The most common findings in group C were ascites, scoliosis, and splenomegaly. There were three abnormal incidental findings that led to causative workup and/or further management (moderate ascites, pericardial and pleural effusion, and venous malformation). The remaining cases with abnormal findings received treatment of their primary conditions only.

CONCLUSIONS

Pediatric abdominal MRA revealed a large number of incidental findings. The large majority were findings without clinical significance. Basal lung atelectasis was the most common overall incidental and nonsignificant finding, whereas ascites was the most common abnormal incidental finding. Although not all abnormal incidental findings affected management, appropriate identification and communication of relevant findings would improve patient care.

Diffusion-weighted MR imaging using FASE sequence for 3T MR system: Preliminary comparison of capability for N-stage assessment by means of diffusion-weighted MR imaging using EPI sequence, STIR FASE imaging and FDG PET/CT for non-small cell lung cancer patients

PURPOSE

To prospectively compare the diagnostic capability of diffusion-weighted MR imaging obtained with fast advantage spin-echo sequence (FASE-DWI) and echo planar imaging sequence (EPI-DWI), short inversion time inversion recovery fast advanced spin-echo (STIR FASE) imaging and FDG PET/CT for N-stage assessment of non-small cell carcinoma (NSCLC) patients.

MATERIALS AND METHODS

95 consecutive operable NSCLC patients underwent STIR FASE imaging, FASE-DWI and EPI-DWI with a 3T system, integrated PET/CT, surgical treatment and pathological and follow-up examinations. Probability of lymph node metastasis was visually assessed using a 5-point visual scoring system. ROC analyses were used to compare diagnostic capability of all methods, while their diagnostic performance was also compared by means of McNemar's test on a per node basis. Finally, McNemar's test was also used for statistical comparison of accuracy of N-stage assessment.

RESULTS

Areas under the curve (Azs) for STIR FASE imaging (Az=0.95) and FASE-DWI (Az=0.92) were significantly larger than those for EPI-DWI (Az=0.78; p<0.0001 for STIR FSE imaging and FASE-DWI) and PET/CT (Az=0.85; p=0.0001 for STIR FSE imaging, p=0.03 for FASE-DWI) on a per node basis analysis. Accuracy of N-stage assessment using STIR FASE imaging (84.2% [80/95]) and FASE-DWI (83.2% [79/95]) was significantly higher than that using EPI-DWI (76.8% [73/95]; p=0.02 for STIR FASE imaging, p=0.03 for FASE-DWI) and PET/CT (73.7% [70/95]; p=0.002 for STIR FSE imaging, p=0.004 for FASE-DWI).

CONCLUSION

Qualitative N-stage assessments of NSCLC patients obtained with FASE-DWI as well as STIR FASE imaging are more sensitive and/or accurate than those obtained with EPI-DWI and FDG PET/CT.

Chest MR imaging in the follow-up of pulmonary alterations in paediatric patients with middle lobe syndrome: comparison with chest X-ray

PURPOSE

The authors evaluated the role of magnetic resonance (MR) imaging of the chest in comparison with chest X-ray in the follow-up of pulmonary abnormalities detected by computed tomography (CT) in paediatric patients with middle lobe syndrome.

MATERIALS AND METHODS

Seventeen patients with middle lobe syndrome (mean age 6.2 years) underwent chest CT at the time of diagnosis (100 kV, CARE dose with quality reference of 70 mAs; collimation 24×1.2 mm; rotation time 0.33 s; scan time 5 s); at follow-up after a mean of 15.3 months, all patients were evaluated with chest MR imaging with a respiratory-triggered T2-weighted BLADE sequence (TR 2,000; TE 27 ms; FOV 400 mm; flip angle 150°; slice thickness 5 mm) and chest X-ray. Images from each modality were assessed for the presence of pulmonary consolidations, bronchiectases, bronchial wall thickening and mucous plugging. Hilar and mediastinal lymphadenopathies were assessed on CT and MR images.

RESULTS

Baseline CT detected consolidations in 100% of patients, bronchiectases in 35%, bronchial wall thickening in 53% and mucous plugging in 35%. MR imaging and chest X-ray identified consolidations in 65% and 35%, bronchiectases in 35% and 29%, bronchial wall thickening in 59% and 6% and mucous plugging in 25% and 0%, respectively. Lymphadenopathy was seen in 64% of patients at CT and in 47% at MR imaging.

CONCLUSIONS

Patients with middle lobe syndrome show a wide range of parenchymal and bronchial abnormalities at diagnosis. Compared with MR imaging, chest X-ray seems to underestimate these changes. Chest MR imaging might represent a feasible and radiation-free option for an overall assessment of the lung in the follow-up of patients with middle lobe syndrome.

Liver fat content determined by magnetic resonance imaging and spectroscopy

Hepatic steatosis as the most prevalent liver disorder can either be related to alcoholic liver disease (ALD) or non-alcoholic fatty liver disease (NAFLD). In both conditions, hepatocytes excessively accumulate fat-containing vacuoles within their cytoplasm, which is the key histological feature. In contrast to ALD, NAFLD is commonly associated with metabolic syndrome, obesity and insulin resistance. To determine increased liver fat content, liver biopsy is currently considered the gold standard. Besides the invasive technique, various other non-invasive techniques have been developed, such as ultrasound, computed tomography (CT), magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) based methods. Among these techniques, ultrasound and CT provide only qualitative information about hepatic steatosis, whereas MRS- or MRI-based methods are able to determine even small amounts of fat accurately. These non-invasive magnetic resonance techniques have already proven their great potential, especially in longitudinal and cross-sectional studies regarding various metabolic conditions and medical treatment regimens. In this review, the most common, non-invasive MRS/MRI techniques for assessment of intrahepatic lipid content are described with their inherent advantages and limitations.

Off-resonance frequency filtered magnetic resonance imaging

One of the main problems with rapid magnetic resonance imaging (MRI) techniques is the artifacts that result from off-resonance effects. The proposed off-resonance frequency filtered MRI (OFF-MRI) method focuses on the elimination of off-resonance components from the image of the observed object. To maintain imaging speed and simultaneously achieve good frequency selectivity, MRI is divided into two steps: signal acquisition and post-processing. After the preliminary phase in which we determine imaging parameters, MRI takes place; the signal from the same object is successively acquired M times. As a result, we obtain M partial signals in k-space, from which we form the image of the observed object in the post-processing phase, after signal acquisition has been completed. This paper demonstrates that with proper selection of acquisition parameters and weighting coefficients in the post-processing phase, OFF-MRI is equivalent to filtering the signal by finite impulse response filter of length M. It is shown that with M successive acquisitions M-1 off-resonance components can be eliminated (filtered-out) from images, and therefore, only two acquisitions are needed to eliminate one off-resonance components. On the other hand, with OFF-MRI, it is also possible to form the image of an arbitrary off-resonance component by eliminating all other off-resonance components, including the on-resonance component. The proposed OFF-MRI method is suitable for MRI where rapid acquisition is required and elimination of off-resonance components can improve reliability of measurements.

Non-invasive assessment and quantification of liver steatosis by ultrasound, computed tomography and magnetic resonance

Hepatic steatosis is the most prevalent liver disorder in the developed world. It is closely associated with features of metabolic syndrome, especially insulin resistance and obesity. The two most common conditions associated with fatty liver are alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD). Liver biopsy is considered the gold standard for the assessment of liver fat, but there is a need for less invasive diagnostic techniques. New imaging modalities are emerging, which could provide more detailed information about hepatic tissue or even replace biopsy. In the present review, available imaging modalities (ultrasound, computed tomography, magnetic resonance imaging and proton magnetic resonance spectroscopy) are presented which are employed to detect or even quantify the fat content of the liver. The advantages and disadvantages of the above-mentioned imaging modalities are discussed. Although none of these techniques is able to differentiate between microvesicular and macrovesicular steatosis and to reveal all features visible using histology, the proposed diagnostic modalities offer a wide range of additional information such as anatomical and morphological information non-invasively. In particular, magnetic resonance imaging and proton magnetic resonance spectroscopy are able to quantify the hepatic fat content hence avoiding exposure to radiation. Except for proton magnetic resonance spectroscopy, all modalities offer additional information about regional fat distribution within the liver. MR elastography, which can estimate the amount of fibrosis, also appears promising in the differentiation between simple steatosis and steatohepatitis.

MRI findings in Shwachman diamond syndrome

Shwachman Diamond Syndrome (SDS) is a rare congenital disorder characterized by pancreatic insufficiency, bone marrow dysfunction, and skeletal changes. Because of the heterogeneous clinical presentation and the limits of laboratory tests that assess pancreatic insufficiency, the diagnosis of SDS can be challenging. Pancreatic lipomatosis, a typical feature of this syndrome, is also difficult to assess by direct tissue sampling. In these circumstances, magnetic resonance imaging (MRI) provides a readily available, noninvasive tool to evaluate the pancreatic fat content. We report a case of a 12-month-old male in which abdominal MRI was used to confirm the clinical diagnosis of SDS.

Total-body MR-imaging in oncology

Although MRI is an effective modality in oncology, state-of-the-art total-body MRI (TB-MRI) in the past was infeasible in the diagnostic work-up, due to the need for repeated examinations with repositioning and separate surface coils to cover all body parts. To overcome this limitation, either a moving table platform in combination with the body-coil or a special designed rolling table platform with one body phased-array coil have been implemented with promising results for both tumor staging and metastases screening. Since 2004, state-of-the-art TB-MR imaging with high spatial resolution has become feasible using a newly developed 1.5 Tesla TB-MRI system with multiple receiver channels. This review gives an overview based on the recent literature as well as our own experience concerning the possibilities, challenges, and limitations of TB-MRI in oncology, emphasizing both oncological staging and early tumor detection in asymptomatic subjects.

Simulated pulmonary nodules implanted in a dedicated porcine chest phantom: sensitivity of MR imaging for detection

PURPOSE

To evaluate the diagnostic accuracy of common magnetic resonance (MR) imaging sequences for detection of small pulmonary nodules by using a chest phantom and porcine lungs containing simulated lesions.

MATERIALS AND METHODS

Fourteen porcine lungs containing 366 porcine myocardial tissue implants were inflated inside a phantom. Two-dimensional (2D) and three-dimensional (3D) gradient-echo (GRE), T2-weighted turbo spin-echo (SE), and T2-weighted single-shot SE train MR sequences were performed. Spiral computed tomography (CT) was performed for comparison. Blinded observers read the images and recorded the sizes and locations of visible nodules by consensus. The sensitivity of each imaging method for depicting single nodules of given sizes was calculated. Specificities, positive predictive values (PPVs), and negative predictive values (NPVs) for detection of one or more nodules of various sizes were calculated.

RESULTS

Sensitivities of 3D GRE, 2D GRE, T2-weighted turbo SE, and T2-weighted single-shot SE train MR imaging and of CT were 0.50, 0.40, 0.12, 0.00, and 0.55, respectively, for detection of 1.4-mm nodules and 0.88, 0.84, 0.69, 0.06, and 0.96, respectively, for detection of 4.2-mm nodules. The 95% CIs for CT and GRE MR imaging overlapped, but those for turbo SE and single-shot SE train MR imaging differed significantly (P <.05). For detection of nodules larger than 5 mm, all examinations except single-shot SE train MR imaging yielded a specificity, PPV, and NPV of 1.00 each. For detection of nodules smaller than 5 mm, diagnostic accuracy of 3D GRE MR imaging was high: Specificity, PPV, and NPV all were approximately 0.90. Two-dimensional GRE MR imaging results were influenced by false-positive findings: Specificity was 0.64; PPV, 0.74; and NPV, 1.00.

CONCLUSION

Common MR imaging sequences such as 3D GRE have high diagnostic accuracy in depicting small pulmonary nodules when artifacts from cardiac and respiratory motion are absent.

Quantification of vena cava blood flow with half Fourier echo-planar imaging

BACKGROUND

Human hemodynamics occurs in very short periods of time. To quantify blood flow under these circumstances, a fast-scan imaging technique is required. Echo-planar imaging can be a good candidate because it is able to acquire images in <50-100 msec. An imaging scheme with these properties can produce real-time images as well as overcome motion artifacts such as blurring and ghosting, which alter image quality. Additionally, echo-planar imaging does not require a calibration protocol to perform flow experiments in the human cardiovascular system. Consequently, echo-planar imaging appears to be the best imaging tool available to quantify blood flow in the vena cava. From a clinical point of view, echo-planar imaging has become a widespread commodity to produce magnetic resonance images in real-time.

METHODS

Flow-encoded half Fourier echo-planar imaging is proposed to determine blood flow in the arteries. This flow sequence was used to investigate vena cava blood flow in healthy volunteers and compared with other diagnostic imaging modalities.

RESULTS

Two-dimensional flow maps were obtained by using the two components (sine and cosine images) resulting from the flow-encoded echo-planar imaging sequence. Velocity profiles of vena cava of two healthy volunteers were calculated from the previous bidimensional blood flow maps.

CONCLUSIONS

We proved that real-time flow imaging of the cardiovascular system can be achieved with flow-encoded echo-planar imaging and a partial Fourier method. It is possible to quantify blood flow in the superior vena cava in humans. We believe that this imaging tool might offer relevant anatomic and physiologic information of the vena cava as well as of the cardiovascular system.

Liver lesion conspicuity: T2-weighted breath-hold fast spin-echo MR imaging before and after gadolinium enhancement--initial experience

PURPOSE

To evaluate the effect of a gadolinium chelate on T2-weighted breath-hold fast spin-echo magnetic resonance images of focal hepatic lesions.

MATERIALS AND METHODS

In 21 patients with focal hepatic lesions, identical T2-weighted breath-hold fast spin-echo images were obtained before and after gadolinium enhancement and were compared regarding lesion-to-liver contrast-to-noise ratio, signal-to-noise ratio, lesion conspicuity, and vascular pulsation artifact. Image review was performed independently, in random order, by two experienced radiologists.

RESULTS

For solid lesions, the lesion-to-liver contrast-to-noise ratio on enhanced images was significantly higher (P <.05) than that on nonenhanced images. For nonsolid lesions, however, there was no significant difference (P =.07). For both readers, lesion conspicuity for solid lesions on enhanced images was significantly higher than on nonenhanced images (P <.05). Severity of vascular pulsation artifact was not significantly different.

CONCLUSION

Solid-lesion contrast on T2-weighted breath-hold fast spin-echo images improves after administration of a gadolinium chelate. These images should be obtained after, rather than before, gadolinium enhancement.

Real-time projection MR angiography: feasibility study

Intraarterial injections of small doses of gadopentetate dimeglumine were combined with a fast spoiled-gradient-echo magnetic resonance (MR) sequence to obtain real-time projection angiographic images of the rabbit aorta and canine coronary arteries. Arterial filling and washout, as well as venous and perfusion phases, were clearly displayed, demonstrating that arterial fluoroscopy in which an MR technique is used is feasible.

Novel technique for three-dimensional visualisation and quantification of deformable, moving soft-tissue body parts

BACKGROUND

Three-dimensional (3D) imaging of delicate, moving soft-tissue body parts is very difficult. Our understanding of the muscles that control lip movements is based largely on histological and cadaveric studies, which provide scant information about dynamic morphology. Our aim was to develop an innovative scanning technique for the imaging and reconstruction of dynamic orofacial morphology by use of 3D and four-dimensional (4D, ie, 3D plus time) ultrasonography.

METHODS

Four volunteers (including one patient) underwent ultrasonography with 3D/4D imaging systems. To avoid deformation of the delicate orofacial structures, a water bath with an acoustic window was devised. The orofacial part was immersed in the bath throughout scanning, and a timer was used to synchronize lip movements with the 4D scan.

FINDINGS

4D views showed the functional differences in superficial and deep muscle groups of the lips, and clearly showed the changes occurring with movement of the lips and mouth. In the patient, a pathological layer and its extension corresponding to surface malformation were clearly identified.

INTERPRETATION

We have developed a prototype device that has made possible 3D and 4D examination of orofacial anatomy and function. With further refinement of the device and improvement in 4D acquisition timing, this technique may offer a new way of dynamically imaging and quantifying many soft-tissue parts in 3D without deforming structure or disturbing function.