Computed tomography of the carotid space and related cervical spaces. Part II: Neurogenic tumors

In order to determine the value of coronal magnetic resonance (MR) in diagnosing thoracic abnormalities, the multisection coronal spin echo MR images were reviewed of 10 normal subjects and 20 patients with thoracic abnormalities. In the abnormal patients, coronal images were compared with transaxial MR images obtained with TR values of 0.5 and 2.0 sec. In general, coronal imaging was of value in several situations. It allowed structures oriented in the coronal plane to be imaged along their longitudinal axes, provided an additional perspective and increased the confidence of diagnosis, and helped clarify anatomic relationships difficult or impossible to resolve on transaxial images because of volume averaging. Specifically, coronal images were sometimes superior to transaxial images in evaluating the aorticopulmonary window and masses at the lung apex or base. Transaxial images were often superior in evaluating the pretracheal space, subcarinal space, and hili. Within the pulmonary hili, lateral hilar masses were better defined on coronal images than were anterior or posterior hilar masses. Coronal images obtained with a TR of 1.0 sec (10 sections) allow evaluation of most node-bearing mediastinal compartments and provide adequate mass/fat contrast.     

MR imaging of the aorta with three-dimensional vessel reconstruction: validation by angiography

Longitudinal vascular structures are difficult to observe on the standard abdominal transaxial magnetic resonance (MR) image sections. To display the information in a three-dimensional reconstruction, an algorithm was written to identify blood flow in a series of transaxial MR sections and was applied to reconstructing images of the aorta and iliac arteries in 12 patients with aortic aneurysm, dissection, or aortoiliac atherosclerosis. Results were validated by angiography. In all patients, the outline of the flow channel in the reconstructed image followed closely the outline of the lumen on angiograms. In aortic dissection, the MR images showed the two lumens more completely than did the angiograms, and in atherosclerosis, sites of vascular stenosis were correctly identified on MR images. The technique is valuable in providing anatomic information as well as functional information on cross-sectional areas and relative flow velocities.     

In-plane vascular imaging: pulse sequence design and strategy

Magnetic resonance (MR) angiography is a noninvasive method of obtaining images without contrast agents. Recent developments in sequence design have allowed images of moving spins to be obtained without a loss of signal by rephasing the spins with three or four gradient pulses to compensate for constant velocity or acceleration, respectively. At longer echo times (TE), this approach allowed for low readout gradients and high signal-to-noise ratios. Angiograms with a resolution of 300 micron were obtained. With additional sequences that allow some dephasing but minimal signal loss, separate images of arteries and veins were obtained. Phase information was used to estimate flow velocity. Application of the rephasing scheme to gradient-echo sequences allowed for ungated, fast MR angiograms. Acceleration correction was important for long TE sequences, but velocity-corrected, gradient-echo sequences with a very short TE were comparable to velocity- and acceleration-corrected, gradient-echo sequences with slightly longer TEs. With ungated three-dimensional, gradient-echo sequences, susceptibility artifacts were minimized and excellent contrast-to-noise ratios were obtained.     

Sagittal MR imaging of the chest: normal and abnormal

To determine the value of sagittal magnetic resonance (MR) in diagnosing thoracic abnormality, we reviewed the multisection sagittal spin-echo MR images of 13 normal subjects and 23 patients with a variety of thoracic abnormalities. In the abnormal subjects sagittal images were compared with transaxial MR performed with repetition time values of both 0.5 and 2.0 s. Sagittal images were most helpful in the evaluation of structures lying in the sagittal plane such as the thoracic aorta. Mediastinal masses in most locations were better seen and evaluated in the transaxial plane. However, the relationship of subcarinal masses to the trachea, left atrium, and pulmonary artery was better appreciated in the sagittal plane. The relationship of hilar masses to hilar vessels, bronchi, and the mediastinum was usually better seen on transaxial images. Also, the relationship of paramediastinal masses to the mediastinum was difficult to evaluate with sagittal scans.     

MR imaging of blood vessels using three-dimensional reconstruction: methodology

Multisection, dual-echo magnetic resonance (MR) transaxial images of blood vessels contain both anatomic and qualitative information about flow. Even so, the images are produced as a series of two-dimensional tomographic sections from which full visualization of connected structures is difficult. A computer algorithm was developed that automatically detects flowing blood based on pixel intensity and calculated T2 and provides reconstructed views of vessels while analyzing and displaying flow characteristics. Images of abdominal vessels, aortic aneurysms, and the heart were encoded by flow and color to demonstrate depth. In addition, these data were reconstructed to derive a more accurate assessment of patency. With this technique, transaxial images can be used to analyze flow patterns, determine patent areas, and visualize all levels of vessels in a single image.     

ECG-synchronized cardiac MR imaging: method and evaluation

An electrocardiographic (ECG) sensing and gating device compatible with a 0.35-tesla (T) magnetic resonance (MR) imager has been developed and used to produce 802 MR images of the heart in 30 patients. The instrument consists of an isolated acquisition module, an electrically floating preamplifier, and a monitor gating module. Two spin-echo images were acquired for each of five, 0.7-cm thick, transaxial sections from the base to the apex of the heart during each ECG-synchronized imaging run. Image quality was assessed in a blind study by two investigators, on a scale from 0 to 3, as diagnostic [2-3] or nondiagnostic [0-1]. There was agreement in 91.4% of their assessments of diagnostic images (68.1% of the images studied). Resolution of heart anatomy on the MR images was adversely affected by prolonged spin-echo time delay, imaging in late diastole, image acquisition at the cardiac apex, irregular triggering, and artifacts. The synchronization of gradient pulses to the ECG at 0.35 T appears safe for patients, permits diagnostic resolution of images, allows image acquisition at distinct points during the cardiac cycle, and enables monitoring of patients during imaging.     

Perfusion magnetic resonance imaging of the lung: characterization of pneumonia and chronic obstructive pulmonary disease. A feasibility study

Perfusion magnetic resonance (MR) imaging is a promising new method for detection of perfusion defects in the diagnosis of pulmonary embolism. In the present study we evaluated the first-pass characteristics of perfusion MR imaging in patients with pneumonia or chronic obstructive pulmonary disease (COPD), frequent differential diagnoses to pulmonary embolism. Dynamic contrast-enhanced MR images of 12 patients with acute pneumonia and 13 patients with exacerbation of COPD were acquired in both the coronal and transaxial planes (an inversion recovery prepared gradient-echo sequence using 0.05 mmol/kg gadodiamide/injection). The MR images and the signal intensity (SI) versus time curves were characterized for each disease entity and compared with normal lung and the findings in pulmonary embolism from our previous study. The perfusion MR images of pneumonia showed distinct regions of increased contrast enhancement; in COPD with signs of emphysema (11 of the 13 COPD patients), the images showed a coarse pattern of reduced contrast enhancement. The SI versus time curves of pneumonia, COPD with signs of emphysema, and normal lung were statistically different, the respective pooled SI values (+/-95% CI) being as follows: mean baseline SI, 20.7 (1.1), 7.4 (0.4), and 8.5 (0.3); mean peak SI, no peak, 12.9 (1.5), and 27 (4.6); and mean max change of SI in percent, 110 (27), 79 (22), and 205 (52). Perfusion MR imaging of pneumonia and COPD with signs of emphysema showed first-pass that were characteristics promising for diagnostic use. Both the MR images and the SI versus time curves were different from the perfusion characteristics in normal lung and pulmonary embolism shown previously.     

MR imaging of normal rat brain at 0.35 T and correlated histology.

A custom-built small-animal transceiver was used for in vivo imaging of normal rat brain at 0.35 T, with the objective of identifying anatomic components by comparison of images with corresponding histologic sections. The cerebrum, cerebellum, brain stem, ventricles, hippocampus, and subarachnoid space were identified and cerebrospinal fluid (CSF) was differentiated from gray matter and white matter on coronal and transaxial magnetic resonance (MR) images. These images compare favorably with those obtained by others at higher field strengths in regard to delineating major neuroanatomic structures. It is concluded that this technique will be useful for investigating small-animal models of human neurologic disease involving morphologic and morphometric changes in gray matter, white matter, and CSF-filled spaces.     

Direct MR arthrography of plica synovialis mediopatellaris

Objective: To evaluate the efficacy of direct MR arthrography for diagnosing plica synovialis mediopatellaris (PSM).Material and Methods: Twenty-two knees of 19 patients were examined by direct MR arthrography. In 19 of those knees, field-echo T2*-weighted transaxial images were obtained with intra-articular administration of 40 ml of saline ("direct MR arthrography"). In the other 3 knees, T1-weighted transaxial images were obtained with intra-articular administration of 40 ml of gadopentetate dimeglumine. Conventional MR images were evaluated prior to direct MR arthrography in 12 of the 22 knees. The results of direct MR arthrography and conventional MR imaging were compared with the arthroscopic findings.Results: Direct MR arthrography demonstrated 17 of 19 PSMs proved on arthroscopy. The features of 16 PSMs on direct MR arthrography corresponded accurately with the arthroscopic classification. Conventional MR demonstrated only 3 plicae.Conclusion: Direct MR arthrography enabled accurate diagnosis of PSMs and could replace diagnostic arthroscopy.     

MR imaging of the normal sacroiliac joint with correlation to histology

Objective The microscopic study of the various components of joints provide a proper basis for understanding the nature of pathologic lesions to which they are subject and their imaging appearances. This study was designed to correlate MR imaging with a systematic histological study of the normal sacroiliac joint (SIJ), which to our knowledge is not available in the literature.Design and patients Five male cadavers, aged 20 to 45 years, and seven male and seven female volunteers, aged 23 to 44 years, were investigated with oblique transaxial and coronal MR imaging of the SIJs. A variety of sequences including pre- and post-contrast T1 fat-saturated studies in the volunteers were used. Cryosectioning was performed in six SIJs of the five cadavers and compared with the MR images for the microscopic joint anatomy and assessed for the presence of abnormalities resembling those associated with sacroiliitis.Results Throughout the SIJ, the hyaline cartilage of the sacral bone and the proximal third of the hyaline iliac cartilage was strongly attached to the surrounding stabilizing ligaments, forming wide margins of fibrocartilage. In the distal one-third of the joint only, the margins of the iliac joint facet resemble that of a synovial joint, which include an inner capsule with synovial cells. The MR anatomy of the ventral and dorsal aspects of the SIJ was only adequately visualized at oblique transaxial MR imaging. No contrast enhancement occurred in the synovial tissue or in the cartilaginous joint space. The dorsal transition between the proximal 2/3 and distal 1/3 of the cartilaginous joint was at microscopy rich in anatomical and histological variants, including osseous clefts, cartilage and subchondral defects, and vascular connective tissue in the bone marrow. These were all recognized at oblique transaxial MR imaging and in coronal MR sectioning may resemble abnormalities. Otherwise, no erosions, bone marrow abnormalities, bone sclerosis or abnormal contrast enhancement occurred in the normal joints.Conclusions The SIJ should be classified anatomically as a symphysis with some characteristics of a synovial joint being confined to the distal cartilaginous portion at the iliac side. Coronal MR imaging does not allow assessment of normal anatomy, variants or abnormalities of the ventral and dorsal margins of the cartilaginous SIJ.     

Magnetic resonance imaging of the hila and mediastinum

In the diagnosis of chest diseases, MR imaging has been shown to be most helpful in the evaluation of hilar and mediastinal lesions. The detection of intrathoracic abnormalities is technique-dependent, and imaging with both short and long repetition and echo times is necessary. ECG gating and sagittal or coronal images supplement transaxial scans in some patients. MR allows the diagnosis of mediastinal vascular lesions and the differentiation of mediastinal mass and vessel without contrast agents. It provides diagnostic information quite similar to that of CT in patients with mediastinal mass, but spatial resolution is somewhat poorer. In patients with hilar mass, MR is superior to CT in identifying the mass and distinguishing it from normal structures.     

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.     

Normal anatomy of the thoracic inlet as seen on transaxial MR images

The thoracic inlet can now be studied with high-resolution MR imaging. Recent advances in fold-over suppression (antialiasing software) allow for small fields of view without the usual problems of aliasing from the shoulders. This pictorial assay shows the normal anatomy that can be seen in this area on transaxial MR images. The vagus, phrenic, and recurrent laryngeal nerves can be seen as discrete entities. MR imaging can be used more often for pathologic conditions involving the lower portion of the neck and the thoracic inlet.     

MR evaluation of Chiari I malformations at 0.15 T

Twelve patients with known or presumed Chiari I malformations and two with clinical diagnoses of multiple sclerosis were examined by magnetic resonance (MR) imaging. MR confirmed or established the diagnosis of Chiari I malformation in all 14 cases. The spin-echo technique with a short time to echo (TE = 40 msec) and a short time to recover (TR = 1000 msec) provided optimum imaging of tonsillar position, hydromyelia cavities, and cervicomedullary "kinking." Long TE (greater than 80 msec) and TR (greater than 2000 msec) increase the signal intensity of cerebrospinal fluid and may obscure the pathology. Sagittal, transaxial, and coronal images provided complementary data; sagittal and coronal views best imaged the abnormal spinal cord and tonsils, but slitlike cavities were best seen on transaxial images. Cervicomedullary kinking was found in 10 (71%) of 14 patients and in 90% of the hydromyelic patients. This high incidence suggests that in other radiologic techniques tonsillar herniation masks the kinking. Symptoms of the Chiari I malformation overlap those of demyelinating diseases and brain tumors. Our early experience suggests MR is the preferred noninvasive procedure for identifying Chiari I malformation. Moreover, the ability to portray the variable cavity morphology of hydromyelia directly offers the potential for improved shunt placement.     

Potential of Gd-DTPA-mannan liposome particles as a pulmonary perfusion MRI contrast agent: an initial animal study

Suga K, Mikawa M, Ogasawara N, et al. Potential of Gd-DTPA-mannan liposome particles as a pulmonary perfusion MRI contrast agent: An initial animal study. Invest Radiol 2001;36:136-145.RATIONALE AND OBJECTIVES: A paramagnetic, particle-type MR contrast agent, (Gd-diethylenetriamine pentaacetic acid [DTPA]-mannan-cholesterol)-coated liposomes, was designed to localize in the lung by the mechanism of capillary blockade, and the potential of this agent for pulmonary perfusion MRI was experimentally investigated. METHODS: Before and up to 60 minutes after slow injection of this contrast agent, MR images were sequentially acquired at 10-second intervals along the same transaxial plane of the lung by using a gradient-echo pulse sequence with a short echo time of 1.2 ms on a 1.5-T MR scanner. After the minimal dose for obtaining a sufficient lung enhancement effect was determined in five rabbits, the time course of the enhancement effect was evaluated in six dogs by arterial blood gas analysis. The efficacy of MRI for detecting perfusion defects was evaluated in seven other dogs with pulmonary embolism. RESULTS: Normal lungs were dose-dependently enhanced by this agent, and with a 2.0 mL/kg dose, dependent lungs were enhanced by more than 201%, with an average half-life of the enhancement effect of 35.7 +/- 5.3 minutes. With less than this dose (1.0-1.5 mL/kg), all of the embolized lung portions were clearly identified as perfusion defects. The prolonged enhancement effect allowed the acquisition of subsequent multisectional lung images, thus facilitating the assessment of anatomic location and extent of the perfusion defects. The reduction of PaO2 in room air after injection was within 5 mm Hg in both normal and embolized animals. CONCLUSIONS: These initial, experimental results show that paramagnetically labeled liposome particles may be a successful MR contrast agent for pulmonary perfusion imaging.     

0.6 T magnetic resonance imaging of the orbit

Magnetic resonance (MR) imaging of the orbit was performed with a 0.6 T superconducting imaging system in 100 patients with normal orbits who were being evaluated for brain pathology and in 21 additional patients with a variety of orbital lesions to determine the efficacy of MR imaging in displaying orbital abnormalities. Usually, MR studies were performed using a multislice technique with multiple spin-echo pulse sequences and 30, 60, and 90 msec echo times and 500, 1500, and 2000 msec repetition times. Using section thicknesses of about 8 mm, imaging was performed in the transaxial, coronal, and sagittal projections. Pixel size was 0.9 X 1.8 mm, and the examination took about 30 min. The MR findings were compared with computed tomographic (CT) findings in all cases. Either combined axial and coronal studies of 5-mm-thick sections or a thin axial study of 1-mm-thick sections followed by reformatting techniques to obtain multiplanar images was used. Contrast enhancement was used in the CT studies. Both MR and CT clearly demonstrated the soft-tissue abnormality in all cases except two, in which MR failed to detect the abnormality. In one, MR failed to detect a small retrobulbar hemorrhage that occurred after a surgical procedure for retinal detachment. In the second case, rather extensive calcification in the posterior choroidal layers and lens was not detected by MR imaging. In several other cases, MR provided information beyond that obtained with CT. MR has the advantage of providing exquisite anatomic detail in multiplanar images, and it appears to be more sensitive than CT in detecting small, subacute and chronic hemorrhage within soft-tissue masses in the orbit and in detecting ischemia of the globe. CT is superior to MR imaging in portraying fine bone detail.     

Correction for vascular artifacts in cerebral blood flow values measured by using arterial spin tagging techniques

?Vascular”? artifacts can have substantial effects on human cerebral blood flow values calculated by using arterial spin tagging approaches. One vascular artifact arises from the contribution of ?tagged”? arterial water spins to the observed change in brain water MR signal. This artifact can be reduced if large bipolar gradients are used to ?crush”? the MR signal from moving arterial water spins. A second vascular artifact arises from relaxation of ?tagged”? arterial blood during transit from the tagging plane to the capillary exchange site in the imaging slice. This artifact can be corrected if the arterial transit times are measured by using ?dynamic”? spin tagging approaches. The mean transit time from the tagging plane to capillary exchange sites in a gray matter region of interest was calculated to be ～0.94 s. Cerebral blood flow values calculated for seven normal volunteers agree reasonably well with values calculated by using radioactive tracer approaches.     

Improving MR image quality in the presence of motion by using rephasing gradients

Numerous techniques exist for suppressing ghosting artifacts due to respiratory motion on MR images. Although such methods can remove coherent ghosting artifacts, motion during gradient pulses also leads to poor image quality. This is due to phase variations at the echo caused by changes in velocity from one phase-encoding view to the next. The effect becomes severe for long sampling times and long TE values and can lead to low estimates of T2. We discuss general, robust modifications of the standard gradient or spin-echo sequences by using rephasing gradients that force the phase of constant-velocity moving spins to be zero at the echo. These sequences lead to a significant reduction in motion artifacts and hence improvement in image quality. They can be applied to multislice, multiecho, water/fat, and gating schemes as well. Since motion problems are universal, it would appear that these modified sequences should come into common usage for MR imaging.     

Diagnosis of femoropopliteal venous thrombosis with MR imaging: a comparison of four MR pulse sequences

In a prospective study, MR images were evaluated in seven patients with femoropopliteal venous thrombosis with symptoms of less than 5 days duration. T1-weighted (600/25 [TR/TE]), intermediate (2000/30), and T2-weighted (2000/100) spin-echo series and a gradient-recalled acquisition in the steady state (GRASS) series were compared. Using venography as the standard for diagnosis, we found GRASS to be the most sensitive of the MR techniques, showing thrombi in all patients. It provided good contrast between the low-intensity thrombus and high-intensity flowing blood and also between thrombus and intermediate- or high-intensity perivascular tissues. The T1-weighted series was the least sensitive technique. All thrombi showed heterogeneity in the transaxial image with differences in signal between the peripheral and central regions. A higher intensity signal in the center than in the periphery at some level of the thrombus was found in six of seven T2-weighted or GRASS images. Heterogeneity in the signal intensity was more frequent in distal portions of thrombi, whereas the most proximal extent was homogeneous in appearance in six of seven cases. The heterogeneous appearance may be related to the greater age of the distal thrombus, because deep venous thrombi are known to begin in the calf and extend proximally over time. We conclude, on the basis of our experience with a small number of patients, that the GRASS MR technique is more sensitive for detecting acute deep venous thrombosis than T1-weighted, intermediate, and T2-weighted MR images.     

A positron-emission transaxial tomograph for nuclear imaging (PETT).

An apparatus was developed for obtaining emission transaxial images of sections of organs containing positron-emitting radiopharmaceuticals. The detection system is a hexagonal array of 24 NaI(T1) detectors connected to coincidence circuits to achieve the "electronic" collimation of annihilation photons. The image is formed by a computer-applied algorithm which provides quantitative reconstruction of the distribution of activity. Computer simulations, phantom and animal studies show that this approach is capable of providing images of better contrast and resolution than are obtained with scintillation cameras. Advantages of positron vs. single photon reconstruction tomography are discussed.