Dynamic contrast-enhanced T2*-weighted MR imaging of tumefactive demyelinating lesions.

PURPOSE: Dynamic contrast-enhanced T2*-weighted MR imaging has been helpful in characterizing intracranial mass lesions by providing information on vascularity. Tumefactive demyelinating lesions (TDLs) can mimic intracranial neoplasms on conventional MR images, can be difficult to diagnose, and often result in surgical biopsy for suspected tumor. The purpose of this study was to determine whether dynamic contrast-enhanced T2*-weighted MR imaging can be used to distinguish between TDLs and intracranial neoplasms that share common features on conventional MR images. METHODS: We retrospectively reviewed the conventional and dynamic contrast-enhanced T2*-weighted MR images and medical records of 10 patients with tumefactive demyelinating disease that was diagnosed by either biopsy or strong clinical suspicion supported by laboratory evaluation that included CSF analysis and evoked potential tests. Twelve TDLs in 10 patients and 11 brain tumors that appeared similar on conventional MR images were studied. Relative cerebral blood volume (rCBV) was calculated from dynamic MR data and was expressed as a ratio to contralateral normal white matter. rCBV values from 11 patients with intracranial neoplasms with very similar conventional MR imaging features were used for comparison. RESULTS: The rCBV values of TDLs ranged from 0.22 to 1.79 (n = 12), with a mean of 0.88 +/- 0.46 (SD). The rCBV values of intracranial neoplasms ranged from 1.55 to 19.20 (n = 11), with a mean of 6.47 +/- 6.52. The difference in rCBV values between the two groups was statistically significant (P =.009). The difference in rCBV values between TDLs and primary cerebral lymphomas (n = 4) was less pronounced but was statistically significant (P =.005). CONCLUSION: Dynamic contrast-enhanced T2*-weighted MR imaging is a useful diagnostic tool in differentiating TDLs from intracranial neoplasms and may therefore obviate unnecessary surgical biopsy.

Magnetic resonance carotid angiography of the neck. Clinical implications.

This article provides a brief review of the pertinent physics of MR angiography. Scanning strategies and common pitfalls are described. Clinical efficacy of MR angiography of the extracranial carotid arteries and the aortic arch are discussed.

Water excitation MPRAGE MRI of VII and VIII cranial nerves.

OBJECTIVE: Our goal was to compare magnetization prepared rapid gradient echo--water excitation (MRPRAGE-WE) with conventional spin echo (CSE) in the evaluation of the VII and VIII cranial nerves. METHODS: One hundred three consecutive patients with symptoms referable to the VII/VIII nerves were studied with CSE T1 and MPRAGE-WE following intravenous gadolinium contrast agent. Each right and left nerve pair was independently evaluated for the presence of an enhancing mass and for visualization of the nerves. RESULTS: On the CSE images, 26 definite and 2 possible lesions were identified, whereas 28 definite and 2 possible abnormalities were seen on the MPRAGE-WE. Four cases were better identified on the MPRAGE-WE and one better seen on the CSE. This difference was not statistically significant (p = 0.19). CSE demonstrated the nerves partially in 23 instances and completely in 6; MPRAGE-WE showed the nerves partially in 35 and completely in 73. This was highly significant (p<0.001). CONCLUSION: With equivalent or slightly improved lesion detection and better visualization of the nerves, MPRAGE-WE may replace CSE in studying the VII/VIII nerves.

MR angiography of the extracranial carotid arteries.

As described earlier, if MR angiography is to replace conventional angiography, the entire course of the carotid arteries should be imaged. De Marco et al describe initial success in imaging the entire circulation from the aortic arch through the circle of Willis, combining 2-D and 3-D TOF methods without gadolinium, electrocardiograph gating or breath holding. The technique requires 1 to 1.5 hours of imaging time, and accuracy in establishing tandem lesions is not yet established. It is likely that future refinements will allow for more widespread acceptance of MR angiography as a sometime alternative to conventional angiography.

MP-RAGE subtraction venography: a new technique.

Preliminary evaluation of a new magnetic resonance (MR) venography technique was performed with data sets from five patients undergoing MR imaging of the brain before and after intravenous administration of gadopentetate dimeglumine. Before contrast agent injection, the patients were imaged with MP-RAGE (magnetization-prepared rapid gradient-echo) and axial turbo T2-weighted sequences. After contrast agent injection, the MP-RAGE sequence was repeated. Images were post-processed with an algorithm that calculates, on a pixel-by-pixel basis, the absolute value of signal intensity of each postcontrast MP-RAGE partition minus that of each precontrast MP-RAGE partition. These subtracted partitions were then subjected to a standard maximum-intensity-projection algorithm to obtain the venogram. In all cases, the new method afforded a high-resolution venogram with clear depiction of venous sinus anatomy. Cortical venous anatomy was also clearly depicted.

High resolution GRASE MRI of the brain and spine: 512 and 1024 matrix imaging.

OBJECTIVE: Our goal was to implement 512 and 1024 matrix MRI of the brain in clinically acceptable imaging times. MATERIALS AND METHODS: With use of the GRASE (gradient-SE) imaging technique, 3 signals are refocused from each of 16 SEs, giving a total of 48 echoes per echo train, for a speed advantage of 1/48 over conventional SE imaging. Images in 1024 matrices are acquired in 2D Fourier transform (FT) multisectional MRI. In related experiments, multislab 3D FT GRASE imaging is performed using 16 partitions/slab, providing thinner 1 to 2 mm sections. In all imaging experiments, higher spatial resolution is obtained with stronger and faster gradients, 24 mT/m in 625 microseconds rise time, on a standard commercial MRI system. RESULTS: The 2D FT 1024 matrix images of the head were acquired in 4:20 min, with 20 sections and TR/TE 7040/115 ms in a rectangular FOV to obtain .28 x .27 mm2 spatial resolution. Small anatomic structures, including cochlea of inner ear, cranial nerves, and vascular detail, are readily demonstrated. The 512 matrix images were obtained in 4:40 min, with 16 sections and TR/TE 3,500/104 ms in a 24 cm FOV. The 3D FT technique substantially increased slice coverage as well as image signal-to-noise ratio. CONCLUSION: The results show that 1024 matrix MRI is technically feasible in clinically acceptable imaging time and offers advantage for high resolution imaging. Optimization of 1024 matrix and 3D FT GRASE imaging should improve the delineation of anatomic regions of interest.

Dual contrast GRASE (gradient-spin echo) imaging using mixed bandwidth.

Equal time spacing of RF pulses in the CPMG sequence imposes a constraint of equal signal read periods in spin-echo train imaging. GRASE imaging differs by using multiple read gradients in each pi-pi time interval, which are not constrained to be equal in number or duration. This additional degree of freedom is developed in dual contrast imaging. Closely spaced read periods are used for the PDW image to reduce T2 decay effects, while fewer low-bandwidth read periods in each of several pi-pi intervals are used to raise the signal-to-noise ratio and avoid signal averaging in the T2-weighted image.

Quantitative analysis of gadopentetate dimeglumine excreted in breast milk.

Specimens of breast milk were obtained from each breast in a lactating patient for a 24-hour period after a gadolinium-enhanced MR imaging examination was performed. A quantitative analysis of gadolinium content in specimens obtained at 2, 11, 17, and 24 hours after the intravenous administration of gadopentetate dimeglumine demonstrated that gadolinium is excreted in small amounts (total of 1.60 mumol during a 24-hour period) and in similar amounts from each breast. These results suggest that a waiting period of 24 hours, with active expression of breast milk from each breast, should provide a reasonable safety margin for allowing resumption of breast-feeding. Additional data will be required before establishing definitive recommendations.

Comparison of magnetic resonance angiography, conventional angiography, and duplex scanning.

BACKGROUND AND PURPOSE: To determine the accuracy of magnetic resonance angiography in assessing patients with cerebrovascular disease, we performed a study comparing the results of conventional cerebral angiography, duplex scanning, and magnetic resonance angiography. METHODS: From 42 patients, a total of 25 carotid arteries were evaluated by all three techniques. The studies were independently read and sorted into five categories according to the degree of stenosis: 0-15%, normal; 16-49%, mild; 50-79%, moderate; 80-99%, severe; and totally occluded. RESULTS: Magnetic resonance angiography correlated exactly with conventional angiography in 39 arteries (52%); duplex scanning correlated with conventional angiography in 49 cases (65%). Compared with conventional angiography, both magnetic resonance angiography and duplex scanning tended to overread the degree of stenosis. The most critical errors associated with magnetic resonance angiography were three readings of total occlusion in vessels found to be patent on conventional angiograms. CONCLUSIONS: Although magnetic resonance angiography offers great hope of providing high-quality imaging of the carotid artery with no risk and at less cost, data from this study suggest that misreading the degree of stenosis, or misinterpreting a stenosis for an occlusion, could lead to errors in clinical decisions. Guidelines for use of magnetic resonance angiography in a clinical setting are offered.

Screening protocol for MR imaging of the internal auditory canal.

We have previously shown that 5 mm axial T1-weighted images following Gd-diethylenetriamine pentaacetic acid (DTPA) administration were sufficient as a screening examination of the internal auditory canal (IAC) and cerebellopontine angle cistern in 80% of cases with sensorineural hearing loss. In the remaining 20% of cases 3 mm slices were necessary for confirmation of the diagnosis. To reduce the number of cases requiring the additional examination, we have modified our protocol to include a coronal "scout" T1-weighted sequence followed by 5 mm axial sections angled through the IACs. Routine axial T2-weighted images of the posterior fossa were also obtained. One hundred eleven patients were studied with this protocol. In all but two of these the 5 mm sections unequivocally provided the diagnosis and in those two cases the coronal images confirmed the diagnosis suspected on the axial study. Three millimeter slices were not necessary in any patient. This three sequence, post-Gd-DTPA protocol, which requires only 12 min of scan time, is recommended as a screening protocol for IAC and cerebellopontine angle disease.

Important considerations in measurement of attractive force on metallic implants in MR imagers.

Almost all reported measurements of the attractive force exerted on metallic implants in magnetic fields of magnetic resonance imagers have been obtained at the portal. This study was done to determine whether the maximum force on an implant is at the portal. Horizontal attractive forces on needles were measured along the axis of the bore at various distances from the portal of a 0.3-T vertical-field magnet and 0.5- and 1.5-T solenoid magnets. Upward vertical forces along the top of the bore were also measured. The horizontal forces 40 cm in from the portals of the 0.5- and 1.5-T magnets were 70%-80% greater than the horizontal forces at the portal. The upward vertical force at the top of the 0.3-T magnet bore, 63 cm in from the portal, was 20 times the horizontal force at the portal and was twice the maximum force measured in the 0.5-T magnet. The maximum force was not at the portal.

Diagnosis of carotid artery stenosis: comparison of 2DFT time-of-flight MR angiography with contrast angiography in 50 patients.

Fifty patients underwent 2DFT time-of-flight MR angiography and intraarterial contrast angiography for evaluation of possible carotid atherosclerotic disease. The MR angiography technique employed contiguous axial flow-sensitive (short TR/TE) slices that were reformatted and postprocessed by using a maximum-intensity projection algorithm to provide 16 angiographic views of the carotid arteries. Both studies were independently reviewed by two observers in a blinded manner. Carotid arteries were categorized as normal, mildly stenotic, moderately stenotic, severely stenotic, or occluded. For the 94 carotid arteries available for review, one observer reported a 70% agreement between the two techniques and the second observer reported a 56% agreement (p = .0001). The best correlation was in the severely stenotic category and the worst was in the occluded category. Agreement between observers was 67% for MR angiography and 72% for contrast angiography, which was similar to that between the two techniques. Although not all carotid atherosclerotic disease was visualized equally well, 2DFT time-of-flight MR angiography had a good overall correlation with the "gold standard" of intraarterial contrast angiography, supporting its use as a screening technique. While further improvements are needed, use of MR angiography as the primary diagnostic tool for many patients with suspected carotid stenosis should continue to increase.

Role of slice thickness in MR imaging of the internal auditory canal.

The relative efficacy of post-Gd-DTPA 5 and 3 mm axial T1-weighted images was compared in the detection of lesions in the internal auditory canal and cerebellopontine angle. One hundred twenty consecutive patients were prospectively evaluated with 5 mm axial T1-weighted slices. If these were negative or questionable. 3 mm axial slices were immediately obtained as the next sequence. Eighteen percent of cases were positive and in none of these was the 5 mm study normal. However, in 22 negative cases and two positive cases, a 3 mm study was necessary for confirmation. Five millimeter axial scanning post Gd-DTPA is recommended as the initial study for detection of masses in the internal auditory canal and cerebellopontine angle. Because this study requires fewer acquisitions than 3 mm sections and can be done satisfactorily on low and midfield systems, there are potential time- and cost-saving benefits to this approach.

Dural "tail" associated with meningiomas on Gd-DTPA-enhanced MR images: characteristics, differential diagnostic value, and possible implications for treatment.

Gadolinium-enhanced magnetic resonance (MR) imaging studies of 30 histologically proved cranial meningiomas revealed a linear enhanced structure or "tail" extending away from the tumor mass along the dural surface in 18 cases (60%). Contrast material-enhanced computed tomographic studies available in 10 of these 18 cases did not depict this structure. Characteristic features of this MR finding were reviewed, and criteria were defined to distinguish this tail from other enhanced structures. To assess the differential diagnostic value of this finding, gadolinium diethylenetriaminepentaacetic acid-enhanced MR studies of a control group of other extraaxial lesions as well as superficial intraaxial tumors that abut the meninges were reviewed. These cases failed to show this MR feature. Pathologic correlation was available in three meningiomas with the finding of a tail, and these demonstrated meningothelial tumor nodules in the samples taken from areas corresponding to the enhanced regions on MR images. This is in agreement with other recent pathologic studies of the dura mater surrounding meningiomas. The authors believe that the linear enhanced structure described may represent tissue containing tumoral nodules. Awareness of this MR sign may be useful in distinguishing meningiomas from other lesions and in planning total resection of the infiltrated dura mater.

Fluid within preexisting pulmonary air-spaces: a potential pitfall in the CT differentiation of pleural from parenchymal disease.

There have been many reports of the ability of CT to distinguish between parenchymal and pleural disease. The purpose of this report is to describe the appearance of seven cases of intraparenchymal fluid-filled air-spaces (bullae or lung cysts) in which the CT findings may resemble those of pleural disease and, thus, cast doubt on the specificity of the established criteria.