Quantitative Contrast-Enhanced MR Imaging of the Optic Nerve

During the acute stages of optic neuritis damage to the blood-optic nerve barrier can be detected using i.v. paramagnetic contrast-enhanced MR imaging. Quantification of the enhancement pattern of the optic nerve, intraorbital fat and muscle was determined in 15 normal subjects using 3 fat-suppression MR imaging methods: T1-weighted spin-echo and spoiled gradient-echo sequences preceded by a fat-frequency selective pulse (FATSAT + SE and FATSAT + SPGR, respectively) and a pulse sequence combining CHOPPER fat suppression with a fat-frequency selective preparation pulse (HYBRID). Pre- and postcontrast-enhanced studies were acquired for FATSAT + SE and FATSAT + SPGR. There was no significant enhancement of the optic nerve by either method (mean increase of 0.96% and 5.3%, respectively), while there was significant enhancement in muscle (mean 118.2% and 108.2%), respectively; p < 0.005) and fat (mean increase of 13% and 37%, respectively; p < 0.05). Postcontrast optic nerve/muscle signal intensity ratios (mean, SD) were 0.51 (0.07), 0.58 (0.05) and 0.75 (0.05) for FATSAT + SE, FATSAT + SPGR and HYBRID, respectively. These results suggest a practical methodology and range of values for normal signal intensity increases and ratios of tissue signal that can be used as objective measures of optic neuritis for natural history studies and treatment trials.

Comparison of 3 T MRI and CT for the measurement of visceral and subcutaneous adipose tissue in humans

OBJECTIVE

CT is considered the gold standard imaging modality for measurement of visceral adipose tissue area. However, as CT imaging exposes subjects to ionising radiation, a comparable imaging technique without this exposure is desirable, such as MRI. Therefore, we compared the agreement of measures of visceral adipose tissue and subcutaneous adipose tissue area from single-slice images obtained at the umbilicus using a 3 T MRI scanner with single-slice images obtained via CT scan.

METHODS

64 images were obtained from 27 subjects who underwent MRI and CT scanning on the same day, after 10-12 hours of fasting. Visceral and subcutaneous adipose tissue depots were manually separated and quantified using a multimodality image-processing software program.

RESULTS

We found good agreement between CT and MRI for the measurement of both visceral adipose tissue and subcutaneous adipose tissue. Bland-Altman difference analysis demonstrated a mean bias of -2.9% (as a portion of total abdominal area) for visceral adipose tissue and +0.4% for subcutaneous adipose tissue, as measured by MRI compared with CT.

CONCLUSION

MRI is a safe, accurate and precise imaging modality for measuring both visceral and subcutaneous adipose tissue, making it a favourable alternative to CT for quantification of these adipose depots.

Temporal lobe magnetic resonance spectroscopic imaging following selective amygdalohippocampectomy for treatment-resistant epilepsy

OBJECTIVES

Magnetic resonance spectroscopic imaging (MRSI) may show circumscribed or extensive decreased brain N-acetyl aspartate (NAA)/creatine and phosphocreatine (Cr) in epilepsy patients. We compared temporal lobe MRSI in patients seizure-free (SzF) or with persistent seizures (PSz) following selective amygdalohippocampectomy (SAH) for medically intractable mesial temporal lobe epilepsy (mTLE). We hypothesized that PSz patients had more extensive temporal lobe metabolite abnormalities than SzF patients.

MATERIALS AND METHODS

MRSI was used to study six regions of interest (ROI) in the bilateral medial and lateral temporal lobes in 14 mTLE patients following SAH and 11 controls.

RESULTS

PSz patients had more temporal lobe ROI with abnormally low NAA/Cr than SzF patients, including the unoperated hippocampus and ipsilateral lateral temporal lobe.

CONCLUSION

Postoperative temporal lobe MRSI abnormalities are more extensive if surgical outcome following SAH is poor. MRSI may be a useful tool to improve selection of appropriate candidates for SAH by identifying patients requiring more intensive investigation prior to epilepsy surgery. Future prospective studies are needed to evaluate the utility of MRSI, a predictor of successful outcome following SAH.

Chemical shift: the artifact and clinical tool revisited

The chemical shift phenomenon refers to the signal intensity alterations seen in magnetic resonance (MR) imaging that result from the inherent differences in the resonant frequencies of precessing protons. Chemical shift was first recognized as a misregistration artifact of image data. More recently, however, chemical shift has been recognized as a useful diagnostic tool. By exploiting inherent differences in resonant frequencies of lipid and water, fatty elements within tissue can be confirmed with dedicated chemical shift MR pulse sequences. Alternatively, the recognition of chemical shift on images obtained with standard MR pulse sequences may corroborate the diagnosis of lesions with substantial fatty elements. Chemical shift can aid in the diagnosis of lipid-containing lesions of the brain (lipoma, dermoid, and teratoma) or the body (adrenal adenoma, focal fat within the liver, and angiomyolipoma). In addition, chemical shift can be implemented to accentuate visceral margins (e.g., kidney and liver).

Aortic and lower-extremity arterial disease: evaluation with MR angiography versus conventional angiography

PURPOSE

To compare magnetic resonance (MR) angiography with conventional angiography in evaluation of the aorta and lower-extremity arterial system.

MATERIALS AND METHODS

Fifty-seven patients were evaluated with femoral conventional and MR angiography. Iliac artery segments were evaluated with two-dimensional and contrast material-enhanced three-dimensional time-of-flight MR angiography. Infrainguinal regions were evaluated with two-dimensional time-of-flight MR angiography with a dedicated lower-extremity coil. Arteries depicted on femoral images were separately interpreted as 20 anatomic segments. Disease classification included normal to moderate disease (0%-50% stenosis), severe stenosis (> 50% stenosis), diffuse disease (more than one severe stenosis), and occlusion. Four readers interpreted the images and rendered treatment recommendations.

RESULTS

Substantial to almost perfect interobserver agreement (kappa, 0.66-1.00) was achieved in most cases for MR angiogram interpretation. The three most experienced readers achieved substantial to almost perfect intraobserver agreement (kappa, 0.61-1.00) between conventional and MR angiogram interpretation in most cases. Among three readers, moderate agreement (kappa, 0.43-0.53) was found between treatment recommendations based on conventional versus MR angiographic findings; for the most experienced reader, this agreement was almost perfect (kappa, 0.90).

CONCLUSION

For experienced readers, there was substantial to almost perfect agreement between conventional and MR angiographic image interpretations of the aorta and lower-extremity arterial system.

Two-point Dixon technique for water-fat signal decomposition with B0 inhomogeneity correction

To separate water and lipid resonance signals by phase-sensitive MRI, a two-point Dixon (2PD) reconstruction is presented in which phase-unwrapping is used to obtain an inhomogeneity map based on only in-phase and out-of-phase image data. Two relaxation-weighted images, a "water image" and a "fat image," representing a two-resonance peak model of proton density, are output. The method is designed for T1- or density-weighted spin-echo imaging; a double-echo scheme is more appropriate for T2-weighted spin-echo imaging. The technique is more time-efficient for clinical fat-water imaging than 3PD schemes, while still correcting for field inhomogeneity.

Alteration of ascending thoracic aorta compliance after treatment with menotropin

OBJECTIVE

Our purpose was to determine whether aortic size and compliance are altered by an exogenously induced rise in estrogen.

STUDY DESIGN

Magnetic resonance imaging was used to determine the aortic cross-sectional area/aortic pressure relationship in nine premenopausal women before and after menotropin therapy. Simultaneous electrocardiograms, carotid pulse tracings, phonocardiograms, and brachial artery pressures were obtained before each magnetic resonance imaging acquisition. Ascending thoracic aorta cross-sectional area was obtained every 32 msec and aligned with brachial artery pressures extrapolated from the carotid pulse tracing, allowing construction of the ascending thoracic aorta cross-sectional area/aortic pressure relationships. Aortic cross-sectional area was normalized to body surface area, and the shifts in the position for the ascending thoracic aorta cross-sectional area/aortic pressure relationship were determined with use of analysis of covariance.

RESULTS

Heart rate and aortic pressure were unchanged before and after menotropin treatment. Initial estradiol levels were < 20 pg/ml. After menotropin treatment (7.4 +/- 1.0 days) estradiol levels rose to 905 +/- 371 pg/ml (p < 0.0001). Ascending thoracic aorta cross-sectional area/body surface area was not significantly increased, adjusted y mean of 389 +/- 7 mm2/m2 before and 403 +/- 7 mm2/m2 after menotropin treatment (p < 0.24). The slope of the ascending aorta cross-sectional area/aortic pressure relationship, an index of aortic compliance, increased from 1.4 +/- 0.6 mm2/m2/mm Hg before to 1.7 +/- 0.6 mm2/m2/mm Hg after menotropin treatment (p < 0.001).

CONCLUSION

In premenopausal women a short-term rise in estrogen induced by menotropin treatment is associated with an increase in aortic compliance. Aorta size is not significantly increased within this time frame.

Evaluation of the iliac arteries: comparison of two-dimensional time of flight magnetic resonance angiography with cardiac compensated fast gradient recalled echo and contrast-enhanced three-dimensional time of flight magnetic resonance angiography

We compared dynamic contrast-enhanced three-dimensional time of flight (3DTOF) magnetic resonance angiography (MRA) with two-dimensional time of flight (2DTOF) MRA with cardiac compensated fast gradient recalled echo (C-MON) and conventional angiography (CA) when it was available. C-MON re-orders the normal data acquisition to minimize ghosting artifacts generated by pulsatile flow. The initial phase of the study involved optimization of parameters and comparison C-MON with no C = MON in eight patients and volunteers. The final phase of the study involved 53 patients who were imaged with contrast-enhanced 3DTOF MRA and 2DTOF MRA with C-MON. Thirty of these patients also had CA. In the initial phase, 2DTOF MRA with C-MON was found to be equal (n = 3) or superior (n = 5) to 2DTOF without C-MON. In the final phase, the agreement among all imaging modalities varied from substantial to almost perfect (Cohen's kappa = .6-.83). The lowest agreement was using 2DTOF to evaluate the external iliac segments. The among suggested treatments varied from substantial to almost perfect for all imaging modalities (Cohen's kappa = .73-93). The diagnostic efficacies of 2DTOF with C-MON and contrast-enhanced 3DTOF were high overall, with the lowest value being a specificity of 63% for one reader in the evaluation of an external iliac segment using 2DTOF. In summary, 2DTOF with C-MON helped to eliminate artifacts due to pulsatility in the iliac arterial segments. In our experience, both dynamic contrast-enhanced 3DTOF MRA and 2DTOF MRA with C-MON performed well in the evaluation of the iliac arteries. Both studies have high interobeserver agreement and high diagnostic efficacy. Contrast-enhanced 3DTOF MRA should be reserved for situations in which the iliac vessels are extremely tortuous or occluded or the external iliac segments are poorly seen.

MR imaging of silicone breast implants: evaluation of prospective and retrospective interpretations and interobserver agreement

MR imaging was used to evaluate the integrity of silicone breast implants in 54 women with 108 implants. MR images were interpreted by relatively inexperienced readers who tried to reproduce the experiences reported in the literature. The study examines the interobserver agreement using different diagnostic signs and the influence of experience on interpretation errors. Prospective and retrospective interpretations were compared with surgical findings at the time of explanation. Diagnostic indicators, including the linguine sign, the inverted tear drop sign, the C sign, water droplets mixed with silicone, and extracapsular globules of silicone, were evaluated for diagnostic efficacy and interobserver agreement. The prospective sensitivity and specificity were 87% and 78%, respectively. With the retrospective interpretations, the sensitivity and specificity increased to 93% and 92%, respectively. Most of the prospective false-positive interpretations were due to misinterpreting radial folds as signs of implant rupture. Six implants interpreted retrospectively as false positives had gross amounts of silicone around the implants at surgery but there were no obvious rents in the implant shells. There was fair to excellent interobserver agreement with the individual diagnostic signs except for extracapsular globules of silicone. All of the signs had specificities of greater than 90%. The sensitivities of the individual signs were less than the overall retrospective sensitivity. With experience, the sensitivity improved from 87% to 93% and the specificity improved from 78% to 92%. This study helps substantiate the use of diagnostic signs used by other authors to detect silicone loss from breast implants by MR imaging; however, questions remain as to the clinical role of MR imaging in evaluating implants for silicone loss.

Coronal fat suppression fast spin echo images of the knee: evaluation of 202 patients with arthroscopic correlation

We optimized fat-suppressed fast spin echo (FS-FSE) parameters for coronal imaging of the knee and then evaluated the technique in a clinical setting. Five volunteers and 12 patients were used to evaluate various repetition (TR), echo (TE), and echo train lengths (ETL). Then, 202 patients underwent both knee MR imaging using coronal plane FS-FSE and arthroscopy. The coronal FS-FSE images were compared with radial multiplanar gradient-recalled echo (MPGRE), axial T1-weighted SE, and parasagittal double echo SE images. Proton density images (2,000/19) with an ETL of 2 best depicted the menisci, ligaments, and capsules. The conspicuity of osteochondral abnormalities depicted by the coronal FS-FSE imaging was significantly higher than for axial T1-weighted SE (p < .003) and parasagittal echo SE images (p < .003). The accuracy of the coronal FS-FSE imaging for medial and lateral meniscal tears was 91.6% and 87.6%, respectively. Combined imaging interpretation of the coronal FS-FSE, axial T1-weighted SE, and radial MPGRE imaging improved the accuracy for meniscal tears slightly over any sequence used alone, but the difference was not statistically significant. Fourteen capsular injuries were demonstrated by the coronal FS-FSE imaging. FS-FSE imaging in the coronal plane is a useful complementary sequence in MR examinations of the knee for the evaluation of meniscal tears, capsular injuries, and osteochondral abnormalities.

Lower extremities: MR angiography with a unilateral telescopic phased-array coil

In a comparison of conventional and magnetic resonance (MR) angiography of the lower extremities, MR imaging was performed with extremity and body coils and a unilateral six-coil telescopic phased-array coil. In one of seven volunteers, average signal-to-noise ratio was 140% higher with the latter (p < .01), only three of five stations were necessary, and examination time was decreased 50%. In the four patients, correlation in findings was 100%.

Detection of discrete white matter lesions after irreversible compression of MR images

PURPOSE

To validate the use of techniques of irreversible compression of images, which can be performed using a block-based discrete cosine transform technique as defined by the Joint Photographic Experts Group, before they can be used in clinical applications, by evaluating the effect of compression on the ability of observers to detect discrete white matter lesions on MR images of the brain.

METHODS

Sixty T2- and intermediate-weighted spin-echo images were compressed with varying degrees of coefficient quantization with compression ratios from 1:1 to more than 40:1, randomized, and evaluated by three observers blinded to the degree of compression.

RESULTS

No significant difference in the number of lesions detected was apparent until compression ratios reached 40:1, despite a significant subjective loss in perceived image quality at 20:1. Only small (< or = 5 mm) lesions were missed at the highest degree of compression. No significant differences were observed in the detection of confluent periventricular white matter disease at any degree of compression tested.

CONCLUSIONS

The use of high degrees of irreversible compression of MR images may be acceptable for diagnostic tasks.

Double-echo three-point-Dixon method for fat suppression MRI

A double-echo two-excitation pulse sequence encoding fat and water signals for a phase-sensitive three-point Dixon-type analysis (DE-3PD) was developed and implemented on a 1.5 T MR imager. Data processing was performed using a previously developed two-dimensional (2D) region-growing algorithm, adapted to use double-echo data. Density-, T1-, and T2-weighted fat suppression images were obtained from six volunteers using the new fat suppression method. The images were compared with corresponding images obtained using frequency-selective excitation fat suppression (FATSAT) and a single-echo three-point-Dixon method (SE-3PD). The results demonstrate that the DE-3PD sequence shortens the imaging time by one-third compared with the SE-3PD method, without loss in image quality. The data also show that a 2D region-growing algorithm effectively unwraps the phase of DE-3PD data sets, and that results of DE-3PD fat signal suppression are consistently better than those obtained using a standard FATSAT method. The authors conclude that the double-echo sequence provides density-, T1-, and T2-weighted images that appear to be promising for routine clinical applications.

Evaluation of the combination of vinblastine and quinidine in patients with metastatic renal cell carcinoma. A phase I study

Quinidine is known to inhibit p-glycoprotein and enhance the activity of vinblastine against cultured renal carcinoma cells. We have combined quinidine and vinblastine in a Phase I trial in patients with metastatic renal cell carcinoma. Twenty-three patients were entered. Prior treatment included nephrectomy (15 patients), radiation (1 patient) and interferons (8 patients). Cohorts of patients were treated at one of three quinidine dose levels (100, 200, and 400 mg); one patient received 300 mg. Quinidine was given orally 4 times daily starting 3 days prior to the first dose of vinblastine of 5 mg/m2 intravenously given once a week. Hematologic parameters, EKG, and quinidine levels were monitored weekly. Mean quinidine levels in each dose tier were 1.58, 2.59, and 4.24 micrograms/ml, respectively. The dose-limiting toxicity was leukopenia, which necessitated dose interruptions in 16 patients. The mean nadir WBC count (x 10(9)/L) was 3.47, 2.3, and 1.73 in each dose tier, respectively. Corresponding values for the mean maximum decrease in WBC count from baseline were 3.85, 5.86, and 6.53, respectively. There was a trend for leukopenia to become more severe with increasing doses of quinidine. Other toxicities included mild nausea and vomiting in all patients, and hypotension and paralytic ileus in one patient each. No cardiac toxicity was observed. One patient had a complete remission and 4 patients had stable disease. We conclude that quinidine and vinblastine may be administered together safely in a clinical setting, with leukopenia being dose-limiting. Further studies are needed to determine any therapeutic advantage over vinblastine alone.

Rotator cuff tendon tears: evaluation with fat-suppressed MR imaging with arthroscopic correlation in 100 patients

PURPOSE

To evaluate the diagnostic accuracy of fat-suppressed magnetic resonance (MR) imaging of rotator cuff tears in a large symptomatic population.

MATERIALS AND METHODS

One hundred patients underwent both MR imaging and arthroscopy of the shoulder. Ninety-two patients underwent fat-suppressed conventional spin-echo MR imaging (repetition time msec/echo time msec = 2,500/20, 60), and eight patients underwent fat-suppressed, fast spin-echo MR imaging (2,000/80).

RESULTS

With data combined for complete and partial tears of the rotator cuff (n = 31), MR imaging had an accuracy of 93%; sensitivity, 84%; and specificity, 97%. Seventeen of 20 complete tears and nine of 11 partial tears were properly identified with MR imaging. Two partial tears were not detected and three complete tears were incorrectly called partial tears at MR imaging. Of two false-positive MR imaging findings, one was called a complete tear and the other, a partial tear.

CONCLUSION

Fat-suppressed MR imaging has high diagnostic accuracy in evaluating tears of the rotator cuff tendon.

Differential permeability and quantitative MR imaging of a human lung carcinoma brain xenograft in the nude rat

This study characterized agent differential permeability, three-dimensional tumor volume, and survival in an LX-1 human small cell lung carcinoma intracerebral xenograft model in the nude rat. The percent accessible tissue space (distribution volume) and the permeability x capillary surface product for aminoisobutyric acid (M(r) 103), methotrexate (M(r) 454), dextran 10 (M(r) 10,000), and dextran 70 (M(r) 70,000) were measured between 8 and 16 days after inoculation of tumor. Magnetic resonance imaging and histology were used to quantitate intracerebral tumor volume (mm3). Accessible tissue space (ml/g) and permeability x capillary surface product in intracranial tumor, surrounding brain, and subcutaneous tumor decreased with increasing molecular weight of the agent, regardless of the number of days after inoculation. Accessible tissue space in intracranial tumor increased between 8 and 16 days for all agents except dextran 70. There was little change in the subcutaneous tumor or other tissues with time. Tumor volume calculations from imaging studies correlated with volumetric measurements from histological sections (r2 = 98.5%) and illustrated natural tumor progression (9 to 225 mm3). These results provide a basis for therapeutic design based on differential permeability of specific agents and the ability to quantitatively measure brain tumor volume for accessing tumor response.

Quantitative contrast-enhanced MR imaging of the optic nerve

During the acute stages of optic neuritis damage to the blood-optic nerve barrier can be detected using i.v. paramagnetic contrast-enhanced MR imaging. Quantification of the enhancement pattern of the optic nerve, intraorbital fat and muscle was determined in 15 normal subjects using 3 fat-suppression MR imaging methods: T1-weighted spin-echo and spoiled gradient-echo sequences preceded by a fat-frequency selective pulse (FATSAT+SE and FATSAT+SPGR, respectively) and a pulse sequence combining CHOPPER fat suppression with a fat-frequency selective preparation pulse (HYBRID). Pre- and postcontrast-enhanced studies were acquired for FATSAT+SE and FATSAT+SPGR. There was no significant enhancement of the optic nerve by either method (mean increase of 0.96% and 5.3%, respectively), while there was significant enhancement in muscle (mean 118.2% and 108.2%, respectively; p < 0.005) and fat (mean increase of 13% and 37%, respectively; p < 0.05). Postcontrast optic nerve/muscle signal intensity ratios (mean, SD) were 0.51 (0.07), 0.58 (0.05) and 0.75 (0.05) for FATSAT+SE, FATSAT+SPGR and HYBRID, respectively. These results suggest a practical methodology and range of values for normal signal intensity increases and ratios of tissue signal that can be used as objective measures of optic neuritis for natural history studies and treatment trials.

Chondromalacia patellae: fat-suppressed MR imaging

PURPOSE

To evaluate the accuracy of fat-suppressed magnetic resonance (MR) imaging in diagnosing chondromalacia patellae.

MATERIALS AND METHODS

Seventy-one patients underwent fat-suppressed MR imaging and arthroscopy of the patellofemoral compartment. Findings were classified as early or advanced chondromalacia or as normal and were correlated with arthroscopic findings.

RESULTS

Early and advanced stages of chondromalacia patellae were reliably detected, with positive predictive values of 85% and 92%, respectively. Specificity in early stages was 94% and in late stages was 98%. However, the overall accuracies did not differ substantially from those reported in studies that did not use fat-suppressed imaging.

CONCLUSION

Axial, fat-suppressed MR imaging accurately depicts changes caused by chondromalacia patellae. Early stages can be seen as intrasubstance changes of increased signal intensity. Results of this study suggest a high degree of specificity in excluding both early and advanced changes.

Effects of Gd-DTPA after osmotic BBB disruption in a rodent model: toxicity and MR findings

OBJECTIVE

This experiment was done to evaluate the gross neurotoxicity of intravenous Gd-DTPA administered in conjunction with osmotic blood-brain barrier (BBB) disruption and to image a human small cell lung carcinoma intracerebral tumor xenograft before and after osmotic BBB disruption.

MATERIALS AND METHODS

Neurotoxicity studies were performed in normal Sprague-Dawley rats following osmotic BBB disruption by the injection of 25% mannitol in the right internal carotid artery and intravenous administration of Gd-DTPA (n = 10). Animals were observed for major neurologic changes such as seizure or substantial motor defects, and after death neuropathologic examination was performed. Human small cell lung carcinoma cells were implanted intracerebrally in athymic nude rats (n = 4). Gadopentetate dimeglumine was injected intravenously and serial T1-weighted images were obtained. Blood-brain barrier disruption was produced in each animal, followed by a second dose of intravenous Gd-DTPA, and imaging studies were repeated.

RESULTS

No gross neurologic toxicity was observed. Tumors showed dense enhancement in a small area, and BBB disruption resulted in marked enhancement in most of the gray matter of the right cerebral hemisphere.

CONCLUSION

Gadopentetate dimeglumine appears to be safe in doses up to 21 mmol/m2 in conjunction with barrier disruption in rats. A human small cell lung carcinoma intracerebral xenograft provides a useful method to study brain tumors.

Phase unwrapping in the three-point Dixon method for fat suppression MR imaging

PURPOSE

TO present an algorithm for performing phase unwrapping for fat and water signal separation in the three-point Dixon (3PD) method of magnetic resonance (MR) imaging.

MATERIALS AND METHODS

The algorithm is based on a two-dimensional region-growing approach, which tracks phase evolution in 3PD images and unwraps the phase when a 2 pi jump is detected. The unwrapped phase data are consecutively used to calculate fat and water signal components on a per-pixel basis. The method was tested in eight volunteers.

RESULTS

The algorithm creates four output images: standard spin-echo, fat suppression, water suppression, and phase map images. This method corrects for the effects of magnetic field inhomogeneities and provides spatially uniform fat signal suppression superior to that achievable with the standard method.

CONCLUSION

This method of 3PD data reconstruction appears to be promising for routine application of fat suppression MR imaging in a clinical setting.

Delivery of virus-sized iron oxide particles to rodent CNS neurons

Delivery of viral particles to the brain is limited by the volume of distribution that can be obtained. Additionally, there is currently no way to non-invasively monitor the distribution of virus following delivery to the central nervous system (CNS). To examine the delivery of virus-sized particles across the blood-brain barrier (BBB), dextran coated, superparamagnetic monocrystalline iron oxide particles, with a hydrodynamic diameter of 20 +/- 4 nm, were delivered to rat brain by direct intracerebral inoculation or by osmotic BBB disruption with hypertonic mannitol. Delivery of these particles was documented by magnetic resonance (MR) imaging and, unexpectedly, neuronal uptake was demonstrated by histochemical staining. Electron microscopy (EM) confirmed iron particle delivery across the capillary basement membrane and localization within CNS parenchymal cells following administration with BBB disruption. This is the first histologic and ultrastructural documentation of the delivery of particles the size of virions across the blood-brain barrier. Additionally, these dextran-coated, iron oxide particles may be useful, in and of themselves, as vectors for diagnostic and/or therapeutic interventions directed at the CNS.

MR imaging of prostate cancer with an endorectal surface coil technique: correlation with whole-mount specimens

PURPOSE

To evaluate the diagnostic accuracy of magnetic resonance (MR) imaging in staging prostate cancer with an endorectal surface coil technique.

MATERIALS AND METHODS

The authors prospectively evaluated MR images obtained with an endorectal surface coil from 70 consecutive patients with known prostate cancer. Gadopentetate dimeglumine was administered to 40 patients. Multiple sequences were used, including conventional and fast spin echo, with and without fat suppression. The readers were blinded to the MR findings unless bone or nodal metastasis was present. MR images were compared with whole-mount sections.

RESULTS

The prospective staging accuracy for MR imaging was 51% (36 of 70 patients). Stage B disease was present in 27 patients (38%), stage C in 42 (60%), and stage D in one (1%). The retrospective staging accuracy was 67% (47 of 70 patients). Of the 42 patients with stage C disease, positive surgical margins were present in 36 (85%). Gadopentetate dimeglumine did not help detect or stage tumors.

CONCLUSION

Further studies must be performed to determine the role of endorectal coil MR imaging in the staging of prostate cancer.

Femoral MR angiography versus conventional angiography: preliminary results

PURPOSE

To develop a bilateral femoral magnetic resonance (MR) angiographic examination that would include the aortic bifurcation to the ankle.

MATERIALS AND METHODS

Thirty-seven patients underwent conventional angiography and the bilateral femoral MR angiographic examination. Two-dimensional time-of-flight angiography was used for all studies.

RESULTS

The femoral MR angiogram could have replaced the conventional angiogram in 57% of patients (21 of 37). In 43% of patients (16 of 37), the femoral MR angiogram could not have replaced the conventional angiogram. Reasons for diagnostic failure with femoral MR angiography included artifact from vascular clips (n = 8) or prosthetic joints (n = 3), overlooked distal aortic stenosis (n = 1), and suboptimal definition of vessels (usually trifurcation vessels) owing to various causes (n = 8). Seven of the patients in this group had bypass grafts.

CONCLUSION

Some of the current limitations of femoral MR angiography could be avoided by supplemental duplex sonography around prostheses and vascular clips. The other limitations will require advances in MR imaging techniques.

Menisci of the knee: radial MR imaging correlated with arthroscopy in 259 patients

Similar to arthrograms, radial plane magnetic resonance (MR) images of the knee provide cross-sectional images of the menisci that are perpendicular to the long axis of the meniscus. The authors evaluated 259 consecutive patients who underwent MR imaging of the knee and arthroscopy. Radial multiplanar gradient-recalled-echo imaging was performed--with repetition time of 700 msec, echo times of 12 and 31 msec (700/12, 31), and a flip angle of 20 degrees--as well as sagittal spin-echo imaging (2,500/20, 80). The radial and sagittal images were interpreted separately and then in combination, and findings were compared with arthroscopic reports. There was no statistical difference between the interpretations of the radial and sagittal images in the evaluation of the menisci. When the interpretations of the radial and sagittal images were combined, the sensitivity and specificity improved slightly. The radial images increased the conspicuity of meniscal tears but at the expense of anatomic detail. The sagittal images enabled better evaluation of the meniscocapsular attachment region and some flap tears of the meniscal free edge. The interpretive accuracies of the combination of sagittal and radial images were 92% and 93% for the medial and lateral menisci, respectively.

Proton (fat/water) chemical shift imaging in medical magnetic resonance imaging. Current status

Fat/water CSI has recently been transformed from an experimental method to a routine clinical MRI approach, particularly for evaluating paramagnetic contrast enhancement in fat-rich regions and as a piggyback method for decreasing MRI artifacts, such as in MR angiographic and echo-planar imaging. The full and appropriate use of CSI in medical MRI requires consideration of the factors and strategies outlined in this review. As the commercial implementation of fat/water CSI continues, we can expect further applications in clinical and experimental studies. For example, in imaging areas where MRI has been of limited efficacy, such as pancreas imaging, skin microimaging and vascular imaging, there are indications that these CSI methods may have practical importance. It seems reasonable to project that for high chemically specific detail, medical fat/water CSI will ultimately be supplanted by SI methods, and certainly by localized spectroscopy. The power of the fat/water CSI method remains its extremely high anatomic resolution, which cannot be achieved by current spectroscopy or SI methods. The fat/water CSI methods provide a means for clinically relevant MRI with more accurate and chemically specific information. Expansion of these methods into three-dimensional and fast imaging formats is already taking place at or near the commercial level. For example, the feasibility of combining echo-planar imaging and CSI methods has already been demonstrated. Methods based on the phase-contrast techniques, such as susceptibility mapping and interferometry, are additional implementations that can provide detailed and more specific information in a high anatomic detail format.

Gadopentetate dimeglumine-enhanced chemical-shift MR imaging of the breast

Standard T1-weighted MR images enhanced with gadopentetate dimeglumine show relatively minimal enhancement of breast lesions due to the high background signal from fat in the breast. Strongly enhancing lesions may become isointense relative to the fat signal and become invisible or indistinct after contrast administration. Fat-suppressed chemical-shift imaging (CSI) combined with administration of gadopentetate dimeglumine improves lesion detection and characterization in other areas of the body where a strong lipid signal is present. We evaluated this technique in the breast. Twenty patients with mammographic lesions were studied with standard unenhanced T1- and T2-weighted images and enhanced T1-weighted images, as well as with CSI before and after administration of gadopentetate dimeglumine. The series were ranked independently for border and matrix characteristics. The border was assessed for a smooth, irregular, or spiculated margin. The matrix or internal substance was evaluated for visibility and type of enhancement, homogeneous or inhomogeneous. The enhanced CSI images were superior to all other images in the depiction of border and matrix characteristics. Of 20 patients, a corresponding mass was detected on MR in 14. In two of the 14 patients, the lesion was seen only in the enhanced CSI images. Chemical-shift artifacts on enhanced T1-weighted images obscured border detail in several cases. Enhanced CSI improves visualization of breast lesions as compared with conventional MR imaging with or without enhancement. The enhanced CSI technique produces differential enhancement between glandular tissue and lesions while suppressing the signal from fat. This improves the visualization of border and matrix characteristics and depicts lesions that otherwise might be obscured.

Optic neuritis and orbital lesions: lipid-suppressed chemical shift MR imaging

A derivative of the Dixon method, chopper fat suppression, was used in the magnetic resonance imaging evaluation of 40 patients: 37 with suspected optic nerve lesions, predominantly optic neuritis, and three with orbital lesions. In patients with optic neuritis, the technique was sensitive, allowing detection of 29 of 34 visual-evoked-response-confirmed lesions on T2-weighted lipid-suppressed images. There were no false-positive studies. Sensitivity for detection of optic neuritis was 89%; specificity, 100%; and accuracy, 86%. The technique was also useful for depicting inflammatory changes in the orbital apex due to fungal abscess and confirming the fatty nature of an extraconal dermoid tumor. A slight increase in noise and mild accentuation of susceptibility effects at interfaces of air, bone, and brain did not degrade images significantly. The chopper-based technique requires only standard imaging time and has usual spin-echo sensitivity. The results are comparable to short-inversion-time inversion-recovery imaging, without the constraints of sequence limitations or artifacts at fat and water interfaces.

Clinical evaluation of stenosis of the carotid bifurcation with magnetic resonance angiographic techniques

We evaluated the images of 60 carotid artery bifurcations in 31 patients suspected to have carotid artery disease who underwent invasive carotid angiography and combined two-dimensional, phase-sensitive and a gradient-echo magnetic resonance angiography. The phase scans consisted of seven serial projections that were obtained at 20 degrees intervals (11.0 minutes) around the carotid bifurcation; the gradient-echo (GRASS) scans were composed of 11 axial images (2.4 minutes) acquired through the bifurcation. The two magnetic resonance angiographic techniques yielded complementary pieces of information and were used together to compare magnetic resonance angiography with invasive angiography. Comparison of magnetic resonance and invasive angiograms of the 60 carotid arteries shows that the sensitivity (86%) and specificity (92%) of the magnetic resonance angiographic techniques we used to diagnose clinically significant carotid stenosis approach but do not reach those of invasive angiography.

Improved detection and delineation of head and neck lesions with fat suppression spin-echo MR imaging

To compare conventional and fat suppression MR imaging in their ability to detect head and neck lesions, we prospectively studied 17 patients with head and neck tumors and one normal volunteer. Five patients had benign tumors (one mixed cell tumor, one hemangioma, one lipoma, and two plexiform neurofibromas), 10 had malignant tumors (six squamous cell carcinomas, two minor salivary gland carcinomas, one lymphoma, and one malignant fibrous histiocytoma), and two had nonspecific lymphadenopathy. All subjects were studied with standard spin-echo T1- and T2-weighted images (T2-weighted imaging was done with and without fat suppression technique). In addition, T1-weighted images with contrast enhancement and fat suppression were obtained in nine patients. A four-point grading system was used for comparison of the conventional and fat suppression images. Grades ranged from 0 (unsatisfactory, the lesion cannot be seen) to 3 (excellent, the lesion and its margins can be seen clearly with sharp contrast from surrounding normal tissue). We found that postcontrast fat suppression T1-weighted images and fat suppression T2-weighted images were most useful; these sequences obtained an average score close to grade 3 (2.77 and 2.85, respectively). On the other hand, the conventional T2-weighted images had an average score of about 2 (1.82) and the conventional T1-weighted image had a score of about 1 (1.33). Fat suppression T2-weighted sequences generally were superior in cases of lymphadenopathies. Postcontrast T1-weighted images were most useful in a case of plexiform neurofibroma, owing to their fibrous component and lower proton density.(ABSTRACT TRUNCATED AT 250 WORDS)

Intra- and paraorbital lesions: value of fat-suppression MR imaging with paramagnetic contrast enhancement

The orbital area of 18 individuals was examined by using a combination of fat-suppression contrast-enhanced MR imaging to determine whether contrast between fat and surrounding tissues could be improved over that obtained with conventional fat-suppression techniques alone. We used a hybrid technique combining two independent methods of fat suppression. Subjects consisted of 16 patients and two normal volunteers. Fifteen individuals received gadopentetate dimeglumine, and conventional T1-weighted, T2-weighted, and fat-suppression T1-weighted images were obtained. The fat-suppressed T1-weighted images obtained after contrast administration provided more information than did the conventional MR images. Intraorbital and paraorbital lesions could be distinguished easily from intraorbital fat that had been suppressed. Cases of chorioretinitis and optic neuritis could be confidently diagnosed only by this technique. Cases of optic nerve meningioma and mixed conal lesions also were better appreciated. Because of sharp contrast between tissue planes, this technique was helpful for detecting any intraorbital invasion from paraorbital lesions. Fat-suppression MR imaging with paramagnetic contrast enhancement can significantly improve the delineation of both normal and abnormal structures and better define lesional margins in the orbit, where large amounts of fat are present. Our results support earlier findings, and we suggest that postcontrast fat-suppressed T1-weighted imaging be used instead of conventional T1-weighted postcontrast imaging in evaluating orbital and paraorbital lesions.

MR imaging of optic nerve lesions: value of gadopentetate dimeglumine and fat-suppression technique

Eleven patients with known or suspected optic nerve lesions and eight normal subjects were examined with spin-echo technique at 1.5 T with unenhanced T1-weighted imaging, IV gadopentetate-dimeglumine-enhanced T1-weighted imaging, and enhanced T1-weighted imaging with fat suppression. Two pathologically proved and four presumed optic nerve meningiomas demonstrated significant enhancement and were best seen with the fat-suppression technique. None of the three presumed optic nerve gliomas nor the optic nerves of normal subjects demonstrated qualitative enhancement. We conclude that the use of a fat-suppression technique with gadopentetate dimeglumine enhancement improves delineation of enhancing optic nerve lesions. This technique should be useful for evaluating other anatomic regions where enhancing tissue marginates fat.

Spin-lattice relaxation time measurements using hybrid CSI--phantom study

The spin-lattice relaxation time T1 of multicomponent tissues is often determined by fitting relaxation data to monoexponential functions. This process can lead to large errors in the relaxation time. We describe a procedure using chemical-shift imaging (CSI) which separates the NMR signal into water and lipid components thus allowing the two signals to be individually analyzed for relaxation times. This procedure yields more representative relaxation times than those obtained by both monoexponential and biexponential fitting schemes.

MR imaging of optic nerve lesions: value of gadopentetate dimeglumine and fat-suppression technique

Eleven patients with known or suspected optic nerve lesions and eight normal subjects were examined with spin-echo technique at 1.5 T with unenhanced T1-weighted imaging, IV gadopentetate-dimeglumine-enhanced T1-weighted imaging, and enhanced T1-weighted imaging with fat suppression. Two pathologically proved and four presumed optic nerve meningiomas demonstrated significant enhancement and were best seen with the fat-suppression technique. None of the three presumed optic nerve gliomas nor the optic nerves of normal subjects demonstrated qualitative enhancement. We conclude that the use of a fat-suppression technique with gadopentetate dimeglumine enhancement improves delineation of enhancing optic nerve lesions. This technique should be useful for evaluating other anatomic regions where enhancing tissue marginates fat.

Variable flip angle imaging and fat suppression in combined gradient and spin-echo (GREASE) techniques

Conventional "proton density" and "T2-weighted" spin-echo images are susceptible to motion induced artifact, which is exacerbated by lipid signals. Gradient moment nulling can reduce motion artifact but lengthens the minimum TE, degrading the "proton density" contrast. We designed a pulse sequence capable of optimizing proton density and T2-weighted contrast while suppressing lipid signals and motion induced artifacts. Proton density weighting was obtained by rapid readout gradient reversal immediately after the excitation RF pulse, within a conventional spin-echo sequence. By analyzing the behavior of the macroscopic magnetization and optimizing excitation flip angle, we suppressed T1 contribution to the image, thereby enhancing proton density and T2-weighted contrast with a two- to four-fold reduction of repetition time. This permitted an increased number of averages to be used, reducing motion induced artifacts. Fat suppression in the presence of motion was investigated in two groups of 8 volunteers each by (i) modified Dixon technique, (ii) selective excitation, and (iii) hybrid of both. Elimination of fat signal by the first technique was relatively uniform across the field of view, but it did not fully suppress the ghosts originating from fat motion. Selective excitation, while sensitive to the main field inhomogeneity, largely eliminated the ghosts (0.21 +/- 0.05 vs. 0.29 +/- 0.06, p less than 0.01). The hybrid of both techniques combined with bandwidth optimization, however, showed the best results (0.17 +/- 0.04, p less than 0.001). Variable flip-angle imaging allows optimization of image contrast which, along with averaging and effective fat suppression, significantly improves gradient- and spin-echo imaging, particularly in the presence of motion.

A comparison of default and reduced bandwidth MR imaging of the spine at 1.5 T

The value of a reduced bandwidth MR imaging technique was tested prospectively in 51 spinal MR examinations by using default (16 kHz) bandwidth, 2000/30, 90 (TR/TEs) and 600/30, and reduced (8 kHz) bandwidth, 2000/48, 92 and 600/30, techniques at 1.5 T. Bandwidth reduction was used to maintain the signal-to-noise ratio for a reduced scan time. Concerns have been raised as to the effect of bandwidth reduction at high field, since a savings in time or an increased signal-to-noise ratio occur at the expense of increased chemical shift misregistration artifact. However, when appreciable, the chemical shift-related artifact in the spine was typically located in the frequency-encoding direction at the vertebral body/disk space interface or the dural sac/epidural fat interface in the lower lumbosacral region and was easily distinguished from pathologic lesions. There were no missed diagnoses with the reduced bandwidth technique. This study suggests that chemical shift-related artifact will rarely be confused with pathology by an experienced reader and suggests a clinical role for the bandwidth technique to decrease scanning time in uncooperative patients or to allow acquisition of additional imaging planes in a reasonable time.

Correction of periodic motion artifacts along the slice selection axis in MRI

The effect of periodic motion of a single magnetic resonance imaging (MRI) slice in the direction of the slice selection axis is modeled as amplitude modulation of the raw data with a motion kernel along the phase encoding direction in the Fourier domain. It is shown that this motion can be detected in 1-D projections of the raw data along the frequency encoding direction which in combination with appropriate filtering leads to the recovery of the motion kernel. It is demonstrated by means of simulation examples that significant reduction in the amplitude of ghost artifacts is obtained when the image is filtered by the inverse of the motion kernel. Some issues to be investigated before the technique can be used in a clinical environment are mentioned.

Reduced-bandwidth MR imaging of the head at 1.5 T1

A decrease in the magnetic resonance (MR) imaging bandwidth can be used to increase the signal-to-noise ratio (S/N) at constant imaging time or to maintain the S/N for reduction of imaging time. The effect of bandwidth reduction from the default value of 16 kHz to 8 kHz was evaluated prospectively in 50 patients referred for MR imaging of the head. On intermediate (2,000/30 [repetition time msec/echo time msec]) and more T2-weighted (2,000/90) studies, there were no definite missed diagnoses and no diagnostically important changes in lesion characteristics when the reduced-bandwidth technique was used to obtain half- or quarter-time studies, excluding differences attributable to unintentional changes in patient position between image acquisitions. Chemical shift misregistration artifacts associated with reduced bandwidth are easily recognized with experience and do not interfere with diagnosis, as the artifacts occur in characteristic locations and diminish in most anatomic locations with increasing echo time. This study suggests the feasibility of reduced-bandwidth techniques in clinical MR imaging of the head at high field strength to achieve an increased S/N, to decrease imaging time, or to obtain images in additional projections.

Fat/water quantitation and differential relaxation time measurement using chemical shift imaging technique

The use of chemical shift imaging for fat and water quantitation and differential measurement of relaxation times for the fat and water component is demonstrated using a hybrid technique. The efficacy of the imaging technique for fat and water quantitation has been tested by comparing the results of imaging to the results of volumetric measurements in phantoms with oil and water homogeneously mixed, fat extraction in ground meat of different grades, and biopsy in preliminary clinical studies. Good correlation is found between the fat and water content measured by imaging and that measured by other means except for the inability to differentiate unsaturated fat protons from water protons. Longitudinal (T1) and transverse (T2) relaxation times for water and fat are also shown to be measurable independently when fat and water signal are suppressed accordingly. The independently measured relaxation times correspond closely to those of the pure samples except that unsaturated protons give decreased water relaxation estimates.

MR fat suppression technique in the evaluation of normal structures of the knee

The chopper fat suppression (CFS) pulse sequence, which is a phase sensitive implementation of the Dixon fat suppression method and the spin echo (SE) pulse sequence, was used in the evaluation of anatomic structures of the normal knee using 48 sets of imaging sequences in six volunteers using a repetition time/echo time combination of 1,500/30, 60 ms. A demonstration of the CFS technique in 10 patients with suspected knee pathology is also presented. A semiquantitative grading scale was established to rate anatomic visualization and used to compare CFS and SE pulse sequence techniques. The results in normal subjects demonstrate that hyaline cartilage is significantly better visualized by fat suppression pulse sequence than by conventional SE pulse sequence in the coronal and sagittal planes of imaging (p less than 0.001). The preliminary results from patients studies suggest that CFS imaging may be useful in the evaluation of meniscal tears, in the differentiation of hyaline cartilage from joint fluid, and in the detection of both soft tissue and bone injuries.

Chemical shift imaging with paramagnetic contrast material enhancement for improved lesion depiction

The depiction of contrast material-enhanced lesions with magnetic resonance imaging can be improved by using chemical shift imaging (CSI) for lipid suppression in combination with gadolinium diethylenetriaminepentaacetic acid (DTPA) enhancement. Gd-DTPA enhancement was combined with the hybrid technique for lipid suppression, which provides water-only images without increasing imaging time or postprocessing. Lesions with high signal intensity due to paramagnetic relaxation enhancement are easily distinguished from low-intensity lipid, which would otherwise dominate T1-weighted images. Preliminary studies were performed to compare Gd-DTPA-CSI images with conventional postcontrast T1-weighted images. In patients examined for orbital, pituitary, and musculoskeletal abnormalities, the Gd-DTPA-CSI technique enabled improved detection and finer anatomic staging of lesions. In theory, a similar result can be achieved by using any chemical shift-selective method that results in true lipid suppression together with paramagnetic contrast agents that generate high signal intensity. This general approach should be applicable to clinical studies in other tissues or organ systems dominated by lipid, including the pelvis, mediastinum, and breast.

Magnetic resonance imaging using a ribbonator: hand and wrist

A modified version of a single-turn solenoid with rectangular symmetry, which we call a ribbonator, provides excellent magnetic resonance images of the hand and wrist when used as both the transmitter and the receiver in a 1.5-T clinical imaging system. The very high RF efficiency provides excellent signal-to-noise and anatomical resolution. Design equations and RF properties of the resonator are discussed.

Hybrid methods of chemical-shift imaging

We propose a family of hybrid chemical-shift sequences which combines two physical principles for water/lipid separation to minimize artifacts introduced by B0 and B1 inhomogeneities. Hybrid sequences provide improved species discrimination over earlier methods without resorting to postprocessing while maintaining a multislice/multiecho capability.

SIMA: simultaneous multislice acquisition of MR images by Hadamard-encoded excitation

We present a method of multislice magnetic resonance imaging that utilizes simultaneous binary-encoded excitation. Signals are acquired from all slices at once, and the images are separated in the reconstruction process. This simultaneous multislice acquisition method has been implemented for multislice spin-echo imaging, and the results are compared with those for a standard interleaved multislice method. Advantages include improved signal-to-noise ratios and flexible slice placement. Phantom and volunteer studies are presented and evaluated in comparison with competing methods.

Fat suppression in the time domain in fast MR imaging

Two gradient-recalled lipid suppression sequences are proposed. A two-excitation sequence cycles the TE interval between excitations to alter the lipid phase which is followed by complex subtraction. A four-excitation variant which improves the extent of lipid suppression by partially compensating for errors resulting from spin-spin relaxation and B0 inhomogeneities is outlined.

Lactate observation in vivo by spectral editing in real time

We have developed a novel in vivo proton MR spectroscopy magnetization transfer method for detection of lactate in ischemic tissue in the presence of interfering fat proton resonances. Pyruvate is magnetically labeled with a saturation pulse and, when converted to lactate, the lactate retains the label. Difference of spectra obtained with and without a saturation pulse contain no fat resonances. High-resolution spectra (determined with a GE 1.5 T Signa) of low lactate levels were obtained in vivo by water suppression using a 2662 composite RF pulse and slice-selective gradients. Spectral subtraction was performed in real time allowing the monitoring of a buildup of the intensity of the lactate peak. Pyruvate-lactate saturation transfer techniques should find wide applicability in the study of ischemia.

Fat-suppression MR imaging of the orbit

The effect of fat suppression on orbital MR imaging was tested by using a derivative of the Dixon method called chopper fat suppression in eight normal volunteers and eight patients with normal conventional orbital MR studies. Chopper fat suppression requires no postacquisition image processing or increased scan time and can be applied through a wide range of T1 to T2 weighting. In normal orbits, fat suppression was found to be advantageous for imaging the lacrimal gland and the optic nerve. Using fat-suppressed T1- or intermediate-weighted sequences, 2000/30 (TR/TE), the optic nerve was recognized by its high signal intensity relative to adjacent CSF, dural sheath, and surrounding fat. The technique minimized loss of anatomic detail by reducing chemical shift misregistration artifact. Disadvantages included an overall lower orbital signal/noise ratio. When used in conjunction with a TR/TE combination carefully selected for both anatomic region of interest and suspected pathology, the fat-suppression technique has the potential for improving the visualization of orbital lesions.

MRI of extremities using perforated single-turn solenoids

A class of single-turn solenoids that permits magnetic imaging of the extremities in horizontal bore magnets with improved coupling between the imaged anatomy and the rf section of the imager is described and demonstrated. These devices differ from more conventional designs primarily by the placement of one or more access holes in the side of the generally cylindrical resonant structure to permit extremity insertion. The image quality is excellent, rf efficiency and homogeneity are good, and signal-to-noise is high, permitting rapid acquisition of magnetic images with small fields of view.