Evaluation of Ultrafast Wave-Controlled Aliasing in Parallel Imaging 3D-FLAIR in the Visualization and Volumetric Estimation of Cerebral White Matter Lesions

BACKGROUND AND PURPOSE

Our aim was to evaluate an ultrafast 3D-FLAIR sequence using Wave-controlled aliasing in parallel imaging encoding (Wave-FLAIR) compared with standard 3D-FLAIR in the visualization and volumetric estimation of cerebral white matter lesions in a clinical setting.

MATERIALS AND METHODS

Forty-two consecutive patients underwent 3T brain MR imaging, including standard 3D-FLAIR (acceleration factor = 2, scan time = 7 minutes 50 seconds) and resolution-matched ultrafast Wave-FLAIR sequences (acceleration factor  = 6, scan time = 2 minutes 45 seconds for the 20-channel coil; acceleration factor = 9, scan time  = 1 minute 50 seconds for the 32-channel coil) as part of clinical evaluation for demyelinating disease. Automated segmentation of cerebral white matter lesions was performed using the Lesion Segmentation Tool in SPM. Student t tests, intraclass correlation coefficients, relative lesion volume difference, and Dice similarity coefficients were used to compare volumetric measurements among sequences. Two blinded neuroradiologists evaluated the visualization of white matter lesions, artifacts, and overall diagnostic quality using a predefined 5-point scale.

RESULTS

Standard and Wave-FLAIR sequences showed excellent agreement of lesion volumes with an intraclass correlation coefficient of 0.99 and mean Dice similarity coefficient of 0.97 (SD, 0.05) (range, 0.84-0.99). Wave-FLAIR was noninferior to standard FLAIR for visualization of lesions and motion. The diagnostic quality for Wave-FLAIR was slightly greater than for standard FLAIR for infratentorial lesions (P < .001), and there were fewer pulsation artifacts on Wave-FLAIR compared with standard FLAIR (P < .001).

CONCLUSIONS

Ultrafast Wave-FLAIR provides superior visualization of infratentorial lesions while preserving overall diagnostic quality and yields white matter lesion volumes comparable with those estimated using standard FLAIR. The availability of ultrafast Wave-FLAIR may facilitate the greater use of 3D-FLAIR sequences in the evaluation of patients with suspected demyelinating disease.

Evaluation of Ultrafast Wave-CAIPI MPRAGE for Visual Grading and Automated Measurement of Brain Tissue Volume

BACKGROUND AND PURPOSE

Volumetric brain MR imaging typically has long acquisition times. We sought to evaluate an ultrafast MPRAGE sequence based on Wave-CAIPI (Wave-MPRAGE) compared with standard MPRAGE for evaluation of regional brain tissue volumes.

MATERIALS AND METHODS

We performed scan-rescan experiments in 10 healthy volunteers to evaluate the intraindividual variability of the brain volumes measured using the standard and Wave-MPRAGE sequences. We then evaluated 43 consecutive patients undergoing brain MR imaging. Patients underwent 3T brain MR imaging, including a standard MPRAGE sequence (acceleration factor [R] = 2, acquisition time [TA] = 5.2 minutes) and an ultrafast Wave-MPRAGE sequence (R = 9, TA = 1.15 minutes for the 32-channel coil; R = 6, TA = 1.75 minutes for the 20-channel coil). Automated segmentation of regional brain volume was performed. Two radiologists evaluated regional brain atrophy using semiquantitative visual rating scales.

RESULTS

The mean absolute symmetrized percent change in the healthy volunteers participating in the scan-rescan experiments was not statistically different in any brain region for both the standard and Wave-MPRAGE sequences. In the patients undergoing evaluation for neurodegenerative disease, the Dice coefficient of similarity between volumetric measurements obtained from standard and Wave-MPRAGE ranged from 0.86 to 0.95. Similarly, for all regions, the absolute symmetrized percent change for brain volume and cortical thickness showed <6% difference between the 2 sequences. In the semiquantitative visual comparison, the differences between the 2 radiologists' scores were not clinically or statistically significant.

CONCLUSIONS

Brain volumes estimated using ultrafast Wave-MPRAGE show low intraindividual variability and are comparable with those estimated using standard MPRAGE in patients undergoing clinical evaluation for suspected neurodegenerative disease.

Balanced Steady-State Free Precession Techniques Improve Detection of Residual Germ Cell Tumor for Treatment Planning

BACKGROUND AND PURPOSE

Identification of a partial/complete chemotherapy response in pediatric patients with intracranial germ cell tumors is clinically important for radiation treatment and management. Partial/complete response is conventionally determined on postcontrast MR imaging sequences. The purpose of this study was to assess the diagnostic utility of a balanced steady-state free precession sequence as an adjunct to standard MR imaging sequences for the detection of residual tumor in pediatric patients on postchemoreduction pre-radiation planning MR imaging.

MATERIALS AND METHODS

This was a retrospective study of pediatric patients with intracranial germ cell tumors undergoing postchemotherapy, preradiotherapy MR imaging. Patients underwent 1.5T or 3T MR imaging with pre- and postcontrast T1WIs, T2WIs, and a balanced steady-state free precession sequence. Two neuroradiologists independently reviewed standard MR imaging sequences without the balanced steady-state free precession sequence, then with the balanced steady-state free precession sequence 1 week later. Assessment for partial/complete response was determined using Response Assessment in Neuro-Oncology criteria. A 5-point Likert scale scored the diagnostic confidence of the neuroradiologist rating each study without/with the balanced steady-state free precession sequence. Rates of residual disease concordance and diagnostic confidence levels without/with the balanced steady-state free precession sequence were calculated.

RESULTS

Thirty-nine patients were included with 31 males and 8 females (mean age, 14.15 ± 4.26 years). Thirty-one patients had single-site disease; 8 patients had multisynchronous disease (47 sites in total). Compared to review of the standard MR sequences alone, the addition of the balanced steady state free precession sequence resulted in higher rates of tumor partial response categorization and greater diagnostic confidence levels (P < .001, P < .001).

CONCLUSIONS

The balanced steady-state free precession sequence improves detection of residual chemotherapy-reduced intracranial germ cell tumors and increases diagnostic confidence of the neuroradiologist. The balanced steady-state free precession sequence may be an important adjunct to the standard MR imaging protocol for radiation planning.

Validation of Highly Accelerated Wave-CAIPI SWI Compared with Conventional SWI and T2*-Weighted Gradient Recalled-Echo for Routine Clinical Brain MRI at 3T

BACKGROUND AND PURPOSE

SWI is valuable for characterization of intracranial hemorrhage and mineralization but has long acquisition times. We compared a highly accelerated wave-controlled aliasing in parallel imaging (CAIPI) SWI sequence with 2 commonly used alternatives, standard SWI and T2*-weighted gradient recalled-echo (T2*W GRE), for routine clinical brain imaging at 3T.

MATERIALS AND METHODS

A total of 246 consecutive adult patients were prospectively evaluated using a conventional SWI or T2*W GRE sequence and an optimized wave-CAIPI SWI sequence, which was 3-5 times faster than the standard sequence. Two blinded radiologists scored each sequence for the presence of hemorrhage, the number of microhemorrhages, and severity of motion artifacts. Wave-CAIPI SWI was then evaluated in head-to-head comparison with the conventional sequences for visualization of pathology, artifacts, and overall diagnostic quality. Forced-choice comparisons were used for all scores. Wave-CAIPI SWI was tested for superiority relative to T2*W GRE and for noninferiority relative to standard SWI using a 15% noninferiority margin.

RESULTS

Compared with T2*W GRE, wave-CAIPI SWI detected hemorrhages in more cases (P < .001) and detected more microhemorrhages (P < .001). Wave-CAIPI SWI was superior to T2*W GRE for visualization of pathology, artifacts, and overall diagnostic quality (all P < .001). Compared with standard SWI, wave-CAIPI SWI showed no difference in the presence or number of hemorrhages identified. Wave-CAIPI SWI was noninferior to standard SWI for the visualization of pathology (P < .001), artifacts (P < .01), and overall diagnostic quality (P < .01). Motion was less severe with wave-CAIPI SWI than with standard SWI (P < .01).

CONCLUSIONS

Wave-CAIPI SWI provided superior visualization of pathology and overall diagnostic quality compared with T2*W GRE and was noninferior to standard SWI with reduced scan times and reduced motion artifacts.

Analysis of noise in phase contrast MR imaging

In this work we analyze the effects of inherent random noise on the detectability of low-contrast vessel structures that possess slow flow. When flow is encoded in more than one direction, the number of independent noise contributions increases in addition to the scan time. In a fast-flow scenario, only the noise contribution from sampling along the direction of flow is of any significance. At slow flow rates, however, it becomes necessary to account for the noise in each encoded Cartesian direction. The degree to which noise affects low-contrast detectability also depends on the method of phase contrast image processing employed. A theoretical analysis of the statistical properties of signal and noise in processed phase contrast magnitude images is presented and verified from experimental MR image data. Results show a progressively increased bias in the processed phase contrast image magnitude at slow flow rates due to contributions from inherent random noise. The amount of this bias increases with the number of physical directions in which flow is encoded and is larger for complex difference processed images than for phase difference processing. Correspondingly, the output signal-to-noise ratio associated with flow is compromised.

Cortical activation in confrontation naming

Functional magnetic resonance imaging was used to detect cortical activation in the right and left perisylvian cortex of seven young adult right-handed volunteers in response to a letter fluency task and to a visual naming task using standardized line drawings. Both letter fluency and visual naming activated left dorsolateral prefrontal cortex (Brodmann's areas 6, 9, 44 and 45). Only visual naming activated area 37 (a cortical region with strong connections to visual association areas), visual association area 19, and areas 39 and 21 previously shown to activate with auditory semantic tasks. This study supports a role for area 37 as participant in a visual lexicosemantic processing network which may otherwise overlap the auditorysemantic network.

Frontal lobe phosphorus metabolism and neuropsychological function in aging and in Alzheimer's disease

31P Magnetic resonance spectroscopy of the frontal lobe was performed in 17 patients with Alzheimer's disease (AD), 8 elderly controls (EC), and 17 young controls (YC). The phosphocreatine/inorganic phosphate (PCr/Pi) ratio in AD (2.32 +/- 0.26 SD) was significantly lower than in EC (2.65 +/- 0.41). In AD patients, a correlation was observed between the PCr/Pi ratio and the dementia rating scale (r = -0.50, p = 0.04). A significant positive correlation between PCr/Pi ratio and age was observed in both AD (r = 0.67, p = 0.003) and YC (r = 0.63, p = 0.006) groups, however, suggesting caution in interpretation of this ratio in AD. We did not find differences between AD, EC, or YC in any other spectroscopic measure. A significant sex difference in the phosphomonoester/phosphodiester ratio (PME/PDE) ratio was observed in AD brain. Females had a lower PME/PDE ratio than males.

Magnetization transfer and high-dose contrast in early brain infection on magnetic resonance

RATIONALE AND OBJECTIVES

The authors studied the effect of contrast dose, use of magnetization transfer (MT), and temporal delay on the visualization of contrast enhancement with gadoteridol (Gd HP-DO3A) in a canine brain abscess model.

METHODS

Alpha streptococcus brain abscesses were studied in five dogs at 1.5 tesla (T) 1 and 5 days after implantation. Scans were performed 1, 11, and 21 minutes after contrast was administered, using an initial dose of 0.1 mmol/kg. A supplemental contrast injection of 0.2 mmol/kg was given (for a cumulative dose of 0.3 mmol/kg), with scans repeated at 31, 41, and 51 minutes.

RESULTS

Lesion conspicuity on day 1 was greater at high-contrast doses (0.3 mmol/kg) compared with standard doses (0.1 mmol/kg), regardless of whether imaging was performed without (0.89 +/- 0.02 compared with 0.26 +/- 0.08) or with (0.97 +/- 0.04 compared with 0.28 +/- 0.06) MT. High-dose, MT, and a delay after contrast was injected all produced a statistically significant improvement. On blinded review of films obtained 11 and 14 minutes after injection, enhancement of the lesion could not be identified with certainty in two of five dogs at a dose of 0.1 mmol/kg, regardless of whether MT was used. Enhancement was seen consistently in all lesions at 0.3 mmol/kg. On day 5, results were comparable, with greater absolute enhancement.

CONCLUSIONS

In early brain infection, high-contrast doses (0.3 mmol/kg), MT, and a moderate delay after injection all improve visualization of lesion enhancement.

Repeat cerebral blood volume assessment with first-pass MR imaging

The feasibility of performing multiple first-pass studies with dynamic, contrast material-enhanced magnetic resonance (MR) imaging was evaluated in a cat model of acute middle cerebral artery (MCA) ischemia. Two dynamic series of SSFP (steady-state free precession) images were acquired in each animal (n = 5) with a conventional 1.5-T imager. The initial first-pass study was acquired at 60 minutes after MCA occlusion, and the second study at 70 minutes, with each performed during an intravenous bolus injection of a 0.5 mmol/kg dose of gadoteridol. In both first-pass studies, differentiation of normal and ischemic gray and white matter was highly statistically significant. At a threshold of P < .01, no statistically significant difference in the peak signal intensity between the first and second studies was noted. A difference between the two studies in the recovery to baseline was seen, presumably due to T1 effects. First-pass MR studies can be repeated within the time frame of a single clinical examination, expanding their utility.

Gadoteridol dose dependence in MR imaging of a liver abscess model

A necrotic liver abscess model was studied with magnetic resonance (MR) imaging at 1.5 T before and after intravenous administration of gadoteridol at doses of 0.1, 0.25, and 0.5 mmol/kg in 24 rabbits. Enhancement characteristics and lesion delineation were assessed with both breath-hold and non-breath-hold imaging techniques. Lesion delineation, as assessed both by signal intensity measurements and evaluations by two image readers blinded to imaging technique, was greatest on high-dose (0.5 mmol/kg) breath-hold images. Lesion rim enhancement was seen consistently only on postcontrast images obtained at a dose of 0.5 mmol/kg and progressed with time after injection of contrast material.

Magnetic resonance contrast agents in neuroimaging. New agents and applications

Of the four gadolinium chelates in clinical use worldwide, only three are available in the United States: gadoteridol, gadodiamide, and gadopentetate dimeglumine. Although gadopentetate dimeglumine is still administered in many clinical practices, a consequence of being the first agent developed (1988) and until recently the only agent available, gadoteridol demonstrates a higher index of safety--at least on theoretical grounds. With linear chelates such as gadopentetate dimeglumine and gadodiamide, greater release of free gadolinium ion occurs in vivo owing to lower thermodynamic and kinetic stability. The concern with respect to demetallation and resultant increased chronic deposition of gadolinium ion in marrow and liver is greater with gadodiamide. Drug safety appears not to be an issue at high dose for gadoteridol, the only agent approved in this regards. The safety of the other chelates at high dose has yet to be established. The nonionic character of the two newer agents (gadoteridol and gadodiamide) is not of great importance in current clinical practice, in which most examinations are still performed with a dose of 0.1 mmol/kg given as a slow infusion. The non-ionic nature of the newer MR contrast agents may become clinically important in the future, with more common application of bolus injection and high dose. High contrast dose (0.3 mmol/kg) is advocated in routine MR clinical practice for the evaluation of intracranial metastatic disease. Only with prior evidence for multiple metastases and in patients for whom the detection of additional lesions would not influence therapy can the use of standard dose (0.1 mmol/kg) be justified.(ABSTRACT TRUNCATED AT 250 WORDS)

Visualization of blood-brain barrier disruption on MR images of cats with acute cerebral infarction: value of administering a high dose of contrast material

OBJECTIVE

The detection of blood-brain barrier disruption in patients with cerebral infarction by means of contrast-enhanced MR images improves the specificity of diagnosis, enables lesion dating, and on occasion improves lesion detection. Accordingly, we performed a study to determine the extent of visualization of disruption of the blood-brain barrier on contrast-enhanced MR images, with specific attention to contrast dose, in a cat model of acute cerebral infarction. We used doses of 0.1 and 0.3 mmol/kg of a gadolinium chelate, gadoteridol, which is characterized by extracellular distribution and renal excretion, and was approved by the Food and Drug Administration for clinical use at these doses.

MATERIALS AND METHODS

Blood flow in the middle cerebral artery was occluded unilaterally in seven cats for 1 hr, followed by 4 hr of reperfusion. T2- and T1-weighted MR images were obtained before the injection of contrast material. After injection, the time course of enhancement was observed for 1 hr by repeated sequential acquisition of T1-weighted images. Five cats received an initial injection of 0.1 mmol/kg of contrast material, supplemented 33 min later by 0.2 mmol/kg (cumulative dose, 0.3 mmol/kg). Two cats received a single injection of contrast material, either 0.1 or 0.3 mmol/kg. The images were reviewed in a prospective fashion by a single observer, who was blinded to the dose of contrast material and the timing of image acquisition, in order to detect abnormal contrast enhancement. Changes in single intensity were quantified by region-of-interest measurements.

RESULTS

Enhancement at 4 and 13 min, respectively, after injection of contrast material was 25 +/- 10% and 38 +/- 7% with 0.1 mmol/kg, vs 80 +/- 12% and 100 +/- 15% with 0.3 mmol/kg (n = 5). The difference between doses was statistically significant (p < .002) for all time points. Abnormal contrast enhancement was visible in three of six cats that received 0.1 mmol/kg and in all cats that received 0.3 mmol/kg. By 13 min after injection, enhancement had peaked with a dose of 0.1 mmol/kg and was within 20% of maximum with a dose of 0.3 mmol/kg.

CONCLUSION

Detection of disruption of the blood-brain barrier in acute cerebral infarction in cats is improved when high doses (0.3 mmol/kg) of contrast material are used. Disruption may not be visualized when 0.1 mmol/kg, the currently accepted standard dose, is used.

Assessment of cerebral perfusion by first-pass, dynamic, contrast-enhanced, steady-state free-precession MR imaging: an animal study

OBJECTIVE

The purpose of this study was to determine whether cerebral perfusion could be assessed with a conventional 1.5-T MR imaging system by applying a steady-state free-precession (SSFP) technique during bolus IV injection of contrast material (gadoteridol). Normal and abnormal perfusion states and the effect of the dose of contrast material were studied in cats.

MATERIALS AND METHODS

Nine healthy anesthetized cats were imaged after administration of 0.5 mmol/kg (n = 5), 0.25 mmol/kg (n = 2), and 0.1 mmol/kg (n = 2) of gadoteridol. Six cats with an acute infarct of the middle cerebral artery (five cats 10 min after and one cat 1 hr after vessel occlusion) were evaluated at a dose of 0.5 mmol/kg. The middle cerebral artery was ligated in each instance by use of a snare placed around the vessel during surgery, which was performed via a transorbital approach. Each animal was imaged with spin-echo T2-weighted (3000/45,90 [TR/TE]) and T1-weighted (500/10) techniques before contrast material was injected. SSFP images (12/18, 80 degrees tip angle) were acquired sequentially (each with a 1-sec acquisition time and no interimage delay) immediately before and for 45 sec after IV injection of a bolus of gadoteridol.

RESULTS

The first-pass effect in both normal gray and white matter was dose dependent, with a greater magnitude of change seen at higher doses of contrast material. A 55% decrease in signal intensity of normal peripheral gray matter was observed during the first pass after bolus injection of 0.5 mmol/kg gadoteridol, compared with a 23% decrease and a 17% decrease at doses of 0.25 and 0.1 mmol/kg, respectively. High temporal (one image per second) and spatial (1.5 x 1.5 x 6.0 mm) resolution was achieved, with sufficient sensitivity that both visual and statistical differentiation of normal and abnormal gray and white matter was possible. Ten minutes after vessel occlusion, the change in signal intensity during the first pass was 45 +/- 5% and 27 +/- 9% for normal central gray and white matter, respectively, as compared with 20 +/- 2% and 11 +/- 5% for ischemic central gray and white matter, respectively.

CONCLUSION

Cerebral perfusion can be assessed on conventional 1.5-T MR imaging systems by combining high-dose IV bolus injection of contrast material (in this instance, using gadoteridol) with dynamic SSFP imaging.

Enhanced liver MR: contrast agents and imaging strategy

Contrast enhancement in liver MR can be achieved by a variety of fundamentally different strategies. The published clinical literature regarding Gd DTPA (gadopentetate dimeglumine), Gd HP-DO3A (gadoteridol), Gd BOPTA (gadobenate dimeglumine), Mn DPDP, and AMI-25 is reviewed, followed by a brief discussion of two new iron particulate agents currently in preclinical trials. Different imaging techniques also must be used for visualization of contrast enhancement depending on the specific type of agent utilized. With both gadolinium and manganese chelates, T1 weighted sequences are used to visualize the effect of the contrast agent. There is positive enhancement (an increase in signal intensity) of normal liver parenchyma post-contrast due to enhanced T1 relaxation. With iron particulate agents, T2 weighted sequences are used. In this instance, there is negative enhancement (a decrease in signal intensity) of normal liver post-contrast due to enhanced T2 relaxation. Clinical use at present is limited to the extracellular gadolinium chelates, with bolus injection and dynamic imaging improving efficacy. Current research also supports the use of a high dose (0.3 mmol/kg) for improved lesion detectability, a finding now clinically relevant due to the recent approval of Gd HP-DO3A at both standard and high doses.

The mucopolysaccharidoses: characterization by cranial MR imaging

PURPOSE

To characterize MR findings in mucopolysaccharidoses (MPS), to aid in diagnosis and categorization, and to define the role of MR in preoperative evaluation.

MATERIALS AND METHODS

Six children with Hurler syndrome (MPS IH), five with Hunter syndrome (MPS II), and three with Sanfilippo A syndrome (MPS IIIA) were studied by routine T1-weighted and T2-weighted images at 1.5 T. MR findings were graded retrospectively.

RESULTS

All had hallmark cribriform changes (sieve-like or multicystic) involving peri- and supraventricular, parietal, white matter (12), corpus callosum (8), and basal ganglia (4), which did not enhance. The cerebellum and brain stem were not involved with these cribriform changes. The most severe degree of cribriform changes occurred in children with Hunter and Hurler syndromes, correlating with non-central nervous system somatic involvement, but inversely related to degree of atrophy, ventricular enlargement, and white matter changes. Mental retardation was most severe in children with Hurler syndrome and correlated with chronicity of the disease. Severity of mental retardation did not correlate with severity of cribriform changes.

CONCLUSIONS

Based on our observations, we postulate that in the natural course of MPS, cribriform changes occurred first, followed by white-matter changes and, last, atrophy. More severe degrees of cribriform changes plus involvement of the corpus callosum may suggest a poorer prognosis. Optimal therapeutic intervention may be at the time of cribriform changes before atrophy has occurred. MR can define and grade these changes.

Contrast-enhanced MR angiography

Magnetic resonance (MR) angiography, although still in its infancy, is recognized as a valuable diagnostic tool. Investigation of the utility of contrast media as applied to MR angiography is, to no surprise, preliminary. In restricted instances, with present techniques, contrast media-enhanced MR angiography can provide additional valuable diagnostic information. Inspection of two-dimensional images (as opposed to three-dimensional projections) and comparison of MR images before and after administration of contrast agent are particularly important. Improved visualization of intracranial aneurysms, arteriovenous malformations, venous anomalies, and arterial occlusions has been demonstrated on three-dimensional time-of-flight MR angiograms after intravenous administration of a gadolinium chelate, relative to studies performed before administration of the contrast agent.

Cerebral infarction: assessment of patterns using ultra-fast MR contrast imaging

We describe a rapid MR imaging technique, applying functional analysis to images obtained during the tissue transit of injected contrast material into the cerebral circulation, which has potential for assessment of the altered hemodynamics in cerebral ischemia. This technique utilized turbo-FLASH imaging maximizing the T1 relaxivity properties of gadopentetate dimeglumine with positive contrast enhancement.

High-dose gadoteridol in MR imaging of intracranial neoplasms

Twelve patients with a high suspicion of brain metastases by previous clinical or radiologic examinations were studied in a phase III investigation with magnetic resonance (MR) imaging at 1.5 T after a bolus intravenous injection of 0.1 mmol/kg gadoteridol followed at 30 minutes by a second bolus injection of 0.2 mmol/kg gadoteridol. All lesions were best demonstrated (showed greatest enhancement) at the 0.3-mmol/kg (cumulative) dose, with image analysis confirming signal intensity enhancement in the majority of cases after the second gadoteridol injection. More lesions were detected with the 0.3-mmol/kg dose than with the 0.1-mmol/kg dose, and more lesions were detected with the 0.1-mmol/kg dose than on precontrast images. In this limited clinical trial, high-dose gadoteridol injection (0.3-mmol/kg cumulative dose) provided improved lesion detection on MR images specifically in intracranial metastatic disease.

Cerebral hemodynamics and cerebral blood volume: MR assessment using gadolinium contrast agents and T1-weighted Turbo-FLASH imaging

PURPOSE

To assess the degree and regional pattern of first-pass brain enhancement using dynamic MR imaging.

MATERIALS AND METHODS

Ultrafast MR imaging (1.06-second acquisition time per image) was performed in 19 healthy subjects following a bolus IV injection of a gadolinium contrast agent; 36 patients with suspected pathology were studied using the same protocol.

RESULTS

Calculated percent blood volumes were 4.9% for right cortical gray matter, 4.8% for left cortical gray matter, and 2.6% for white matter. Subtraction images were obtained that depicted the first pass "blood pool" pattern of enhancement (gray and white matter) which was significant.

CONCLUSION

Preliminary evidence suggests utility for cerebral "blood pool" imaging, especially if reduced image acquisition times can be achieved.

High-dose applications of gadolinium chelates in magnetic resonance imaging

Administration of gadolinium chelates at doses greater than 0.1 mmol/kg IV can potentially improve both lesion detection and the assessment of tissue perfusion. Preliminary results are presented in clinical patients and two animal models. In human intracranial metastatic disease, administration of 0.3 (cumulative dose) mmol/kg gadoteridol (Gd HP-DO3A) has permitted detection of additional lesions not visualized at 0.1 mmol/kg. In a rabbit model of focal liver disease, 0.5 mmol/kg IV provided superior enhancement of both normal parenchyma and lesion rim compared to doses of 0.25 and 0.1. Dynamic imaging (T1-weighted turbo-FLASH) immediately following bolus injection of 0.5 mmol/kg permitted direct visualization (on unsubtracted images) of an acute perfusion defect in the cat brain not visible on conventional T1- and T2-weighted scans.

Clinical comparison of three-dimensional MP-RAGE and FLASH techniques for MR imaging of the head

Three-dimensional (3D) MP-RAGE (magnetization-prepared rapid gradient-echo) imaging was evaluated as a high-resolution 3D T1-weighted brain imaging technique for patients with suspected neurologic disease. Fourteen patients were studied. In five, 3D MP-RAGE images were compared with 3D FLASH (fast low-angle shot) images. Signal difference--to-noise ratios and T1 contrast were not statistically different for 3D MP-RAGE images as opposed to 3D FLASH images. Advantages intrinsic to the application of 3D MP-RAGE sequences include decreased imaging time and decreased motion artifact. With this technique, it is possible to perform a relatively motion-insensitive, T1-weighted screening brain study with voxel resolution of 1.0 x 1.4 x 2.0 mm or smaller, in an imaging time of 5.9 minutes or less--permitting offline (poststudy) reconstruction of high-resolution images in any desired plane.

Basic principles of magnetic resonance contrast agents

The use of contrast agents in MRI is well established as a means to improve diagnosis. MRI differs from other imaging modalities because signal and contrast are multiparametric in both the properties of the tissue and the method of measurement. Contrast depends on differences in proton-spin density, magnetic susceptibility, molecular diffusion and perfusion, and T1 and T2 relaxation times. Relaxivity contrast agents, those that focus on shortening relaxation times, are most commonly employed in the form of paramagnetic chelates and depend on a variety of mechanisms, including concentration, number of ion-coordination sites, spin quantum number, magnetic moment, ion-to-proton distance, and correlation time constants characteristic of the chemical and molecular structure. A sound understanding of the principles of general contrast mechanisms, contrast agent design, and MRI techniques used in conjunction with contrast agents is vital to ensure proper enhancement and optimal diagnostic results.