Use of MR imaging contrast agents in the brain

Evolving Characteristics of Gadolinium-Based Contrast Agents for MR Imaging: A Systematic Review of the Importance of Relaxivity

Gadolinium-based contrast agents (GBCAs) are widely and routinely used to enhance the diagnostic performance of magnetic resonance imaging and magnetic resonance angiography examinations. T1 relaxivity (r1) is the measure of their ability to increase signal intensity in tissues and blood on T1-weighted images at a given dose. Pharmaceutical companies have invested in the design and development of GBCAs with higher and higher T1 relaxivity values, and "high relaxivity" is a claim frequently used to promote GBCAs, with no clear definition of what "high relaxivity" means, or general concurrence about its clinical benefit. To understand whether higher relaxivity values translate into a material clinical benefit, well-designed, and properly powered clinical studies are necessary, while mere in vitro measurements may be misleading. This systematic review of relevant peer-reviewed literature provides high-quality clinical evidence showing that a difference in relaxivity of at least 40% between two GBCAs results in superior diagnostic efficacy for the higher-relaxivity agent when this is used at the same equimolar gadolinium dose as the lower-relaxivity agent, or similar imaging performance when used at a lower dose. Either outcome clearly implies a relevant clinical benefit. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 3.

A proposal for a tailored protocol for focal suppurative infection of the central nervous system: analysis of an institutional experience in pediatric patients

OBJECTIVE

The incidence of focal suppurative infections (FSIs) of the brain has significantly decreased owing to the better health and fundamental conditions of the population on the one hand and earlier detection and the availability of more potent antibiotics on the other. Interestingly, the antibiotic protocols have not been well defined in terms of duration despite a prompter diagnosis, definitive management of the etiology, and the advent of various higher-generation antibiotics. In this study, the authors evaluated the current treatment protocol. Their aim was to optimize management protocols for FSIs of the central nervous system based on clinical parameters.

METHODS

The study was a retrospective analysis of all children who had undergone surgical management for an FSI at the Division of Paediatric Neurosurgery, Amrita Institute of Medical Sciences and Research Centre, Kochi, Kerala, in the period from January 2001 to February 2018. In addition to demographic characteristics, the details of culture reports and antibiotic regimens were collected. The response to treatment was compared to changes in the infective profile (C-reactive protein) and imaging. Instances of reaspiration were compared with clinical signs, imaging findings, and infective profiles. Treatment response was separated into two groups: responders within or at 2 weeks and responders beyond 2 weeks. The clinical characteristics of these two groups were compared.

RESULTS

Forty-eight children were treated in the study period. Nineteen patients benefited from the 2-week (short-term) protocol of intravenous antibiotics. Twenty-nine patients required more than 2 weeks (approximately 4 weeks; long-term protocol) for resolution. Of those requiring more than 2 weeks, 69% had cardiogenic etiology. All patients were followed up with a minimum of 3 weeks of oral antibiotics. In a comparative analysis between short-term and long-term responders, only etiology was significantly different. None of the patients who had the short-term protocol had a recurrence.

CONCLUSIONS

A shorter antibiotic protocol can be used successfully in carefully selected patients who are surgically treated and followed up. It is clear that the 2-week intravenous antibiotic protocol is more suitable for immunocompetent patients who have a noncardiogenic etiology.

MR imaging evaluation of seizures

It is imperative for a radiologist to determine the type of seizure a patient has prior to magnetic resonance (MR) imaging to optimally provide the clinician with the information he or she requires. Specifically, complex partial seizures require evaluation of the frontal lobes and the hippocampus (for mesial temporal sclerosis). These are best evaluated with fluid-attenuated inversion recovery (FLAIR) imaging; the use of intravenously administered contrast material is not required. Other types of chronic seizures are best evaluated with nonenhanced FLAIR or T2-weighted imaging for low-grade tumors, vascular malformations, gliosis after infarction, inflammation, or trauma. The presence of new-onset seizures in an adult or the worsening of chronic seizures warrants T2-weighted or FLAIR imaging and gadolinium-enhanced T1-weighted imaging (to look for primary or metastatic tumors, infections, or inflammatory lesions). If available, echo-planar diffusion imaging should be used also (to look for acute infarcts).

MRI measurement of brain tumor response: comparison of visual metric and automatic segmentation

An automatic magnetic resonance imaging (MRI) multispectral segmentation method and a visual metric are compared for their effectiveness to measure tumor response to therapy. Automatic response measurements are important for multicenter clinical trials. A visual metric such as the product of the largest diameter and the largest perpendicular diameter of the tumor is a standard approach, and is currently used in the Radiation Treatment Oncology Group (RTOG) and the Eastern Cooperative Oncology Group (EGOG) clinical trials. In the standard approach, the tumor response is based on the percentage change in the visual metric and is categorized into cure, partial response, stable disease, or progression. Both visual and automatic methods are applied to six brain tumor cases (gliomas) of varying levels of segmentation difficulty. The analyzed data were serial multispectral MR images, collected using MR contrast enhancement. A fully automatic knowledge guided method (KG) was applied to the MRI multispectral data, while the visual metric was taken from the MRI films using the T1 gadolinium enhanced image, with repeat measurements done by two radiologists and two residents. Tumor measurements from both visual and automatic methods are compared to "ground truth," (GT) i.e., manually segmented tumor. The KG method was found to slightly overestimate tumor volume, but in a consistent manner, and the estimated tumor response compared very well to hand-drawn ground truth with a correlation coefficient of 0.96. In contrast, the visually estimated metric had a large variation between observers, particularly for difficult cases, where the tumor margins are not well delineated. The inter-observer variation for the measurement of the visual metric was only 16%, i.e., observers generally agreed on the lengths of the diameters. However, in 30% of the studied cases no consensus was found for the categorical tumor response measurement, indicating that the categories are very sensitive to variations in the diameter measurements. Moreover, the method failed to correctly identify the response in half of the cases. The data demonstrate that automatic 3D methods are clearly necessary for objective and clinically meaningful assessment of tumor volume in single or multicenter clinical trials.

Evaluation of the clinical safety of gadodiamide injection, a new nonionic MRI contrast medium for the central nervous system: a European perspective

Gadodiamide injection is a new nonionic paramagnetic, extracellular contrast medium. Its safety at a dose of 0.1 mmol/kg body weight was evaluated in a large European multicentre trial on adults referred for contrast-enhanced MRI of the central nervous system. Safety analysis was performed on 2102 patients, in whom adverse events during and up to 24 h after injection were recorded. Adverse events related or possibly related to gadodiamide injection were observed in 102 patients. Injection-associated reactions classified as discomfort (sensation of heat or coldness, pain or pressure at the injection site) occurred in 37 patients (1.8%) and other adverse events (e.g. headache, nausea) were observed in 65 patients (3.1%). No serious adverse event was reported. Efficacy analysis, performed on 2273 patients, and based on comparison of T1- and T2-weighted images before and T1-weighted images after injection showed that more diagnostic information was obtained after gadodiamide injection in 1424 (62.6%) patients: management of 386 (17.0%) patients was affected by the new information given and that a new diagnosis was made in 755 (33.3%) patients. Gadodiamide injection was shown to be safe and well tolerated. It represents a nonionic alternative to the current products for MRI of the central nervous system.

Neurotolerability of nonionic X-ray contrast media. The role of chemotoxicity

RATIONALE AND OBJECTIVES

Because small quantities of x-ray contrast agents can cross the blood-brain barrier, the authors evaluate the properties that contribute to neurotoxicity.

METHODS

The acute toxicity of various monomer and dimer contrast media was assessed after intracerebroventricular (ICV) injection to mice and intracisternal (ICI) injection to rats.

RESULTS

In mice, median lethal dose (LD50) values for monomer contrast media apart from iohexol were higher than those for dimer contrast media. In rats, iopentol and iopromide were more neurotoxic than all other contrast media. The signs of toxicity for all contrast media included convulsions, dyspnea, hypoactivity, and sedation. Hypertonic D-mannitol solution was tolerated as well as artificial cerebrospinal fluid. Neither the hydrophilicity of the molecules nor the physicochemical properties of their solutions explain the toxicities satisfactorily.

CONCLUSIONS

Neurotoxicity of monomer or dimer contrast media depends more on chemical structure characteristics other than hydrophilicity than on the physicochemical characteristics of their solutions.

The use of gadolinium-BOPTA on magnetic resonance imaging in brain infection

RATIONALE AND OBJECTIVES

The use of gadolinium (Gd)-BOPTA as a magnetic resonance contrast agent for central nervous system disease was studied in a canine brain abscess model.

METHODS

A Streptococcus faecalis brain abscess was evaluated in five dogs at 1.5T. Imaging was performed during the late cerebritis stage, at 5 to 7 days after surgery. Magnetic resonance scans were acquired before and at 1, 5, 15, 30, 45, and 60 minutes after contrast administration, using a dose of 0.1 mmol/kg. Scans also were acquired both before and after contrast injection with the implementation of magnetization transfer.

RESULTS

Lesion enhancement, quantified by region-of-interest measurement, peaked at 5 minutes after contrast injection. Both the increase in lesion enhancement from 1 to 5 minutes after injection and the decrease from 5 to 15 minutes after injection, although small, were statistically significant (P < 0.004 and P < 0.03, respectively). The application of magnetization transfer improved lesion enhancement, as measured by signal difference/noise, by 39%. This result also was statistically significant (P < 0.001).

CONCLUSIONS

In intraparenchymal brain infection, Gd-BOPTA provides effective lesion enhancement when used at a dose of 0.1 mmol/kg. Further research is needed to compare the magnitude of enhancement achieved with Gd-BOPTA, which has weak protein binding and both hepatobiliary and renal excretion, with that with Gd chelates, which have pure renal excretion.

Magnetic resonance imaging of the brain with gadopentetate dimeglumine-DTPA: comparison of T1-weighted spin-echo and 3D gradient-echo sequences

Short TR, short TE, high resolution, 3D gradient-recalled echo (GRE) imaging was evaluated for lesion detection in the brain. High resolution 3D GRE data acquisition was used to reduce partial volume effects and flow artifacts, to better visualize smaller structures, to minimize signal losses caused by field inhomogeneities, and to allow better image reformatting. Spin-echo (SE) and 3D GRE approaches were compared for lesion detection after the administration of an MR contrast agent, gadopentetate dimeglumine. Preliminary clinical studies demonstrated that the signal-to-noise ratio (SNR) in each slice of the GRE scan was worse than that of the SE scan because of the much thicker slices acquired with the SE technique. However, by averaging two adjacent 3D slices, the SNR of the two methods was essentially equivalent. In the averaged GRE slices, large lesions were seen just as well as in the SE images. More importantly, small lesions were better visualized in the thin 3D GRE images than in the thick SE images for the lesions studied in this work and the protocols used. These observations were confirmed by theoretical simulations.

MRI segmentation: methods and applications

The current literature on MRI segmentation methods is reviewed. Particular emphasis is placed on the relative merits of single image versus multispectral segmentation, and supervised versus unsupervised segmentation methods. Image pre-processing and registration are discussed, as well as methods of validation. The application of MRI segmentation for tumor volume measurements during the course of therapy is presented here as an example, illustrating problems associated with inter- and intra-observer variations inherent to supervised methods.

Dynamics of tumor imaging with Gd-DTPA-polyethylene glycol polymers: dependence on molecular weight

Macromolecular contrast media offer potential advantages over freely diffusible agents in magnetic resonance (MR) imaging outside the central nervous system. To identify an optimum molecular weight for macromolecular contrast media, the authors studied a novel macromolecular contrast agent, gadolinium diethylenetriaminepentaacetic acid polyethylene glycol (DTPA-PEG), synthesized in seven polymer (average) molecular weights ranging from 10 to 83 kd. Twenty-eight rabbits bearing V2 carcinoma in thighs underwent T1-weighted spin-echo imaging before injection and 5-60 minutes and 24 hours after injection of the Gd-DTPA-PEG polymers or Gd-DTPA at a gadolinium dose of 0.1 mmol/kg. Tumor region-of-interest measurements were obtained at each time point to determine contrast enhancement dynamics. Blood-pool enhancement dynamics were observed for the Gd-DTPA-PEG polymers larger than 20 kd. Polymers smaller than 20 kd displayed dynamics similar to those of the freely diffusible agent Gd-DTPA. Above the 20 kd threshold, tumor enhancement was more rapid for smaller polymers. The authors conclude that the 21.9-kd Gd-DTPA-PEG polymer is best suited for clinical MR imaging.