John E. Kirsch

An Image-based Approach to Understanding the Physics of MR Artifacts

As clinical magnetic resonance (MR) imaging becomes more versatile and more complex, it is increasingly difficult to develop and maintain a thorough understanding of the physical principles that govern the changing technology. This is particularly true for practicing radiologists, whose primary obligation is to interpret clinical images and not necessarily to understand complex equations describing the underlying physics. Nevertheless, the physics of MR imaging plays an important role in clinical practice because it determines image quality, and suboptimal image quality may hinder accurate diagnosis. This article provides an image-based explanation of the physics underlying common MR imaging artifacts, offering simple solutions for remedying each type of artifact. Solutions that have emerged from recent technologic advances with which radiologists may not yet be familiar are described in detail. Types of artifacts discussed include those resulting from voluntary and involuntary patient motion, magnetic susceptibility, magnetic field inhomogeneities, gradient nonlinearity, standing waves, aliasing, chemical shift, and signal truncation. With an improved awareness and understanding of these artifacts, radiologists will be better able to modify MR imaging protocols so as to optimize clinical image quality, allowing greater confidence in diagnosis.

Evaluation of a modified Stejskal-Tanner diffusion encoding scheme, permitting a marked reduction in TE, in diffusion-weighted imaging of stroke patients at 3 T.

Purpose:To evaluate a modified Stejskal-Tanner diffusion gradient pulsing scheme that applies diffusion encoding during the entire time between the 2 requisite radiofrequency pulses, shortening TE. Materials and Methods:Seventeen healthy volunteers and 15 patients with acute and early subacute infarcts were evaluated at 3 T utilizing: a conventional bipolar gradient double spin echo planar imaging diffusion weighted imaging with a parallel imaging factor of 2 (p2) and a modified Stejskal-Tanner short TE (sTE) SE echo planar imaging diffusion weighted imaging with parallel imaging factors of 2, 3, and 4. Signal-to-noise ratio (SNR) and susceptibility-induced spatial distortions were quantified, and a blinded reader ranked scans in terms of susceptibility artifact and overall preference. Results:The sTE sequence allowed a shortening in TE of 18 to 28 milliseconds versus the standard bipolar gradient sequence. SNRs were generally not significantly different among the sTE scans because of compensation by number of scan averages. By using twice the number of averages, the SNR with the bipolar gradient sequence was not significantly different from that of the sTE sequences in patients. sTE scans with higher parallel imaging factors demonstrated less susceptibility-related artifact. The blinded reader ranked the p3 or p4 sTE scans most preferred and the bipolar gradient scans least or tied for least preferred in every case. Conclusions:Utilization of the sTE modified Stejskal-Tanner sequence markedly improves SNR—an increase that may be used with parallel imaging to improve overall scan quality whereas maintaining reasonable scan times and SNR.

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 (Brodmanns 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.

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 (IMT), and temporal delay on the visualization of contrast enhancement with gadotcridol (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 highcontrast 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, NT, 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.