W. Thomas Dixon

Incidental magnetization transfer contrast in standard multislice imaging

Radio frequency magnetic fields from 5 to 100 kHz off resonance reduce longitudinal magnetization of water in some tissues without affecting the magnetization of simple solutions (Wolff, S.D.; Balaban, R.S. Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo. Magn. Reson. Med. 10:135-144; 1989). We demonstrate that off resonant fields used in day to day multislice imaging have a significant effect. Extra slices reduced brain (gray and white matter) and muscle intensities 10%-20% relative to fat, water, and paramagnetic solutions, a larger effect than produced by slice overlap. Implications for image contrast and quantitation are discussed.

MRI techniques for cardiovascular imaging.

Over the last several years, cardiovascular MRI has benefited from a number of technical advances which have improved routine clinical imaging techniques. As a result, MRI is now well positioned to realize its longstanding promise of becoming the comprehensive cardiac imaging test of choice in many clinical settings. This may be achieved using a combination of basic advanced techniques. In this overview, the basic cardiac MRI techniques which are clinically useful are reviewed, and the recent technical advances which are clinically promising are described. These advances include routine black blood and cine bright blood techniques that are high speed (<10s per black blood image or cine slice), multislice whole heart perfusion imaging methods, and recently emerging real‐time imaging methodologies. J Magn. Reson. Imaging 1999;10:590–601.

Measuring diffusion in inhomogeneous systems in imaging mode using antisymmetric sensitizing gradients

Abstract A principle for self-diffusion pulse-sequence design which cancels harmful cross terms involving both imaging and sample susceptibility-induced gradients by using antisymmetric diffusion-sensitizing-gradient pulses is proposed. A water diffusion coefficient of 1.94 × 10−5 cm2 s−1 was obtained by the cross-term-free sequence, a bipolar sensitizing-gradient pulse (BGP) scheme, while the conventional method gave 2.22 and 1.60 × 10−5 cm2 s−1 with sensitizing gradient running parallel and antiparallel to the imaging gradient, respectively. A diffusion coefficient of 1.38 × 10−5 cm2 s−1 was obtained from an apple, in fair agreement with 1.56 × 10−5 cm2 s−1 for its juice, using the BGP sequence. Although the conventional technique gave similar values for apple juice, it gave only about half as large a value for the whole apple, presumably because of internal gradients from inhomogeneous susceptibility. The rms value of these local gradients in the apple was estimated as 6.0 kHz/cm in our 200 MHz instrument.

Magnetic resonance imaging of rectal cancer: staging and restaging evaluation

Abstract Magnetic resonance imaging is used to non-invasively stage and restage rectal adenocarcinomas. Accurate staging is important as the depth of tumor extension and the presence or absence of lymph node metastases determines if an individual will undergo preoperative neoadjuvant chemoradiation. Accurate description of tumor location is important for presurgical planning. The relationship of the tumor to the anal sphincter in addition to the depth of local invasion determines the surgical approach used for resection. High-resolution T2-weighted imaging is the primary sequence used for initial staging. The addition of diffusion-weighted imaging improves accuracy in the assessment of treatment response on restaging scans. Approximately 10%–30% of individuals will experience a complete pathologic response following chemoradiation with no residual viable tumor found in the resected specimen at histopathologic assessment. In some centers, individuals with no residual tumor visible on restaging MR who are thought to be at high operative risk are monitored with serial imaging and a “watch and wait” approach in lieu of resection. Normal rectal anatomy, MR technique utilized for staging and restaging scans, and TMN staging are reviewed. An overview of surgical techniques used for resection including newer, minimally invasive endoluminal techniques is included.

Colorectal Cancer Initial Diagnosis: Screening Colonoscopy, Diagnostic Colonoscopy, or Emergent Surgery, and Tumor Stage and Size at Initial Presentation

INTRODUCTION/BACKGROUND Rates of colorectal cancer screening are improving but remain suboptimal. Limited information is available regarding how patients are diagnosed with colorectal cancer (for example, asymptomatic screened patients or diagnostic workup because of the presence of symptoms). The purpose of this investigation was to determine how patients were diagnosed with colorectal cancer (screening colonoscopy, diagnostic colonoscopy, or emergent surgery) and tumor stage and size at diagnosis. PATIENTS AND METHODS Adults evaluated between 2011 and 2014 with a diagnosis of colorectal cancer were identified. Clinical notes, endoscopy reports, surgical reports, radiology reports, and pathology reports were reviewed. Sex, race, ethnicity, age at the time of initial diagnosis, method of diagnosis, presenting symptom(s), and primary tumor size and stage at diagnosis were recorded. Colorectal cancer screening history was also recorded. RESULTS The study population was 54% male (265 of 492) with a mean age of 58.9 years (range, 25-93 years). Initial tissue diagnosis was established at the time of screening colonoscopy in 10.7%, diagnostic colonoscopy in 79.2%, and during emergent surgery in 7.1%. Cancers diagnosed at the time of screening colonoscopy were more likely to be stage 1 than cancers diagnosed at the time of diagnostic colonoscopy or emergent surgery (38.5%, 7.2%, and 0%, respectively). Median tumor size was 3.0 cm for the screening colonoscopy group, 4.6 cm for the diagnostic colonoscopy group, and 5.0 cm for the emergent surgery group. At least 31% of patients diagnosed at the time of screening colonoscopy, 19% of patients diagnosed at the time of diagnostic colonoscopy, and 26% of patients diagnosed at the time of emergent surgery had never undergone a screening colonoscopy. CONCLUSION Nearly 90% of colorectal cancer patients were diagnosed after development of symptoms and had more advanced disease than asymptomatic screening patients. Colorectal cancer outcomes will be improved by improving rates of colorectal cancer screening.

NMR measurements of different tissue perfusion parameters

The traditional perfusion rate is based on the amount of fluid flowing longitudinally down the capillaries, often given in ml/100 g tissue/min. In principle, fluid flowing radially out of or into the capillaries can be measured as well. Radial and longitudinal perfusion have the same units but independent values. If spins leave and reenter a capillary several times during a transit, radial perfusion can exceed longitudinal perfusion and it probably does. Radial perfusion measures capillary wall function. Progressively increasing the time between two gradient pulses in a proposed radial perfusion imaging sequence allows more magnetization to move into the vasculature to be dephased or removed. Progressive decreases in gradient strength keep diffusion effects constant allowing isolation of the desired perfusion effects.

Referenced MR thermometry using three-echo phase-based fat water separation method

A three-point image reconstruction method for internally referenced MR thermometry was developed. The technique exploits the fact that temperature-induced changes in the water resonance frequency are small relative to the chemical shift difference between water and fat signals. This property enabled the use of small angle approximations to derive an analytic phase-based fat-water separation method for MR thermometry. Ethylene glycol and cream cool-down experiments were performed to validate measurement technique. Over a cool-down temperature range of 20 °C, maximum deviation between probe and MR measurement (averaged over 1.3 cm3 region surrounding probe) was 0.6 °C and 1.1 °C for ethylene glycol and cream samples, respectively.