Andrew C. Larson

Self‐gated cardiac cine MRI

The need for ECG gating presents many difficulties in cardiac magnetic resonance imaging (CMRI). Real‐time imaging techniques eliminate the need for ECG gating in cine CMRI, but they cannot offer the spatial and temporal resolution provided by segmented acquisition techniques. Previous MR signal‐based techniques have demonstrated an ability to provide cardiac gating information; however, these techniques result in decreased imaging efficiency. The purpose of this work was to develop a new “self‐gated” (SG) acquisition technique that eliminates these efficiency deficits by extracting the motion synchronization signal directly from the same MR signals used for image reconstruction. Three separate strategies are proposed for deriving the SG signal from data acquired using radial k‐space sampling: echo peak magnitude, kymogram, and 2D correlation. The SG techniques were performed on seven normal volunteers. A comparison of the results showed that they provided cine image series with no significant differences in image quality compared to that obtained with conventional ECG gating techniques. SG techniques represent an important practical advance in clinical MRI because they enable the acquisition of high temporal and spatial resolution cardiac cine images without the need for ECG gating and with no loss in imaging efficiency. Magn Reson Med 51:93–102, 2004.

Preliminary investigation of respiratory self-gating for free-breathing segmented cine MRI.

Segmented cine MRI generally requires breath‐holding, which can be problematic for many patients. Navigator echo techniques, particularly successful for free‐breathing coronary MRA, are incompatible with the acquisition strategies and SSFP pulse sequences commonly used for cine MRI. The purpose of this work is to introduce a new self‐gating technique deriving respiratory gating information directly from the raw imaging data acquired for segmented cine MRI. The respiratory self‐gating technique uses interleaved radial k‐space sampling to provide low‐resolution images in real time during the free‐breathing acquisition that are compared to target expiration images. Only the raw data‐producing images with high correlation to the target images are included in the final high‐resolution reconstruction. The self‐gating technique produced cine series with no significant differences in quantitative image sharpness to series produced using comparable breath‐held techniques. Because of the difficulties associated with breath‐holding, the respiratory self‐gating technique represents an important practical advance for cardiac MRI. Magn Reson Med 53:159–168, 2005. Published 2004 Wiley‐Liss, Inc.

Irreversible Electroporation Therapy in the Liver: Longitudinal Efficacy Studies in a Rat Model of Hepatocellular Carcinoma

Irreversible electroporation (IRE) is an innovative local-regional therapy that involves delivery of intense electrical pulses to tissue to induce nanoscale cell membrane defects for tissue ablation. The purpose of this study was to investigate the feasibility of using IRE as a liver-directed ablation technique for the treatment of hepatocellular carcinoma (HCC). In the N1-S1 rodent model, hepatomas were grown in 30 Sprague-Dawley rats that were divided into treatment and control groups. For treatment groups, IRE electrodes were inserted and eight 100-mus 2,500-V pulses were applied to ablate the targeted tumor tissues. For both groups, magnetic resonance imaging scans were performed at baseline and 15-day follow-up intervals to determine tumor sizes (one-dimensional maximum diameter, D(max); estimated two-dimensional cross-sectional area, C(max)) as a tactic to assess longitudinal outcomes. Additional groups of treated animals were sacrificed at 1-, 3-, and 7-day intervals posttherapy for pathology assessment of treatment response. Magnetic resonance images showed significant tumor size reductions within 15 days posttherapy (32 +/- 31% D(max) and 52 +/- 39% C(max) decreases compared with 110 +/- 35% D(max) and 286 +/- 125% C(max) increases for untreated tumors). Pathology correlation studies documented progression from poorly differentiated viable HCC tissues before treatment to extensive tumor necrosis and full regression in 9 of 10 treated rats 7 to 15 days after treatment. Our findings suggest that IRE can be an effective strategy for targeted ablation of liver tumors, prompting its further evaluation for HCC therapy.

Automated rectilinear self-gated cardiac cine imaging.

ECG‐based gating in cardiac MR imaging requires additional patient preparation time, is susceptible to RF and magnetic interference, and is ineffective in a significant percentage of patients. “Wireless” or “self‐gating” techniques have been described using either interleaved central k‐space lines or projection reconstruction to obtain MR signals synchronous with the cardiac cycle. However, the interleaved, central line method results in a doubling of the acquisition time, while radial streak artifacts are encountered with the projection reconstruction method. In this work, a new self‐gating technique is presented to overcome these limitations. A retrospectively gated TrueFISP cine sequence was modified to acquire a short second echo after the readout and phase gradients are rewound. The information obtained from this second echo was used to derive a gating signal. This technique was compared to ECG‐based gating in 10 healthy volunteers and shown to have no significant difference in image quality. The results indicate that this method could serve as an alternative gating strategy without the need for external physiological signal detection. Magn Reson Med 52:782–788, 2004.

Multimodality Imaging Following 90Y Radioembolization: A Comprehensive Review and Pictorial Essay

Radioembolization with yttrium 90 (90Y) microspheres represents an emerging transarterial therapy for the treatment of liver malignancies that continues to generate interest in the medical community. The classic indication of treatment response is a reduction in tumor size; however, parenchymal changes (eg, necrosis, lack of enhancement, specific findings at positron emission tomography and functional magnetic resonance imaging) and other benign findings (pleural effusions, perivascular edema, contralateral hypertrophy, ring enhancement, perihepatic fluid, fibrosis) may occur following treatment, requiring proper image interpretation. With classic imaging findings and surrogates (time to progression, duration of response, disease-free interval), response rates range from 20% to 80% in patients treated for hepatocellular carcinoma or metastatic disease to the liver. Complications of 90Y radioembolization include cholecystitis, abscess, and bilomas and should be recognized early in the imaging follow-up of these patients. Radiologists who are involved in the posttreatment assessment of patients undergoing 90Y radioembolization should be familiar with the imaging findings and potential imaging pitfalls associated with this therapy.

Diffusion-weighted MR Imaging for Determination of Hepatocellular Carcinoma Response to Yttrium-90 Radioembolization

Early detection of the response of hepatocellular carcinoma (HCC) to yttrium-90 radioembolization therapy may be important to permit repeat radioembolization or alternative treatment options. Water-mobility measurements with use of diffusion-weighted (DW) magnetic resonance (MR) imaging are useful for noninvasive interrogation of microstructural tissue properties. Findings of DW MR imaging may serve as an early biomarker of HCC response. This study tested the hypothesis that DW MR imaging can detect changes in tumor tissue water diffusion in response to (90)Y therapy. In each of six patients with HCC included in the study, tumor water diffusion increased significantly after therapy. DW MR imaging is a promising technique for noninvasive assessment of tumor response to (90)Y radioembolization.

Imaging of Hepatocellular Carcinoma After Treatment with Yttrium-90 Microspheres

OBJECTIVE Yttrium-90 radioembolization is an emerging therapy for unresectable hepatocellular carcinoma (HCC). Although therapeutic response based on size has been evaluated in numerous studies, necrosis has been used as a criterion of response in only a few studies. The purpose of our study was to describe the imaging features of HCC after 90Y treatment and to compare size criteria (World Health Organization [WHO] and Response Evaluation Criteria in Solid Tumors [RECIST]) with necrosis criteria and combined criteria (RECIST and necrosis) for assessment of response. MATERIALS AND METHODS CT images of 42 patients with 76 90Y-treated HCC lesions were analyzed. We used four response criteria: WHO size, RECIST size, necrosis, and combined criteria (RECIST and necrosis). Imaging features of treated lesions included both nodular and peripheral rim enhancement. Survival was assessed with the Kaplan-Meier method. RESULTS The response rate was 23% according to RECIST criteria, 26% according to WHO criteria, 57% according to necrosis criteria, and 59% according to combined criteria. Response according to necrosis and combined criteria was detected earlier than response according to size criteria alone. Ten responding lesions initially increased in size. After therapy, enhancing peripheral nodules increased in size in 10 lesions, decreased in size in two lesions, and disappeared in two lesions. Twenty-one of 25 lesions with thin rim enhancement after 90Y administration responded to treatment. The median survival times were 660 and 236 days for Okuda stage I and Okuda stage II disease, respectively. CONCLUSION Use of combined size and necrosis criteria may lead to more accurate assessment of response to 90Y therapy than use of size criteria alone. Imaging features after 90Y treatment, including size, necrosis, peripheral enhancing nodules, and thin rim enhancement, are described.

Motion-Corrected Free-Breathing Delayed Enhancement Imaging of Myocardial Infarction

Following administration of Gd‐DTPA, infarcted myocardium exhibits delayed enhancement and can be imaged using an inversion‐recovery sequence. A conventional segmented acquisition requires a number of breath‐holds to image the heart. Single‐shot phase‐sensitive inversion‐recovery (PSIR) true‐FISP may be combined with parallel imaging using SENSE to achieve high spatial resolution. SNR may be improved by averaging multiple motion‐corrected images acquired during free breathing. PSIR techniques have demonstrated a number of benefits including consistent contrast and appearance over a relatively wide range of inversion recovery times (TI), improved contrast‐to‐noise ratio, and consistent size of the enhanced region. Comparison between images acquired using segmented breath‐held turbo‐FLASH and averaged, motion‐corrected, free‐breathing true‐FISP show excellent agreement of measured CNR and infarct size. In this study, motion correction was implemented using image registration postprocessing rather than navigator correction of individual frames. Navigator techniques may be incorporated as well. Magn Reson Med 53:194–200, 2005. Published 2004 Wiley‐Liss, Inc.

Tumor Response after Yttrium-90 Radioembolization for Hepatocellular Carcinoma: Comparison of Diffusion-weighted Functional MR Imaging with Anatomic MR Imaging

PURPOSE Anatomic magnetic resonance (MR) imaging assessment of hepatocellular carcinoma (HCC) response to yttrium-90 ((90)Y) radioembolization may require 3 months before therapeutic effectiveness can be determined. The relationship between anatomic MR and diffusion-weighted imaging (DWI) changes after (90)Y therapy is unclear. The present study tested the hypothesis that apparent diffusion coefficient (ADC) values on DWI at 1 month precede anatomic tumor size change at 3 months after (90)Y radioembolization. MATERIALS AND METHODS In this prospective study, 20 patients with HCC (16 men) enrolled between April 2005 and July 2006 underwent lobar (90)Y therapy with mean doses of 141 Gy (right lobe) and 98 Gy (left lobe). Anatomic 1.5-T MR imaging (gadolinium-enhanced T1-weighted gradient-recalled echo) and DWI (single-shot spin-echo echo-planar imaging; b value of 0, 500 sec/mm(2)) were performed at baseline (0-3 weeks before (90)Y therapy) and at 1 and 3 months after (90)Y therapy. Tumor size and ADC values were measured and compared, and the percentage change in ADC was compared to the change in tumor size (minimum >5% change in size), with use of a paired t test (alpha = .05). RESULTS Yttrium-90 therapy was successfully delivered in all patients. The mean baseline ADC of 1.64 x 10(-3) mm(2)/sec +/- 0.30 significantly increased to 1.81 x 10(-3) mm(2)/sec +/- 0.37 at 1 month (P = .02), and to 1.82 x 10(-3) mm(2)/sec +/- 0.23 at 3 months (P = .02). The mean baseline tumor size of 83.0 cm(2) +/- 63.7 did not change statistically at 1 month (84.1 cm(2) +/- 62.1; P = .75) or 3 months (74.0 cm(2) +/- 57.0; P = .10). The overall mean ADC percentage change at 1 month of 10.5% +/- 23.1% preceded an overall mean tumor size percentage change at 3 months of -18.5% +/- 31.5% (P = .03). CONCLUSIONS HCC tumor response assessed with DWI at 1 month preceded anatomic size changes at 3 months after (90)Y therapy. DWI may assist in early determination of the response or failure of (90)Y therapy for HCC.

Multishot diffusion-weighted PROPELLER magnetic resonance imaging of the abdomen

Objective:The objective of this study was to evaluate the feasibility of using multishot PROPELLER for diffusion-weighted imaging (DWI) of the abdomen. Materials and Methods:Diffusion-weighted abdominal imaging was performed in 9 healthy volunteers and 3 patients using both single-shot DW-SE-EPI and multishot DW-PROPELLER (BLADE sequence). We compared ADC measurements in phantoms, liver and pancreatic tissues and performed qualitative comparisons of the diffusion-weighted images and ADC maps provided by these 2 techniques. Results:DW-PROPELLER significantly improved image quality (P < 0.05) with reduced geometric distortion and artifact. The ADC values of phantoms and abdominal organs measured by DW-PROPELLER were generally greater than those measured by single-shot DW-SE-EPI. The ADC values measured by both DWI techniques were significantly different for liver tissues but not for pancreatic tissues (P < 0.05). Preliminary patient studies demonstrated clearly distinguished lesion areas from surrounding normal liver tissues in the DW-PROPELLER images. DW-PROPELLER offers the potential for high-resolution DWI of the abdomen. Conclusions:The multishot DW-PROPELLER sequence is a promising technique for DWI of abdominal organs. Future clinical studies will evaluate the use of DW-PROPELLER technique for abdominal oncologic imaging applications.