This article reviews the requirements for successful compressed sensing (CS), describes their natural fit to MRI, and gives examples of four interesting applications of CS in MRI. The authors emphasize on an intuitive understanding of CS by describing the CS reconstruction as a process of interference cancellation. There is also an emphasis on the understanding of the driving factors in applications, including limitations imposed by MRI hardware, by the characteristics of different types of images, and by clinical concerns.

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Compressed Sensing MRI

Productive research in patients with peritoneal carcinomatosis has moved slowly over several decades. In large part this was due to lack of standardized assessments by which cancer responses could be quantitated in a reliable and statistically evaluable manner. Recently, both laboratory and clinical assessments have been developed. Clinical research has shown that the prognosis of patients with peritoneal carcinomatosis is dependent upon preoperative and intraoperative variables [1]. These are the histologic grade of the malignancy, the preoperative tumor volume as assessed at surgery, the distribution of tumor at surgery, and the completeness of cytoreductive surgery. Also the preoperative abdominal and pelvic computed tomography (CT) scan has shown predictive value in assessment of the completeness of cytoreduction in patients with mucinous carcinomatosis [2]. The clinical research tools that are currently used in peritoneal carcinomatosis studies are reviewed in this manuscript.

Clinical research methodologies in diagnosis and staging of patients with peritoneal carcinomatosis

Iron-chelating therapy and the treatment of thalassemia

Liver resection currently represents the only potentially curative therapeutic option for hepatic colorectal metastasis (CRM), and 5-year survival rates of 25% to 58% have been reported.1–6 Traditionally, primary tumor stage, preoperative carcinoembryonic antigen (CEA) level, hepatic tumor size, number of hepatic metastases, time from primary tumor treatment to diagnosis of hepatic metastases, and presence of extrahepatic disease have been reported to be independent predictors of survival after resection.7,8

Surgical margin status is an additional factor that has been evaluated for its influence on long-term survival after resection of CRM, but its significance remains controversial. Several series concerning liver resection for colorectal liver metastasis have reported that surgical margins of less than 1 cm are an absolute9,10 or relative contraindication to surgery.11 Cady et al10 have reported that a surgical margin less than 1 cm was associated on univariate analysis with a significantly shorter disease-free survival. As a result, major centers have adopted a 1-cm margin as a target during resection to minimize hepatic recurrence and improve survival after resection of CRM.12,13 In fact, a 1-cm margin has been proposed as the minimally acceptable margin even for ablative techniques.14,15

Despite the emphasis on a 1-cm margin, some investigators16 have reported that the actual width of the surgical margin has no effect on survival as long as the margin is negative. Altendorf-Hofmann and Scheele16 noted that patients with a microscopically positive margin (R1) had a worse prognosis compared with patients who had a microscopically negative margin (R0), but survival was not associated with the width of the negative surgical margin. More recently, Adam et al17 reported a 5-year survival rate of 33% in 138 patients, among whom 67% had less than 1-mm surgical margins. All of these studies, however, examined only the effect of surgical margin status on survival but not local recurrence.

If margin status or width of margin is important, it has practical implications. Specifically, margin considerations may dictate which patients are resectable, the extent of resection, and the treatment of residual positive margins at the time of surgery.10,18 The objective of the current study was to evaluate the influence of surgical margin status on survival and site of recurrence in patients treated with hepatic resection for CRM at 3 hepatobiliary centers.

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Effect of surgical margin status on survival and site of recurrence after hepatic resection for colorectal metastases.

An MR angiographic technique, referred to as 3D TRICKS (3D time-resolved imaging of contrast kinetics) has been developed. This technique combines and extends to 3D imaging several previously published elements. These elements include an increased sampling rate for lower spatial frequencies, temporal interpolation of k-space views, and zero-filling in the slice-encoding dimension. When appropriately combined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2-6 s. Acquiring a temporal series of images offers advantages over the current contrast-enhanced 3D MRA techniques in that it I) increases the likelihood that an arterial-only 3D image set will be obtained. II) permits the passage of the contrast agent to be observed, and III) allows temporal-processing techniques to be applied to yield additional information, or improve image quality.

Time-resolved contrast-enhanced 3D MR angiography

Magnetic resonance (MR) imaging methods with good spatial and contrast resolution are often too slow to follow the uptake of contrast agents with the desired temporal resolution. Imaging can be accelerated by skipping the acquisition of data normally taken with strong phase-encoding gradients, restricting acquisition to weak-gradient data only. If the usual procedure of substituting zeros for the missing data is followed, blurring results. Substitutinit instead reference data taken before or well after contrast agent injection reduces this problem. Volunteer and patient images obtained by using such reference data show that imaging can be us~eiklly accelerated severalfold. Cortical and medullary regions of interest and whole kidney regions were studied, and both gradientanld spin-echo images are shown. The method is believed to be coimpatible with other acceleration methods such as half-Fourier reconstruction and reading of mare than one line of k space per excitation.

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“Keyhole” method for accelerating imaging of contrast agent uptake

Forty-three patients with known primary or secondary neoplastic involvement of the liver underwent evaluation to determine the number, size, and location of focal lesions before possible tumor resection. Imaging studies included computed tomography (CT) during arterial portography (CTAP), delayed CT, and magnetic resonance (MR) imaging at various pulse sequences. Results of radiologic studies were compared with surgical and pathologic findings. In the combined group of surgical and nonsurgical patients, CTAP was significantly more sensitive (85%) than all other techniques except 1.5-T T2-weighted spin-echo imaging (64%). Combining the information from all MR pulse sequences yielded a cumulative sensitivity of 68%. Combining the information from two modalities yielded sensitivity of 96% for CTAP plus MR imaging, 85% for CTAP plus delayed CT, and 77% for delayed CT plus MR imaging. The authors conclude that when it is vital to know the precise number, size, and location of focal hepatic lesions before tumor...

Hepatic tumors: comparison of CT during arterial portography, delayed CT, and MR imaging for preoperative evaluation.

The purpose of our study was to determine the frequency of detection of small hepatic lesions (less than or equal to 15 mm) in outpatients who had abdominal CT and to assess the significance of these lesions in the presence or absence of known malignant tumors. Contrast-enhanced abdominal CT scans in 1454 patients were reviewed. In 254 patients (17%), hepatic lesions 15 mm or smaller were detected. In 51% of these patients, lesions were judged benign on the basis of other imaging studies, biopsy results, or stability for at least 6 months as shown by CT. Lesions were judged malignant on the basis of progression seen on radiologic studies or biopsy in 22%. The other 27% of the patients had lesions that could not be classified. The majority of patients with small hepatic lesions (82%) were known to have a malignant tumor; in 51% of these patients, lesions were diagnosed as benign. No patient without a known malignant tumor had a small hepatic lesion that was determined to be malignant. Multiple small lesions were more likely to represent malignant disease than were single small lesions. We conclude that small hepatic lesions are common (seen in 17% of our patients), and that there is a high probability that hepatic lesions smaller than 15 mm are benign, even in patients known to have an extrahepatic malignant tumor.

The frequency and significance of small (less than or equal to 15 mm) hepatic lesions detected by CT.

Diagnostic approach to hepatic hemangiomas.

The authors retrospectively studied 36 hepatic masses in 20 patients who underwent computed tomography during arterial portography (CTAP) and subsequent hepatic tumor resection. The authors used the right main and left main portal veins as landmarks for the transverse scissura, along with the hepatic veins, the gallbladder fossa, and the umbilical fissure to blindly predict the segmental location of each tumor confirmed at surgery. The right main and left main portal veins were found to be consistently near the middle sections through the liver. CTAP findings and surgical descriptions agreed on the primary segmental location of 33 of 36 focal lesions (92%) but disagreed on the extent of 11 of 36 lesions (31%). Further review of the CTAP scans of the 11 lesions revealed that the extent of the lesion was more correctly described at surgery in six masses and at CTAP in four masses; in one lesion, opposite margins of the same mass were correctly described at both surgery and CTAP. Since it may be difficult or...

Judith L. Chezmar

Papers

“Keyhole” method for accelerating imaging of contrast agent uptake

Hepatic tumors: comparison of CT during arterial portography, delayed CT, and MR imaging for preoperative evaluation.

The frequency and significance of small (less than or equal to 15 mm) hepatic lesions detected by CT.

Diagnostic approach to hepatic hemangiomas.

Preoperative localization of focal liver lesions to specific liver segments: utility of CT during arterial portography.