MR images play an important role in the detection and characterization of diseases of the kidney (1–6). With conventional T2-weighted spin echo (SE) sequences, the longer imaging times and the inherent motion of the abdomen result in motion artifacts that degrade image quality, which has been one of the barriers to the use of MR as a primary abdominal imaging examination. Imaging time could be reduced significantly by the RARE sequence implemented as fast SE or turbo SE (TSE) sequences. Rydberg et al. (7) recently refined the TSE technique to allow breath-hold T2-weighted imaging of the liver, providing improved anatomic detail and lesion conspicuity. In addition, recent reports suggest that the abdominal multicoil (phased array coil) significantly improves imaging over use of the standard body coil (8). However, with these breath-hold imaging techniques, images are not free of motion artifacts, and some compromise with limited resolution cannot be avoided. With use of state-of-the-art MR techniques, fast imaging of the upper abdomen including the kidney can be performed by using a single-shot FSE sequence implemented as half-Fourier single-shot TSE (HASTE) sequences (9,10). The HASTE sequence generates fast heavily T2-weighted images with large echo train lengths. Images can be obtained in a very short time and have demonstrated great potential in MR cholangiopancreatography and MR urography (11,12). Recently, modified HASTE sequences with short echo space and high bandwidth with smaller echo train length (=64) have allowed increased contrast resolution and decreased acquisition time (0.3 s/slice) and may thus be suitable for imaging of the kidney.
In this study, we report our clinical experiences of using the HASTE sequence for T2-weighted MRI of the kidney. We especially focused on whether this fast sequence can be used as a substitute for the TSE sequence in terms of signal-to-noise ratio (SNR), lesion conspicuity, image quality, and diagnostic value.
MATERIALS AND METHODS
The prospective study group consisted of 45 consecutive patients (38 to 81 years old, mean age 59 years) referred for MRI with suspected abnormalities of the kidney by ultrasound, CT, or both between April 1997 and September 1998. A total of 53 lesions were evaluated: 22 renal cell carcinomas (RCCs), 15 complicated cysts, 14 simple cysts, and 2 metastases. In patients with multiple cysts, the largest lesions were evaluated. The diagnosis of the lesions was based mainly on the histologic findings (n = 27). In 22 RCCs, 13 tumors were <3 cm in diameter. Pseudocapsules were found in 11 of 22 RCCs. The diagnosis of some benign cystic lesions was made on the basis of the patterns seen in typical ultrasound and CT images.
All MR examinations were performed on a 1.5 T superconductive magnet (Siemens Magnetom Vision) with a phased array coil (CP Body Array be 01-1H; Siemens). For T2-weighted imaging, both HASTE and TSE sequences were performed. For T1-weighted imaging, fast low-angle shot (FLASH) sequence was performed.
The basic principle of the HASTE sequence is similar to that of the RARE sequence, which creates multiple spin echoes after one 90° excitation pulse to cover all the k space. In contrast to RARE, slightly more than half of the k space is acquired, and the image is created with a half-Fourier reconstruction technique. The echo ordering in the k space is such that the early echoes are in the center of the k space with progressively longer TEs toward the upper half of the k space. In our HASTE sequence, the echo train length was limited to 64 to generate a stronger last echo (the 64th echo) in an echo space of 4.2 ms to fill half of the k space. The higher gradient strength (25 mT/0.6 ms) resulted in higher soft tissue contrast resolution and less blurring in the tissues with a short T2 value in comparison with previous HASTE sequences used for MR cholangiopancreatography or MR urography (11,12).
The sequence was obtained with an effective TE of 60 ms and a 255 × 340 acquisition matrix, and each slice was obtained in 0.3 s, image acquisition being performed sequentially. The number of sections determined the total imaging time. The acquisition time for HASTE was 0.3 s/slice (14 slices/12 s). In this study, we obtained 9–11 sections with 8 mm thickness with breath-hold.
T2-weighted respiratory-triggered TSE imaging was performed with TR varying from 3,180 to 8,638 ms (mean 4,657 ms) according to the period of the respiratory cycle, an effective TE of 90 ms, an echo train of 7, and one excitation. The trigger level was set at 50% of the tidal volume (maximum minus baseline) during expiration. The TR for respiratory triggering is approximately twice that used for breath-hold MRI. Acquisition time varied depending on TR and ranged from 115 to 249 s (mean 185 s) for 14 sections.
In all sequences, the same asymmetric (5/8) field of view, section thickness (8 mm), and the intersection gap (20%) were used. The section position in terms of table coordinates was the same for respiratory-triggered TSE and HASTE breath-hold imaging, although the numbers of slices were not the same. Saturation bands superior and inferior to the imaging volume were used in the respiratory-triggered TSE sequences to reduce the presence of flow-related artifacts.
Image Analysis and Statistics
Quantitative analysis was performed using an electronic cursor for measurements of regions of interest (ROIs) in the kidney parenchyma and lesions. Noise was measured on each image using a cursor positioned just ventral to the anterior abdominal wall. The areas with the most prominent ghost were not included. The size of the cursor was chosen so as to include a large representative portion of the organ or lesion. The values of the signal intensities (SIs) of the kidney were divided by the corresponding SD of noise to derive the SNR. The kidney-to-lesion (solid or cystic mass) contrast-to-noise ratio (CNR) was calculated as |lesion SI − organ SI|/noise SD.
Qualitative analysis was performed by two radiologists who were blind to the clinical history and pathologic results. Initially, images of the two sequences were randomly presented to the radiologists, and image quality was scored with regard to lesion conspicuity, internal details, marginal conspicuity, and focal lesion contrast. Each radiologist independently gave numerical scores of 1–5, representing nondiagnostic, poor, fair, good, and excellent, respectively. Artifacts (blurring, motion, pulsation, and chemical shift) were graded on a scale of 1–4 (1, severe; 2, moderate; 3, mild; 4, none).
Table 1 gives the values for the SNR of the kidney and CNR of each lesion obtained with HASTE and respiratory-triggered TSE techniques. The values with HASTE sequence were higher than those obtained with the TSE sequences for cystic lesions, but there were no statistically significant differences for solid masses (Fig. 1).
Table 2 summarizes the results of lesion conspicuity in both solid and cystic lesions. Visualization of the cystic lesions was significantly better on HASTE sequences than on TSE sequences (Fig. 2). The details of the wall of the cyst were more clearly visualized on HASTE sequences. For solid lesions, lesion conspicuity was not significantly different between the two sequence types, although the image resolution with the TSE sequences was better than that with the HASTE images. Visualization of tumor capsules and wall irregularity of the cystic was better on the HASTE sequences because of the lack of motion artifacts and chemical shift artifacts (Figs. 1–3). Table 3 summarizes the detection of the pseudocapsule of the RCC on both sequences.
Table 4 shows the results for artifacts. Motion-related artifacts and pulsation artifacts were negligible, and chemical shift artifacts were not seen in any patients with the HASTE sequences (Fig. 4). There were significant differences in respiratory motion and chemical shift artifacts. Blurring artifacts were higher with the HASTE sequences than with the TSE sequences (Fig. 5).
MRI has been accepted as a problem-solving modality for the evaluation of renal masses. For evaluation of cystic renal lesions and tumor capsules of renal masses, T2-weighted images are useful (13–15). Non-breath-hold T2-weighted MRI by SE or TSE sequences of the kidney is often plagued by poor quality images caused by motion artifacts, because most T2-weighted imaging techniques require several minutes—a long acquisition time that provides ample opportunity for various artifacts to become manifest. Respiratory triggering is an effective approach for imaging of the abdomen on TSE sequence, but blurring artifacts are often encountered. Despite the very short imaging time compared with conventional TSE methods, the HASTE sequence provided acceptable resolution and absence of artifacts for the clinical use (Figs. 1 and 3). Surprisingly, the lesion-to-organ contrast of this HASTE sequence was superior to the conventional TSE sequences. This net increment in contrast resolution can be attributed to several factors: a wider bandwidth implemented in the HASTE sequence (650 Hz/pixel for HASTE and 135 Hz/pixel for TSE); a moderately T2-weighted image generated by the shortened effective TE, giving rise to increased SNR and conspicuity of solid mass lesions; and, probably most importantly, a lack of motion artifacts and image noise caused by bright abdominal fat. However, it would be difficult to evaluate complex cysts with a combination of protein and hemorrhage with the HASTE sequence because of the lack of soft tissue contrast. Contrast-enhanced study with Gd-DTPA may be required for the evaluation of morphologic details of those complex cysts.
Due to the very rapid acquisition (0.3 s/slice), high bandwidth, and multiple 180° pulses, the HASTE sequence is free of motion-related, chemical shift, and susceptibility artifacts (9). As the resolution of the HASTE sequence is good for tissues with long T2 values, this technique was most valuable in the imaging of cystic renal lesions (10). The internal architecture of the cystic lesions was clearly visualized in comparison with conventional TSE sequence (Fig. 2). Even for solid lesions such as small renal carcinomas, HASTE imaging provided high contrast images, and diagnostic information was almost comparable between TSE and HASTE imaging. The pseudocapsules were depicted on almost all cases of small RCCs with the HASTE sequence (Fig. 1). This might be a useful finding for renal parenchyma, offering the possibility of avoiding unnecessary surgery.
There are several limitations to the current HASTE sequence. First, because of the limitation of the gradient power, the small field of view and high resolution image matrix that were used on the TSE sequence could not be achieved, and direct comparison of SNR was therefore difficult. If the SNRs are corrected by the pixel size, higher values will be obtained with the TSE sequence. However, there was a clear scan time advantage with the HASTE sequence. Second, the effective TE (=60 ms) seems to be slightly shorter for T2-weighted imaging. However, this shorter TE might have compensated for the potentially lower SNR of the HASTE sequence, and water-rich lesions appeared very bright owing to the T2-filtering effect and magnetization transfer caused by multiple 180° pulses.
In conclusion, the fast HASTE sequence generates higher kidney-to-lesion contrast and comparable image quality in comparison with respiratory-triggered TSE, with negligible image degradation due to respiratory motion artifacts in a short breath-hold time. In particular, the HASTE sequence offers an advantage in the evaluation of cystic renal masses. We believe that this sequence can play an important role in T2-weighted MRI of the kidney and will gain widespread application. Refinement of this type of sequence may have a great potential for imaging of the kidney.
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Index Terms: Magnetic resonance imaging; Kidney, neoplasms© 2001 Lippincott Williams & Wilkins, Inc.