MRI Protocol for Planning NSS

Localizer images are obtained to plan the diagnostic sequences. Typically, fast T1-weighted images are acquired in three planes. Then, T1-weighted in-phase and out-of-phase images are obtained using a 2-D fast gradient echo sequence without fat saturation. With a slice thickness of 5 to 6 mm, only about 20 slices are needed to cover the kidneys in a single breath-hold. On a 1.5 Tesla system, the out-of-phase time to echo (TE) is approximately 2 msec and the in-phase TE is approximately

Figure 2 (Figure on facing page) MPR reconstructions created for surgical planning. These images are created from thin-section data obtained in the parenchymal phase. (A) Standard axial image shows a hypodense left renal mass (arrow) in the lateral interpolar kidney. (B) Sagittal and (C) oblique coronal MPR images help to delineate the position of the tumor (arrow) with respect to the remaining normal renal parenchyma and are easy to create at the CT scanner console. Abbreviations: CT, computed tomography; MPR, multiplanar reformatted.

Exophytic Renal Cyst

Figure 4 Precontrast T1- and T2-weighted MRI images. (A) Coronal and (B) axial precon-trast T2-weighted images (half-Fourier single-shot turbo spin echo) show the heterogeneous mixed intensity of this exophytic renal cell carcinoma (arrow), a homogeneously hyperintense simple cortical cyst (arrowhead), and a distended collecting system in the right kidney (small arrow). (C) Axial in-phase and (D) out-of-phase precontrast T1-weighted images show the generally low precontrast T1-weighted signal of a renal cell carcinoma (arrow). Abbreviation: MRI, magnetic resonance imaging.

Figure 4 Precontrast T1- and T2-weighted MRI images. (A) Coronal and (B) axial precon-trast T2-weighted images (half-Fourier single-shot turbo spin echo) show the heterogeneous mixed intensity of this exophytic renal cell carcinoma (arrow), a homogeneously hyperintense simple cortical cyst (arrowhead), and a distended collecting system in the right kidney (small arrow). (C) Axial in-phase and (D) out-of-phase precontrast T1-weighted images show the generally low precontrast T1-weighted signal of a renal cell carcinoma (arrow). Abbreviation: MRI, magnetic resonance imaging.

4 msec. If available, the use of a double echo technique to acquire both in-phase and out-of-phase images during a single breath-hold is advantageous for two reasons. First, it ensures precise registration of the in-phase and out-of-phase images, and second, it reduces the number of breath-holds the patient must perform. On this sequence, voxels containing both fat and water will have a degree of signal cancellation leading to signal intensity loss or signal dropout on the out-of-phase images when compared to the in-phase images. Thus, such tissue as lipid-rich adrenal adenomas and liver with fatty infiltration with intracellular or microscopic fat can be identified due to its signal dropout on the out-of-phase images (Fig. 5). A T1-weighted sequence with frequency-specific fat saturation is also employed to identify regions of bulk or macroscopic fat, as seen in angiomyolipoma. This is one of the same sequences used after contrast administration. Two goals are achieved by acquiring pre- and postcon-trast data using the same sequence: first, the precontrast images identify bulk and macroscopic fat; and, second, postprocessing can be performed using the precontrast sequence as a mask for image subtraction.

Figure 5 Axial (A) in-phase and (B) out-of-phase precontrast T1-weighted images of an incidental adrenal mass (arrow) in a patient with a renal tumor. There is signal drop-out on the out-of-phase image, indicating fat, and in this case, a lipid-rich adrenal adenoma.

Next, T2-weighted images are obtained to detect and evaluate areas of fluid, including cystic lesions and the renal collecting systems. Single-shot techniques are employed to obtain T2-weighted images in a single breath-hold and to image the entire region of interest with adequate resolution. Twenty slices can be obtained in approximately 20 seconds by utilizing a half-Fourier single-shot technique (HASTE). Thus, the kidneys and adrenal glands can be scanned using a slice thickness of 4 to 5 mm. If desired, axial imaging using the same technique can be performed with slice thickness and positioning corresponding to the in-phase and out-of-phase images.

A standard MRI contrast dose of 0.1 mmol/kg of gadolinium is adequate for most diagnostic studies and most MRI angiograms. However, as previously stated, it is helpful to increase the contrast dose when studies are performed for NSS surgical planning. A dose of 0.15mmol/kg (1.5 times the standard dose, or, typically, 30mL) is used to obtain better venous opacification for surgical planning studies. Contrast administration is ideally performed utilizing a power injector at a rate of 2mL/sec followed by a saline flush at the same rate.

Because the intravenous contrast volume is small in MRI, there is typically only a 10-second window of ideal arterial opacification. Therefore, proper timing for scans acquired during the arterial phase of imaging is critical. More specifically, filling the data in the more ''contrast-sensitive'' center of k-space during optimal arterial opacification is necessary for best-quality imaging. A timing examination has been proven to be useful for obtaining images consistently during the arterial phase of contrast enhancement. The timing examination is performed by injecting a small amount of contrast (typically 1 mL) followed by a 20 mL saline flush and then obtaining images at fixed intervals (typically every 1-2 seconds) following the start of the injection. Usually, multiple images are obtained at the level of the kidneys for a period of 60 seconds, thereby defining the time course of contrast administration. Evaluation of these images allows for the easy determination of the delay needed to achieve peak arterial enhancement. In addition, evaluation of the enhancement of the renal parenchyma during this sequence provides a preliminary assessment of renal parenchymal perfusion.

Postcontrast imaging is obtained in multiple phases using T1-weighted gradient echo 3-D interpolated, fat saturated sequences. These sequences allow a slice thickness of 1.5 mm in the coronal plane and 2 mm in the axial plane. Since the resolution of each image is at or below the slice thickness, the voxels are nearly isotropic allowing for high quality multiplanar reconstructions in a manner similar to helical CT. Both arterial and venous phase imaging in the coronal plane are obtained using an angio-graphic sequence [such as fast low-angle shot (FLASH)]. This sequence tends to suppress background tissue signal in order to highlight vascular structures (36). Following this, anatomic imaging in the axial plane is obtained during the cortico-medullary phase of renal enhancement using a more tissue-sensitive sequence [volume interpolated breath-hold examination (VIBE)] (37). Accurate assessment of the vascu-lature and accurate characterization of renal lesions are possible by combining these different techniques.

Coronal images are then obtained about 5 to 10 minutes after the initiation of the contrast material injection to obtain an MRI urogram. In patients whose lesions approach the renal sinus, it is desirable to distend the calyces in order to better determine whether the calyces are involved. Administering a small dose of intravenous fur-osemide during the timing examination will promote diuresis, distending the collecting system to achieve a better-quality MRI urogram. In patients who are not on chronic furosemide therapy, a dose of 10 mg given intravenously is almost always adequate. For patients who are on chronic therapy, dosage adjustments must be made.

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