Clinical Note

Chemical Shift Artifact along the Section-Select Axis' Ronald H. Wachsberg, MD2 Donald G. Mitchell, MD Simon Vinitski, PhD Talin A. Tasciyan, PhD Chemical shift artifact (CSA), familiar to radiologists along the frequency-encodingaxis, also occurs along the sectionselect axis. The authors observed a case in which CSA mimicked a renal mass. Subsequent retrospective analysis of 50 abdominal magnetic resonance (MR)imaging studies was performed to assess occurrence of CSA adjacent to the upper and lower renal poles. CSA along the section-select axis was observed in 76%of cases and adjacent to 39%of all renal poles imaged. CSA along the section-selectaxis is common in abdominal MR imaging and may occasionally mimic disease.

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Index terms: Artifact Chemlcal shift imaging * Kidney. MR. 81.1214

JMRI 1992; 2:589-591

From the Department ofRadlology. Thornas Jefferson University Hospital, 1096 Main Bldg, 132 S 10th St, Philadelphia. PA 19107. Received February 4: revision requested May 8: revision received May 28: accepted June 29. Address reprint requests to D.G.M. Current address: Department of Radiology. University Hospital-UMDNJ, Newark, NJ. Current address: Department of Radiol-

o w , Albany Medical College, Albany. NY. 1l

SMRI. 1992

Matthew D. Rifkin, MD3

CHEMICAL SHIFT ARTIFACT (CSA)is commonly observed in individual sections along the frequency-encoding axis in magnetic resonance (MR)imaging (1,2).It is also known to occur along the section-select axis, although less frequentlyreported (3). We observed a case in which a CSA along the section-select axis mimicked a renal mass. To assess this phenomenon, we reviewed 50 MR studies of the abdomen. 0

METHODS

Fifty random abdominal MR studies were retrospectively analyzed by two reviewers (R.H.W.,D.G.M.) in anonblinded fashion. N o patient was suspected of having focal renal disease. Images were obtained on a Signa 1.5-T system (GE Medical Systems, Milwaukee). The section thickness was 10 mm, with an intersection gap of 2 mm. The receiver bandwidth (with respect to the frequency-encoding axis)was held constant at 32 kHz ( - 16 to + 16 kHz) for all echoes, or 125 Hz per pixel. The section-selective radio-frequency bandwidth was 1,250 Hz, resulting in a 1.9-mm displacement of water relative to fat along the section-select axis. The upper and lower renal poles and renal hila were analyzed for CSA along the section-select axis on long TR images (TR msec/TE msec = 2,500/50. 100).This form of CSA was deemed present if we noted a region with higher signal intensity than that of fat or kidney immediately superior to the upper renal pole (Fig 11, a region less intense than fat or kidney immediately inferior to the lower renal pole (Fig 2). or a region less intense than fat or kidney at the renal hilum (Fig 3).

RESULTS Of 135 renal poles included in the 50 MR studies, 39% demonstrated CSA along the section-select axis.This artifact was observed adjacent to at least

0

one renal pole in 76% of the 50 studies. The left lower pole was most likely to generate this form of CSA, seen in 1 1 (61%)of left lower poles imaged (Table). Section-select CSA was most frequent in individuals with abundant body fat. Factors mitigating CSA in some cases included paucity of fat, hypointense kidneys (such as in iron overload), and adjacent bowel. In general, CSA adjacent to upper poles was more striking on long TR/TE images, whereas CSA adjacent to lower poles was best appreciated on long TR/short TE images. In one case. a hypointense pseudomass at the renal hilum was generated by cephalic misregistration of the upper pole relative to hilar fat (Fig 3).Coronal MR images and renal sonograms in this patient confirmed the absence of a true mass. DISCUSSION CSA is due to the slightly different resonant frequencies of fat and water protons, causing an apparent spatial shift of water with respect to fat. The resulting misregistration of water relative to fat along the frequency-encoding axis is well known ( 1,2).CSA along the section-select ads is less familiar, but it has been used to achieve fat suppression (3.4).Section-select CSA also may obscure the more familiar frequencyencoding CSA adjacent to lesions such as ovarian dermoids (5).Frequencyencoding CSA results from misreading of the frequency-encoding data, whereas section-select CSA results from misexcitation of fat and water protons at slightly different locations but with the same range of precessional frequencies. The severity of section-select CSA depends on the bandwidth of radio-frequency section selection but not on the receiver bandwidth or the thickness of intersection gaps. Misregistration of perinephric fat with respect to renal water manifests as a region that is either brighter or darker 0

589

a. b. Figure 1. Spin-echo images (2,500/100)of two sections (ais one section inferior to b)through kidney show summation artifact (arrows)hyperintense to either kidney or fat, just cephalad to the left lower renal pole. than these two tissues. Hyperintense CSA (Fig 1)represents a summation of fat and kidney signal intensity: mere partial-volume averaging would produce intermediate signal intensity between that of fat and kidney. Similarly, hypointense CSA (Fig 2) cannot reflect a partial-volume effect, since it is darker than either of these two tissues. We observed one patient in whom section-select CSA produced a low-signal-intensity pseudomass adjacent to the left renal hilum. This is of particular relevance in view of reports of hypointense renal cell carcinoma (6-8). The appearance of this artifact is reminiscent of the “hilar lip” pseudomass artifact related to partial-volume averaging seen at computed tomography (9).We are aware of only one other report of section-select CSA masqueradingas an abnormality, in which misregistration of calvarial marrow fat mimicked bilateral subdural hematomas (10). Section-select CSA is common where soft tissue abuts fat. It was most common at the left lower pole, presumably because there is more fat inferior to the left kidney, owing to its higher location within the abdomen relative to the right kidney in most individuals. The hyperintense artifact adjacent to the upper renal poles was most conspicuous on long TE images, on which renal and adipose tissue appear most similar and summation of these two tissue is most prominent. The hypointense artifact adjacent to the lower renal poles was most conspicuous on short TE images, on which it contrasted most sharply with high-signal-intensiw fat. Familiarity with the appearance of section-select CSA may help prevent confusion of normal findings with disease. 0

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a. b. Figure 2. Spin-echo images (2,500/50)of two sections (ais one section inferior to b)through left kidney show artifact (arrows)hypointense to either kidney or fat, immediately caudad to left lower renal pole.

CSA on MR Images of Kidney ~~~~

September/October 1992

Renal Pole

No. of Poles Imaged

No. of Poles with CSA

Right upper Right lower Left upper Left lower Total

43 26 48 18 135

14 9 19 11

53

CSA Frequency (%I 33

35 40 61 39

a. b. C. Figure 3. (a)Spin-echo image (417/12)at left renal hilus shows "mass" [arrow)posterior to renal pedicle, intermediate in intensity relative to kidney and fat. With this sequence, partial-volume averaging might account for this appearance. (b)Corresponding 2,500/ 100 spin-echo image shows pseudomass (arrow)as hypointense to kidney and fat. It is caused by signal void due to chemical shift misregistration along the section-select axis. (c) On T2-weighted image obtained just cephalad to b, configuration of lip of renal tissue (arrow)conforms to contour of artifact.

Acknowledgment: The authors acknowledge the assistance of Daya R. Varma. MD.

References 1. Soila K, Viamonte M. Starewicz P. Chemical shift misregistration effect in magnetic resonance imaging. Radiology 1984: 153:819-820. 2. Dick BW. Mitchell DG. Burk DL J r . Levy DV, Vinitski S , Rifkin MD. Effect of chemical shift misregistration on cortical bone thickness on MR imaging. AJR 1988: 151:537-538. 3. Park HW.Kim DJ. Cho ZH. Gradient reversal technique and its applications

to chemical shift related NMR imaging. Magn Reson Med 1987; 4:526-536. 4. Gomori J M , Holland GA, Grossman RI, Gefter WB, Lenkinski RE. Fat suppression by section-select gradient reversal on spin-echoMR imaging. Radiology 1988: 168:493-495. 5 . Smith RC. Lange RC, McCarthy SM. Chemical shift artifact: dependence on shape and orientation of the lipid-water interface. Radiolo@ 1991; 181:225229. 6. Kulkarni MV, ShaffMI. Sandler MP, et al. Evaluation of renal masses by MR imaging. J Comput Assist Tomogr 1984: 8:861-865.

7. Fein AB, Lee JKT, Balfe DM, et al. Diagnosis and staging of renal cell carcinoma: a comparison of MR imaging and CT.AJR 1987; 148:749-753. 8. Sussman S K , Glickstein MF. Krzymowski GA. Hypointense renal cell carcinoma: MR imaging with pathologic correlation. Radiology 1990: 177:495497. 9. Zeman RK, Cronan JJ. Rosenfeld AT. et al. Computed tomography of renal masses: pitfalls and anatomic variants. RadioGraphics 1986: 6351-371. 10. Henkelman RM. Bronskill MJ. Rev Ma@ Reson Med 1987: 2: 1-126.

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Chemical shift artifact along the section-select axis.

Chemical shift artifact (CSA), familiar to radiologists along the frequency-encoding axis, also occurs along the section-select axis. The authors obse...
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