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Abdominal MRI at 3.0 T: The Basics Revisited

Elmar M. Merkle1 and Brian M. Dale2

1 Department of Radiology, Duke University Medical Center, Duke North, Rm. 1417, Box 3808, Erwin Rd., Durham, NC 27710.
2 Siemens Medical Solutions, USA, Cary, NC 27519.


Figure 1
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  		Fig. 1A —Diagrammatic illustrations of relationships of relaxation times of liver tissue and main magnetic field strength (B0). Note that data of de Bazelaire et al. [1] are within confidence interval obtained by Bottomley et al. [7] even though 3.0-T data point is outside scope of article by Bottomley et al. Graph shows relationship of longitudinal relaxation time, T1, of liver tissue and main magnetic field strength. Data are shown from meta-analysis based on more than 800 study samples performed by Bottomley et al. [7] ({blacksquare}), together with data based on six volunteers acquired by de Bazelaire et al. [1] ({blacktriangleup}). Note nonlinear increase of T1 of liver tissue with main magnetic field strength. Also shown are nonlinear regression (solid line) and 95% confidence interval for data (dashed lines) as described by Bottomley et al.

 

Figure 2
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  		Fig. 1B —Diagrammatic illustrations of relationships of relaxation times of liver tissue and main magnetic field strength (B0). Note that data of de Bazelaire et al. [1] are within confidence interval obtained by Bottomley et al. [7] even though 3.0-T data point is outside scope of article by Bottomley et al. Graph shows relationship of transverse relaxation time, T2, of liver tissue and main magnetic field strength. Data are shown from meta-analysis based on more than 250 study samples performed by Bottomley et al. [7] ({blacktriangleup}), together with data based on six volunteers acquired by de Bazelaire et al. [1] ({blacksquare}). Note that no obvious relationship is seen between T2 of liver tissue and main magnetic field strength. Also shown are mean (solid line) and 95% confidence interval (dashed lines) for data as described by Bottomley et al.

 

Figure 3
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  		Fig. 2A —Chemical shift artifacts of the first and second kinds at various magnetic field strengths in same patient, 32-year-old woman. Axial gradient-echo in-phase image acquired at field strength of 1.5 T shows minimal chemical shift artifacts of the first kind along frequency-encoding axis.

 

Figure 4
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  		Fig. 2B —Chemical shift artifacts of the first and second kinds at various magnetic field strengths in same patient, 32-year-old woman. Axial gradient-echo in-phase image acquired at field strength of 3.0 T shows marked chemical shift artifacts of the first kind along frequency-encoding axis that appear bright toward higher part of readout gradient field (long arrow) and dark along lower part (short arrow).

 

Figure 5
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  		Fig. 2C —Chemical shift artifacts of the first and second kinds at various magnetic field strengths in same patient, 32-year-old woman. Axial gradient-echo opposed-phase image acquired at field strength of 1.5 T shows chemical shift artifact of the second kind in all pixels along fat-water interface.

 

Figure 6
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  		Fig. 2D —Chemical shift artifacts of the first and second kinds at various magnetic field strengths in same patient, 32-year-old woman. Axial gradient-echo opposed-phase image acquired at field strength of 3.0 T shows chemical shift artifact of the second kind, which is not significantly different from artifact seen at standard magnetic field strength of 1.5 T (compare withC).

 

Figure 7
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  		Fig. 3A —Effect of receiver bandwidth on size of chemical shift artifacts of the first kind at 3.0 T in 40-year-old man. Axial in-phase T1-weighted gradient-echo image through kidney shows significant chemical shift artifact (arrow) (receiver bandwidth, 210 Hz; pixel shift, 1.9; SNR [signal-to-noise ratio]LIVER, 75).

 

Figure 8
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  		Fig. 3B —Effect of receiver bandwidth on size of chemical shift artifacts of the first kind at 3.0 T in 40-year-old man. Axial in-phase T1-weighted gradient-echo image through kidney with twice receiver bandwidth as in A shows smaller chemical shift artifact (arrow) (receiver bandwidth, 415 Hz; pixel shift, 1.0; SNRLIVER, 56). However, SNR is also decreased by approximately 30%.

 

Figure 9
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  		Fig. 3C —Effect of receiver bandwidth on size of chemical shift artifacts of the first kind at 3.0 T in 40-year-old man. Axial in-phase T1-weighted gradient-echo image through kidney with four times receiver bandwidth as in A shows markedly smaller chemical shift artifact (arrow) (receiver bandwidth, 850 Hz; pixel shift, 0.5; SNRLIVER, 41). However, SNR is again substantially decreased by another 30%.

 

Figure 10
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  		Fig. 4 —Effect of magnetic field strength on size of susceptibility artifacts in various MR sequences in vitro.

A, Photograph shows phantom setup with water-filled straw (arrow) embedded in gelatin. Three pairs of surgical clips (arrowheads) are embedded at various distances from straw.

B, HASTE image acquired at 1.5 T (TR/TE, 1,010/128; field of view, 250 mm2; slice thickness, 2.8 mm; matrix, 256 x 205; bandwidth, 490 Hz) shows typical susceptibility artifacts caused by surgical clips.

C, HASTE image acquired at 3.0 T (1,010/128; field of view, 250 mm2; slice thickness, 2.8 mm; matrix, 256 x 205; bandwidth, 490 Hz) shows larger susceptibility artifacts caused by surgical clips when compared with B.

D, Gradient-echo image acquired at 1.5 T (118/2.4; field of view, 280 x 210 mm2; slice thickness, 3.0 mm; matrix, 256 x 154; bandwidth, 385 Hz) shows typical susceptibility artifacts caused by surgical clips.

E, Gradient-echo image acquired at 3.0 T (118/2.4; field of view, 280 x 210 mm2; slice thickness, 3.0 mm; matrix, 256 x 154; bandwidth, 385 Hz) shows larger susceptibility artifacts caused by surgical clips when compared with D. Note that artifact size increases by approximately 100% in terms of volume.

 

Figure 11
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  		Fig. 5A —Effect of magnetic field strength on size of metal-related susceptibility artifacts in vivo in 58-year-old man. Gradient-echo opposed-phase image acquired at 1.5 T (TR/TE, 200/2.2) shows typical susceptibility artifacts caused by surgical clips (arrows).

 

Figure 12
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  		Fig. 5B —Effect of magnetic field strength on size of metal-related susceptibility artifacts in vivo in 58-year-old man. Gradient-echo opposed-phase image acquired at 3.0 T (200/1.5) shows larger susceptibility artifacts (arrows) caused by surgical clips when compared with A despite shorter TE.

 

Figure 13
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  		Fig. 6A —Negative effect of magnetic field strength on size of gas-related susceptibility artifacts in vivo in 52-year-old man. Gradient-echo in-phase image acquired at 1.5 T (TR/TE, 200/4.4) shows minor susceptibility artifacts (arrows) in hepatic flexure and transverse colon caused by colonic gas.

 

Figure 14
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  		Fig. 6B —Negative effect of magnetic field strength on size of gas-related susceptibility artifacts in vivo in 52-year-old man. Gradient-echo in-phase image acquired at 3.0 T (200/4.4) shows larger susceptibility artifacts (arrows) in colon when compared with A. Note that these susceptibility artifacts obscure colonic wall.

 

Figure 15
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  		Fig. 7A —Positive effect of magnetic field strength on size of gas-related susceptibility artifacts in vivo in 65-year-old woman. Gradient-echo in-phase image acquired at 1.5 T (TR/TE, 200/4.4) shows minor susceptibility artifact (arrow) in left hepatic bile duct caused by pneumobilia that may be misinterpreted as a branch of portal venous system.

 

Figure 16
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  		Fig. 7B —Positive effect of magnetic field strength on size of gas-related susceptibility artifacts in vivo in 65-year-old woman. Gradient-echo in-phase image acquired at 3.0 T (200/4.4) shows markedly larger susceptibility artifact (arrow) in left hepatic bile duct caused by pneumobilia when compared with A. Enlarged susceptibility artifact makes misinterpretation less likely.

 

Figure 17
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  		Fig. 8 —Physical basis of radiofrequency shielding. In step A, rapidly varying magnetic field (black arrows) induces a circulating electric field (white arrows). In presence of a conductive medium (step B), circulating electric field leads to a circulating current (gray arrows). In step C, circulating current acts as electromagnet to produce magnetic field in opposite direction; and in step D, amplitude of overall magnetic field is reduced. Note that steps B and C require a conductive medium and that effect is stronger in more conductive medium. In imaging, this effect can be noticed in patients with ascites or fetal imaging, in which circulating currents can be established in relatively large regions of highly conductive fluid. Resulting artifacts are generally more visible in sequences that use a large number of radiofrequency pulses to generate contrast.

 

Figure 18
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  		Fig. 9A —Severe standing wave and conductivity artifact in 38-year-old woman with liver cirrhosis and ascites during ultrahigh-field-strength MRI at 3.0 T. Coronal HASTE image shows marked signal loss in center of image (arrows). Fluid accumulations in peritoneal cavity enlarge abdomen and increase electrical conductivity in field of view, causing severe artifacts.

 

Figure 19
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  		Fig. 9B —Severe standing wave and conductivity artifact in 38-year-old woman with liver cirrhosis and ascites during ultrahigh-field-strength MRI at 3.0 T. Axial HASTE image again shows marked signal loss in center of image (arrows) and represents severe standing wave and conductivity artifact.

 

Figure 20
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  		Fig. 9C —Severe standing wave and conductivity artifact in 38-year-old woman with liver cirrhosis and ascites during ultrahigh-field-strength MRI at 3.0 T. Contrast-enhanced axial gradient-echo T1-weighted image acquired at same level as B shows normal anatomy in center of field of view (arrows) and no evidence of susceptibility artifacts.

 

Figure 21
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  		Fig. 10 —Severe standing wave and conductivity artifact in 28-year-old pregnant woman during fetal ultrahigh-field-strength MRI at 3.0 T. Coronal HASTE image shows marked signal loss (long arrows). This artifact is caused by large amount of amniotic fluid and increased size of abdomen, which increase electrical conductivity in field of view. Short thin arrows mark placenta; thick arrows mark fetal torso. (Courtesy of Jones B, Cincinnati, OH)

 

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