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Chemical Shift: The Artifact and Clinical Tool Revisited

¹ Department of Radiology and Nuclear Medicine, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd, Bethesda, MD 20814-4799 (M.N.H., V.B.H., J.G.S.)
² Department of Radiology, Oregon Health Sciences University, Portland (J.S.)

View larger version (11K): [in a new window]   Figure 1.  Equation illustrates the relationship between chemical shift and field strength, where fcs = resonant frequency shift in parts per million (ppm), = relative chemical shift frequency difference, and 0 = main magnetic field resonant frequency. Given that human lipid and water have a resonant frequency difference of 3.5 ppm (ie, fcs = 3.5 ppm), the chemical shift frequency change () will be approximately 224 Hz at a field strength of 1.5 T (ie, 0 = 64 MHz).

View larger version (157K): [in a new window]   Figure 2.  Coronal T1-weighted spin-echo image (repetition time msec/TE msec = 666/10) of a 52-year-old woman with an anteverted uterus demonstrates a chemical shift artifact around the uterus. The superior border of the uterus (U) has a bright band (black arrow), whereas the inferior border of the bladder has a dark band (white arrow). The severe lipid-water interface between the uterus and the pelvic fat produces a strong shift of the protons at the margin of the bladder.

View larger version (173K): [in a new window]   Figure 3.  Coronal T1-weighted fast multiplanar spoiled gradient-echo (FMPSPGR) image (repetition time msec/TE msec, flip angle = 103/5.6, 80°) obtained with right-to-left frequency encoding of a 61-year-old man demonstrates chemical shift artifact in many of the abdominal lipid-water interfaces. Dark bands (arrowheads) are seen along the lateral right psoas muscle, lateral right renal, medial splenic, and medial left renal margins. Bright bands are visualized at the opposing lipid-water interfaces of the lateral left psoas muscle, medial right renal, lateral splenic, and lateral left renal margins.

View larger version (29K): [in a new window]   Figure 4.  Schematic depicts chemical shift along the frequency-encoding direction. Lipid signal is shifted to a lower-frequency position when it is surrounded by water, which leaves a signal void (dark band) on the high-frequency side of the lipid structure and an increase in signal (bright band) on the low-frequency side.

View larger version (66K): [in a new window]   Figure 5a.  T1-weighted FMPSPGR images (51/2.1, 90°) of a phantom with inner water (w) and outer oil (o) components illustrate the chemical shift artifact, which is perpendicular to the frequency-encoding direction. (a) On the image obtained with left-to-right frequency encoding (open arrow), the dark band of the chemical shift artifact is noted along the superior-to-inferior water-oil interfaces (solid arrow). (b) On this image, the frequency encoding is switched to inferior to superior (open arrow), and the dark band of the artifact is noted along the right-left water-oil interfaces (solid arrow). The bright band is poorly visualized in both images because it is superimposed over the already bright signal of the surrounding oil.

View larger version (68K): [in a new window]   Figure 5b.  T1-weighted FMPSPGR images (51/2.1, 90°) of a phantom with inner water (w) and outer oil (o) components illustrate the chemical shift artifact, which is perpendicular to the frequency-encoding direction. (a) On the image obtained with left-to-right frequency encoding (open arrow), the dark band of the chemical shift artifact is noted along the superior-to-inferior water-oil interfaces (solid arrow). (b) On this image, the frequency encoding is switched to inferior to superior (open arrow), and the dark band of the artifact is noted along the right-left water-oil interfaces (solid arrow). The bright band is poorly visualized in both images because it is superimposed over the already bright signal of the surrounding oil.

View larger version (14K): [in a new window]   Figure 6. Figures 6, 7. (6) Equation demonstrates the relationship between chemical shift, frequency shift, field of view (FOV), and bandwidth. (7) Pulse sequence diagram shows the differences in readout (frequency) window length with a change in the receiver bandwidth. The readout window must be open longer to preserve the field of view, which in turn, increases the chemical shift artifact.

View larger version (10K): [in a new window]   Figure 7. Figures 6, 7. (6) Equation demonstrates the relationship between chemical shift, frequency shift, field of view (FOV), and bandwidth. (7) Pulse sequence diagram shows the differences in readout (frequency) window length with a change in the receiver bandwidth. The readout window must be open longer to preserve the field of view, which in turn, increases the chemical shift artifact.

View larger version (63K): [in a new window]   Figure 8a.  T1-weighted FMPSPGR images (51/2.1, 90°) of a phantom with inner water and outer oil components illustrate the effect of bandwidth on the degree of chemical shift seen in images. (a) This image, obtained with a bandwidth of 7.8 kHz, shows a dark band on the inferior border (arrow) and a bright band on the superior border (arrowhead) of the water portion of the phantom. (b) On this image, obtained with a 31.2-kHz bandwidth, the dark and bright chemical shift bands are barely noticeable.

View larger version (65K): [in a new window]   Figure 8b.  T1-weighted FMPSPGR images (51/2.1, 90°) of a phantom with inner water and outer oil components illustrate the effect of bandwidth on the degree of chemical shift seen in images. (a) This image, obtained with a bandwidth of 7.8 kHz, shows a dark band on the inferior border (arrow) and a bright band on the superior border (arrowhead) of the water portion of the phantom. (b) On this image, obtained with a 31.2-kHz bandwidth, the dark and bright chemical shift bands are barely noticeable.

View larger version (164K): [in a new window]   Figure 9a.  In-phase FMPSPGR image (90/4.2, 90°) of a 57-year-old man with focal scarring of his right kidney more accurately demonstrates enhancement (a), compared with the out-of-phase FMPSPGR image (90/2.2, 90°) (b), which better delineates the renal contours. The out-of-phase gradient-echo image better delineates the focal scar in the middle of the right kidney (arrow).

View larger version (178K): [in a new window]   Figure 9b.  In-phase FMPSPGR image (90/4.2, 90°) of a 57-year-old man with focal scarring of his right kidney more accurately demonstrates enhancement (a), compared with the out-of-phase FMPSPGR image (90/2.2, 90°) (b), which better delineates the renal contours. The out-of-phase gradient-echo image better delineates the focal scar in the middle of the right kidney (arrow).

View larger version (14K): [in a new window]   Figure 10.  Schematic shows lipid and water protons precessing in and out of phase with respect to each other over time at 1.5 T. When lipid and water are in phase with each other, the net signal is additive, as shown at time points A, C, and E. Out-of-phase lipid and water signals nearly cancel each other out, as shown by the drop in the sine curve at time points B and D. The sine wave depicts the signal intensity within a voxel over time as the water and lipid protons oscillate between in phase and out of phase after the radio-frequency excitation pulse is delivered. The curve also drops in overall signal intensity due to T2* decay. (Modified and reprinted, with permission, from reference 5.)

View larger version (16K): [in a new window]   Figure 11.  Schematic illustrates chemical shift misexcitation. During the slice-selection radio-frequency excitation pulse, protons are subjected to a range of frequencies or bandwidths (BW). The chemical shift between lipid and water actually shifts the lipid protons along the slice-selection axis. The lipid protons are excited in a position that is shifted slightly lower along the slice-selection axis than the water protons in the section, which causes a misexcitation to occur in the slice-selection direction. (Modified and reprinted, with permission, from reference 5.)

View larger version (116K): [in a new window]   Figure 12a.  (a) Computed tomographic (CT) scan of a 1-year-old boy with a left external ear deformity shows an intracranial lipoma as a hypoattenuating region (*). (b) Sagittal T1-weighted image (617/20) displays the fatty nature of the intracranial lipoma (L) as a homogeneously high-signal-intensity structure within the brain. (c) Axial proton-density–weighted image (3,500/20) obtained with inferior-to-superior frequency encoding shows a small amount of chemical shift artifact (arrow) occurring between the lipoma and the cerebrospinal fluid in the foramen magnum. (d) T2-weighted image (3,500/100) obtained with inferior-to-superior frequency encoding shows a much larger chemical shift artifact.

View larger version (117K): [in a new window]   Figure 12b.  (a) Computed tomographic (CT) scan of a 1-year-old boy with a left external ear deformity shows an intracranial lipoma as a hypoattenuating region (*). (b) Sagittal T1-weighted image (617/20) displays the fatty nature of the intracranial lipoma (L) as a homogeneously high-signal-intensity structure within the brain. (c) Axial proton-density–weighted image (3,500/20) obtained with inferior-to-superior frequency encoding shows a small amount of chemical shift artifact (arrow) occurring between the lipoma and the cerebrospinal fluid in the foramen magnum. (d) T2-weighted image (3,500/100) obtained with inferior-to-superior frequency encoding shows a much larger chemical shift artifact.

View larger version (130K): [in a new window]   Figure 12c.  (a) Computed tomographic (CT) scan of a 1-year-old boy with a left external ear deformity shows an intracranial lipoma as a hypoattenuating region (*). (b) Sagittal T1-weighted image (617/20) displays the fatty nature of the intracranial lipoma (L) as a homogeneously high-signal-intensity structure within the brain. (c) Axial proton-density–weighted image (3,500/20) obtained with inferior-to-superior frequency encoding shows a small amount of chemical shift artifact (arrow) occurring between the lipoma and the cerebrospinal fluid in the foramen magnum. (d) T2-weighted image (3,500/100) obtained with inferior-to-superior frequency encoding shows a much larger chemical shift artifact.

View larger version (141K): [in a new window]   Figure 12d.  (a) Computed tomographic (CT) scan of a 1-year-old boy with a left external ear deformity shows an intracranial lipoma as a hypoattenuating region (*). (b) Sagittal T1-weighted image (617/20) displays the fatty nature of the intracranial lipoma (L) as a homogeneously high-signal-intensity structure within the brain. (c) Axial proton-density–weighted image (3,500/20) obtained with inferior-to-superior frequency encoding shows a small amount of chemical shift artifact (arrow) occurring between the lipoma and the cerebrospinal fluid in the foramen magnum. (d) T2-weighted image (3,500/100) obtained with inferior-to-superior frequency encoding shows a much larger chemical shift artifact.

View larger version (131K): [in a new window]   Figure 13a.  (a) Axial T1-weighted image (400/10) of a 48-year-old woman, in whom CT had revealed a focal hypoattenuating mass in the left lateral hepatic lobe, shows the lesion as a homogeneous, mildly high-signal-intensity area (arrow) without mass effect, findings suggestive of focal fat. The axial in-phase gradient-echo image (not shown) also depicted the lesion as hyperintense compared with the rest of the liver. (b) Axial out-of-phase FMPSPGR image (34/2.3, 90°) helps confirm that the mass is focal fat by demonstrating the same area as being very dark (arrow). Because of destructive interaction between fat and water signals on out-of-phase images, fatty lesions appear as a signal void.

View larger version (148K): [in a new window]   Figure 13b.  (a) Axial T1-weighted image (400/10) of a 48-year-old woman, in whom CT had revealed a focal hypoattenuating mass in the left lateral hepatic lobe, shows the lesion as a homogeneous, mildly high-signal-intensity area (arrow) without mass effect, findings suggestive of focal fat. The axial in-phase gradient-echo image (not shown) also depicted the lesion as hyperintense compared with the rest of the liver. (b) Axial out-of-phase FMPSPGR image (34/2.3, 90°) helps confirm that the mass is focal fat by demonstrating the same area as being very dark (arrow). Because of destructive interaction between fat and water signals on out-of-phase images, fatty lesions appear as a signal void.

View larger version (147K): [in a new window]   Figure 14a.  (a) Axial in-phase FMPSPGR T1-weighted, dynamic gadolinium-enhanced image (89.5/4.2, 80°) of a 46-year-old woman with diffuse fatty liver and focal nodular hyperplasia shows an enhancing mass, the focal nodular hyperplasia (arrow), in the right lobe of the liver (the mass was suspected from appearances on previous T1- and T2-weighted images). (b) Corresponding out-of-phase FMPSPGR image (78.3/2.3, 80°) poorly illustrates the mass (arrow). Because of suppression of the signal within surrounding fatty liver, the enhancing mass was less apparent on the out-of-phase image (b) than on the in-phase image (a). A small second lesion, a hemangioma (arrowhead), is incidentally noted just posterior to the focal nodular hyperplasia.

View larger version (150K): [in a new window]   Figure 14b.  (a) Axial in-phase FMPSPGR T1-weighted, dynamic gadolinium-enhanced image (89.5/4.2, 80°) of a 46-year-old woman with diffuse fatty liver and focal nodular hyperplasia shows an enhancing mass, the focal nodular hyperplasia (arrow), in the right lobe of the liver (the mass was suspected from appearances on previous T1- and T2-weighted images). (b) Corresponding out-of-phase FMPSPGR image (78.3/2.3, 80°) poorly illustrates the mass (arrow). Because of suppression of the signal within surrounding fatty liver, the enhancing mass was less apparent on the out-of-phase image (b) than on the in-phase image (a). A small second lesion, a hemangioma (arrowhead), is incidentally noted just posterior to the focal nodular hyperplasia.

View larger version (151K): [in a new window]   Figure 15a. Figures 15, 16. (15) Axial in-phase FMPSPGR (89/4.2, 90°) (a) and out-of-phase FMPSPGR (89/3.1, 90°) (b) images of a 62-year-old woman show a 3-cm left adrenal mass, which had lower signal intensity (arrow) with the out-of-phase pulse sequence. Region-of-interest measurements of the mass were performed for both images. A substantial difference in mean signal intensity was measured between the images (in phase = 115, out of phase = 66; 42% decrease in signal intensity), which helped confirm the presence of fat within the lesion and the diagnosis of benign adrenal adenoma. (16) Axial in-phase FMPSPGR (74/4.2, 60°) (a) and out-of-phase (74/2.3, 60°) (b) images of a 76-year-old man with metastatic non-small cell and squamous cell carcinoma of the lung demonstrate a left adrenal mass (*). No differences in signal are seen on the two images. Metastases generally do not contain fat and do not exhibit the signal loss caused by chemical shift effects.

View larger version (155K): [in a new window]   Figure 15b. Figures 15, 16. (15) Axial in-phase FMPSPGR (89/4.2, 90°) (a) and out-of-phase FMPSPGR (89/3.1, 90°) (b) images of a 62-year-old woman show a 3-cm left adrenal mass, which had lower signal intensity (arrow) with the out-of-phase pulse sequence. Region-of-interest measurements of the mass were performed for both images. A substantial difference in mean signal intensity was measured between the images (in phase = 115, out of phase = 66; 42% decrease in signal intensity), which helped confirm the presence of fat within the lesion and the diagnosis of benign adrenal adenoma. (16) Axial in-phase FMPSPGR (74/4.2, 60°) (a) and out-of-phase (74/2.3, 60°) (b) images of a 76-year-old man with metastatic non-small cell and squamous cell carcinoma of the lung demonstrate a left adrenal mass (*). No differences in signal are seen on the two images. Metastases generally do not contain fat and do not exhibit the signal loss caused by chemical shift effects.

View larger version (151K): [in a new window]   Figure 16a. Figures 15, 16. (15) Axial in-phase FMPSPGR (89/4.2, 90°) (a) and out-of-phase FMPSPGR (89/3.1, 90°) (b) images of a 62-year-old woman show a 3-cm left adrenal mass, which had lower signal intensity (arrow) with the out-of-phase pulse sequence. Region-of-interest measurements of the mass were performed for both images. A substantial difference in mean signal intensity was measured between the images (in phase = 115, out of phase = 66; 42% decrease in signal intensity), which helped confirm the presence of fat within the lesion and the diagnosis of benign adrenal adenoma. (16) Axial in-phase FMPSPGR (74/4.2, 60°) (a) and out-of-phase (74/2.3, 60°) (b) images of a 76-year-old man with metastatic non-small cell and squamous cell carcinoma of the lung demonstrate a left adrenal mass (*). No differences in signal are seen on the two images. Metastases generally do not contain fat and do not exhibit the signal loss caused by chemical shift effects.

View larger version (143K): [in a new window]   Figure 16b. Figures 15, 16. (15) Axial in-phase FMPSPGR (89/4.2, 90°) (a) and out-of-phase FMPSPGR (89/3.1, 90°) (b) images of a 62-year-old woman show a 3-cm left adrenal mass, which had lower signal intensity (arrow) with the out-of-phase pulse sequence. Region-of-interest measurements of the mass were performed for both images. A substantial difference in mean signal intensity was measured between the images (in phase = 115, out of phase = 66; 42% decrease in signal intensity), which helped confirm the presence of fat within the lesion and the diagnosis of benign adrenal adenoma. (16) Axial in-phase FMPSPGR (74/4.2, 60°) (a) and out-of-phase (74/2.3, 60°) (b) images of a 76-year-old man with metastatic non-small cell and squamous cell carcinoma of the lung demonstrate a left adrenal mass (*). No differences in signal are seen on the two images. Metastases generally do not contain fat and do not exhibit the signal loss caused by chemical shift effects.

View larger version (157K): [in a new window]   Figure 17a.  (a) Coronal in-phase FMPSPGR image (57/4.2, 80°) of a 70-year-old woman obtained after intravenous administration of a gadolinium-chelate contrast agent reveals several nonenhancing simple renal cysts in the left kidney. (b) Coronal out-of-phase image (57.4/2.4, 80°) better delineates the renal margins and relative locations of the cysts.

View larger version (150K): [in a new window]   Figure 17b.  (a) Coronal in-phase FMPSPGR image (57/4.2, 80°) of a 70-year-old woman obtained after intravenous administration of a gadolinium-chelate contrast agent reveals several nonenhancing simple renal cysts in the left kidney. (b) Coronal out-of-phase image (57.4/2.4, 80°) better delineates the renal margins and relative locations of the cysts.

View larger version (153K): [in a new window]   Figure 18a.  (a) Coronal gadolinium-enhanced in-phase FMPSPGR image (90/4.2, 90°) of a 44-year-old woman demonstrates an enhancing renal mass (arrow). (Previous CT scan revealed a 2-cm complex low-attenuation lesion in the inferior pole of the left kidney that was not consistent with a simple renal cyst.) (b) Coronal out-of-phase image (90/2.3, 90°) obtained less than 30 seconds after a reveals a dramatic reduction in the signal intensity of the mass, despite the gadolinium enhancement.

View larger version (164K): [in a new window]   Figure 18b.  (a) Coronal gadolinium-enhanced in-phase FMPSPGR image (90/4.2, 90°) of a 44-year-old woman demonstrates an enhancing renal mass (arrow). (Previous CT scan revealed a 2-cm complex low-attenuation lesion in the inferior pole of the left kidney that was not consistent with a simple renal cyst.) (b) Coronal out-of-phase image (90/2.3, 90°) obtained less than 30 seconds after a reveals a dramatic reduction in the signal intensity of the mass, despite the gadolinium enhancement.

View larger version (124K): [in a new window]   Figure 19a. Figures 19, 20. (19a) Axial CT scan of a 69-year-old woman shows a mass in the left adrenal gland that appears to invade the left kidney (arrow). (19b) On an out-of-phase FMPSPGR image (90/6.3, 90°), a black boundary surrounding the adrenal gland and the kidney can be seen. On a subsequent follow-up MR image (not shown), the left adrenal mass had decreased in size, a finding consistent with an adrenal hemorrhage. (20) Sagittal out-of-phase FMPSPGR image (50/6, 90°) of a 57-year-old man with known colon cancer shows a mass (m) invading through the Gerota fascia and into the anterior aspect of the lower pole of the right kidney (arrow), as evidenced by the interruption of the dark cortical line that demarcates the renal margin. The mass has also invaded the liver, as shown by the absence of the dark line that should be seen along the inferior margin of the liver.

View larger version (136K): [in a new window]   Figure 19b. Figures 19, 20. (19a) Axial CT scan of a 69-year-old woman shows a mass in the left adrenal gland that appears to invade the left kidney (arrow). (19b) On an out-of-phase FMPSPGR image (90/6.3, 90°), a black boundary surrounding the adrenal gland and the kidney can be seen. On a subsequent follow-up MR image (not shown), the left adrenal mass had decreased in size, a finding consistent with an adrenal hemorrhage. (20) Sagittal out-of-phase FMPSPGR image (50/6, 90°) of a 57-year-old man with known colon cancer shows a mass (m) invading through the Gerota fascia and into the anterior aspect of the lower pole of the right kidney (arrow), as evidenced by the interruption of the dark cortical line that demarcates the renal margin. The mass has also invaded the liver, as shown by the absence of the dark line that should be seen along the inferior margin of the liver.

View larger version (183K): [in a new window]   Figure 20. Figures 19, 20. (19a) Axial CT scan of a 69-year-old woman shows a mass in the left adrenal gland that appears to invade the left kidney (arrow). (19b) On an out-of-phase FMPSPGR image (90/6.3, 90°), a black boundary surrounding the adrenal gland and the kidney can be seen. On a subsequent follow-up MR image (not shown), the left adrenal mass had decreased in size, a finding consistent with an adrenal hemorrhage. (20) Sagittal out-of-phase FMPSPGR image (50/6, 90°) of a 57-year-old man with known colon cancer shows a mass (m) invading through the Gerota fascia and into the anterior aspect of the lower pole of the right kidney (arrow), as evidenced by the interruption of the dark cortical line that demarcates the renal margin. The mass has also invaded the liver, as shown by the absence of the dark line that should be seen along the inferior margin of the liver.