Principles, techniques, and applications of T2*-based MR imaging and its special applications

Abstract

T2* relaxation refers to decay of transverse magnetization caused by a combination of spin-spin relaxation and magnetic field inhomogeneity. T2* relaxation is seen only with gradient-echo (GRE) imaging because transverse relaxation caused by magnetic field inhomogeneities is eliminated by the 180 degrees pulse at spin-echo imaging. T2* relaxation is one of the main determinants of image contrast with GRE sequences and forms the basis for many magnetic resonance (MR) applications, such as susceptibility-weighted (SW) imaging, perfusion MR imaging, and functional MR imaging. GRE sequences can be made predominantly T2* weighted by using a low flip angle, long echo time, and long repetition time. GRE sequences with T2*-based contrast are used to depict hemorrhage, calcification, and iron deposition in various tissues and lesions. SW imaging uses phase information in addition to T2*-based contrast to exploit the magnetic susceptibility differences of the blood and of iron and calcification in various tissues. Perfusion MR imaging exploits the signal intensity decrease that occurs with the passage of a high concentration of gadopentetate dimeglumine through the microvasculature. Change in oxygen saturation during specific tasks changes the local T2*, which leads to the blood oxygen level-dependent effect seen at functional MR imaging. The basics of T2* relaxation, T2*-weighted sequences, and their clinical applications are presented, followed by the principles, techniques, and clinical uses of four T2*-based applications, including SW imaging, perfusion MR imaging, functional MR imaging, and iron overload imaging.

Figure 1

Graph shows T2 and T2* relaxation curves. T2* is shorter than T2.

Figure 2

Cerebral hematoma. Axial MR image acquired with hemorrhagic GRE pulse sequence (Hemo; Siemens Healthcare, Malvern, Pa) (800/26; flip angle, 20°; bandwidth, 80 Hz/pixel; voxel size, 1.1 × 0.8 × 5.0 mm; field of view [FOV], 210 × 210 mm) shows a moderate-sized hematoma with dark borders (arrows) in the right posterior frontal region.

Figure 3a

Cavernoma with bleeding. (a) Axial multiplanar GRE MR image (MPGR; GE Healthcare, Chalfont St Giles, United Kingdom) (600/20; flip angle, 20°; bandwidth, 122 Hz/pixel; voxel size, 0.78 × 0.9 × 5.0 mm; FOV, 200 × 200 mm) shows a hematoma (arrowhead) with dark border and edema around it in the right occipital lobe. (b) Follow-up axial T2*-weighted fast GRE MR image (T2 FFE; Philips) (665/23; flip angle, 18°; bandwidth, 108.6 Hz/pixel; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm) shows reduction in the size of the acute hematoma. A residual low-signal-intensity area (arrowhead) is seen at the site of the hematoma, which was a complication of a cavernoma. A new small cavernoma (arrow) is seen in the left frontal lobe. T2*-weighted images depict more cavernomas than T2-weighted fast spin-echo images.

Figure 3b

Cavernoma with bleeding. (a) Axial multiplanar GRE MR image (MPGR; GE Healthcare, Chalfont St Giles, United Kingdom) (600/20; flip angle, 20°; bandwidth, 122 Hz/pixel; voxel size, 0.78 × 0.9 × 5.0 mm; FOV, 200 × 200 mm) shows a hematoma (arrowhead) with dark border and edema around it in the right occipital lobe. (b) Follow-up axial T2*-weighted fast GRE MR image (T2 FFE; Philips) (665/23; flip angle, 18°; bandwidth, 108.6 Hz/pixel; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm) shows reduction in the size of the acute hematoma. A residual low-signal-intensity area (arrowhead) is seen at the site of the hematoma, which was a complication of a cavernoma. A new small cavernoma (arrow) is seen in the left frontal lobe. T2*-weighted images depict more cavernomas than T2-weighted fast spin-echo images.

Figure 4a

Familial multiple cavernomas in a 16-year-old girl. (a) Axial T2-weighted fast spin-echo MR image (Philips) (4209/200; flip angle, 90°; bandwidth, 212 Hz/pixel; voxel size, 0.6 × 0.78 × 5 mm; FOV, 220 × 220 mm) shows multiple cavernomas (arrows) in both occipital lobes. (b) On axial T2*-weighted fast GRE MR image (T2 FFE; Philips) (665/23; flip angle, 18°; bandwidth, 108.6 Hz/pixel; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm), the hypointense lesions (arrows) are seen better because of the greater sensitivity of the T2*-weighted sequence for hemorrhagic products.

Figure 4b

Familial multiple cavernomas in a 16-year-old girl. (a) Axial T2-weighted fast spin-echo MR image (Philips) (4209/200; flip angle, 90°; bandwidth, 212 Hz/pixel; voxel size, 0.6 × 0.78 × 5 mm; FOV, 220 × 220 mm) shows multiple cavernomas (arrows) in both occipital lobes. (b) On axial T2*-weighted fast GRE MR image (T2 FFE; Philips) (665/23; flip angle, 18°; bandwidth, 108.6 Hz/pixel; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm), the hypointense lesions (arrows) are seen better because of the greater sensitivity of the T2*-weighted sequence for hemorrhagic products.

Figure 5a

Tumoral hemorrhage in a 17-year-old male adolescent with thalamic glioma. (a) Axial fluid-attenuated inversion-recovery MR image (Philips) (7000/140/2300 [TR msec/TE msec/inversion time msec]; flip angle, 90°; bandwidth, 220 Hz/pixel; voxel size, 0.75 × 0.81 × 5 mm; FOV, 220 × 220 mm) shows a hyperintense tumor in the right thalamus, with a hypointense area of hemorrhage (arrow). (b) Axial T2*-weighted fast GRE MR image (T2 FFE; Philips) (665/23; flip angle, 18°; bandwidth, 108.6 Hz/pixel; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm) shows blooming of the area of hemorrhage (arrow) in the tumor. T2*-weighted sequences should always be included in brain tumor imaging to detect tumoral bleeding.

Figure 5b

Tumoral hemorrhage in a 17-year-old male adolescent with thalamic glioma. (a) Axial fluid-attenuated inversion-recovery MR image (Philips) (7000/140/2300 [TR msec/TE msec/inversion time msec]; flip angle, 90°; bandwidth, 220 Hz/pixel; voxel size, 0.75 × 0.81 × 5 mm; FOV, 220 × 220 mm) shows a hyperintense tumor in the right thalamus, with a hypointense area of hemorrhage (arrow). (b) Axial T2*-weighted fast GRE MR image (T2 FFE; Philips) (665/23; flip angle, 18°; bandwidth, 108.6 Hz/pixel; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm) shows blooming of the area of hemorrhage (arrow) in the tumor. T2*-weighted sequences should always be included in brain tumor imaging to detect tumoral bleeding.

Figure 6a

Diffuse axonal injury in two patients imaged with multishot GRE echo-planar sequence (GRE-EPI; Philips) (3500/30; flip angle, 90°; bandwidth, 35.8 Hz/pixel; echo-planar imaging factor, 15; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm). (a) Axial multishot GRE echo-planar MR image shows multiple foci of low signal intensity (arrows) consistent with petechial hemorrhages in subcortical and periventricular white matter in both frontal and right occipital lobes. (b) Axial multishot GRE echo-planar MR image in another patient with head injury shows petechial hemorrhage (arrow) in the left parietal subcortical white matter and intraventricular hemorrhages (arrowheads) in the left occipital horn and third ventricle. T2*-weighted sequences form an essential part of the MR imaging done for diffuse axonal injury because this sequence can show small petechial hemorrhages, a characteristic finding of diffuse axonal injury, better than spin-echo or fast spin-echo sequences can.

Figure 6b

Diffuse axonal injury in two patients imaged with multishot GRE echo-planar sequence (GRE-EPI; Philips) (3500/30; flip angle, 90°; bandwidth, 35.8 Hz/pixel; echo-planar imaging factor, 15; voxel size, 0.9 × 1.1 × 5.0 mm; FOV, 220 × 192 mm). (a) Axial multishot GRE echo-planar MR image shows multiple foci of low signal intensity (arrows) consistent with petechial hemorrhages in subcortical and periventricular white matter in both frontal and right occipital lobes. (b) Axial multishot GRE echo-planar MR image in another patient with head injury shows petechial hemorrhage (arrow) in the left parietal subcortical white matter and intraventricular hemorrhages (arrowheads) in the left occipital horn and third ventricle. T2*-weighted sequences form an essential part of the MR imaging done for diffuse axonal injury because this sequence can show small petechial hemorrhages, a characteristic finding of diffuse axonal injury, better than spin-echo or fast spin-echo sequences can.

Figure 7a

Hemophilic arthropathy. Coronal T2*-weighted fast GRE MR images (T2 FFE; Philips) (695/14; flip angle, 25°; bandwidth, 108.6 Hz/pixel; voxel size, 0.58 × 0.73 × 4.0 mm; FOV, 178 × 170 mm) of middle (a) and posterior (b) parts of ankle joints in a hemophiliac patient show dark areas of hemosiderin deposition (arrows in a and b) in both joints. The right talus bone shows irregularity and osteochondral change (arrowhead in a). T2*-weighted GRE MR images are useful in evaluation of hemophilic arthropathy because they show hemosiderin deposition, articular cartilages, and osteochondral changes well.

Figure 7b

Hemophilic arthropathy. Coronal T2*-weighted fast GRE MR images (T2 FFE; Philips) (695/14; flip angle, 25°; bandwidth, 108.6 Hz/pixel; voxel size, 0.58 × 0.73 × 4.0 mm; FOV, 178 × 170 mm) of middle (a) and posterior (b) parts of ankle joints in a hemophiliac patient show dark areas of hemosiderin deposition (arrows in a and b) in both joints. The right talus bone shows irregularity and osteochondral change (arrowhead in a). T2*-weighted GRE MR images are useful in evaluation of hemophilic arthropathy because they show hemosiderin deposition, articular cartilages, and osteochondral changes well.

Figure 8a

Pigmented villonodular synovitis. (a) Sagittal multiplanar GRE MR image (MPGR; GE Healthcare) (400/15; flip angle, 30°; bandwidth, 122 Hz/pixel; voxel size, 0.51 × 0.67 × 4.0 mm; FOV, 130 × 130 mm) of the knee joint shows moderate to severe joint effusion and a Baker cyst (arrowhead). Synovium shows irregular nodular thickening with dark areas of hemosiderin deposition (arrows). (b) Sagittal contrast-enhanced T1-weighted spin-echo fat-saturated MR image (Philips) (500/10; flip angle, 90°; bandwidth, 108.5 Hz/ pixel; voxel size, 0.57 × 1.1 × 4 mm; FOV, 220 × 220 mm) shows diffuse irregular enhancement of the synovium.

Figure 8b

Pigmented villonodular synovitis. (a) Sagittal multiplanar GRE MR image (MPGR; GE Healthcare) (400/15; flip angle, 30°; bandwidth, 122 Hz/pixel; voxel size, 0.51 × 0.67 × 4.0 mm; FOV, 130 × 130 mm) of the knee joint shows moderate to severe joint effusion and a Baker cyst (arrowhead). Synovium shows irregular nodular thickening with dark areas of hemosiderin deposition (arrows). (b) Sagittal contrast-enhanced T1-weighted spin-echo fat-saturated MR image (Philips) (500/10; flip angle, 90°; bandwidth, 108.5 Hz/ pixel; voxel size, 0.57 × 1.1 × 4 mm; FOV, 220 × 220 mm) shows diffuse irregular enhancement of the synovium.

Figure 9a

Osteochondritis dissecans. Coronal (a) and sagittal (b) T2*-weighted fast GRE MR images (T2 FFE; Philips) (695/14; flip angle, 25°; bandwidth, 108.6 Hz/pixel; voxel size, 0.58 × 0.73 × 4.0 mm; FOV, 178 × 170 mm) of the left knee joint show a lesion (arrow in a and b) of osteochondritis dissecans on the articular surface of the lateral femoral condyle. Bones are dark on T2*-weighted images because of increased susceptibility among trabeculae, while joint fluid is bright because of the long T2. This makes these sequences useful for evaluation of articular cartilage, which is seen as a structure with signal intensity intermediate between signal intensity of bones and that of joint fluid.

Figure 9b

Osteochondritis dissecans. Coronal (a) and sagittal (b) T2*-weighted fast GRE MR images (T2 FFE; Philips) (695/14; flip angle, 25°; bandwidth, 108.6 Hz/pixel; voxel size, 0.58 × 0.73 × 4.0 mm; FOV, 178 × 170 mm) of the left knee joint show a lesion (arrow in a and b) of osteochondritis dissecans on the articular surface of the lateral femoral condyle. Bones are dark on T2*-weighted images because of increased susceptibility among trabeculae, while joint fluid is bright because of the long T2. This makes these sequences useful for evaluation of articular cartilage, which is seen as a structure with signal intensity intermediate between signal intensity of bones and that of joint fluid.

Figure 10a

Axial SW MR images obtained at 3 T (20/30; flip angle, 15°; bandwidth, 100 Hz/pixel; voxel size, 0.5 × 1.0 × 2.0 mm; FOV, 256 × 256 mm). Original magnitude image (a), original phase image (b), processed single-section magnitude image (c), and minimum intensity projection through eight sections (d) show enhancement of the veins and globi pallidi (arrowheads in b–d), which are seen as dark structures. There is also increased visibility of cortical fibers connecting adjacent sulci (arcuate fibers) (arrows in b–d), especially on phase (b) and processed magnitude (c) images. The thick (16-mm) minimum intensity projection SW image (d) shows dark vessels, with increased depiction of small veins.

Figure 10b

Axial SW MR images obtained at 3 T (20/30; flip angle, 15°; bandwidth, 100 Hz/pixel; voxel size, 0.5 × 1.0 × 2.0 mm; FOV, 256 × 256 mm). Original magnitude image (a), original phase image (b), processed single-section magnitude image (c), and minimum intensity projection through eight sections (d) show enhancement of the veins and globi pallidi (arrowheads in b–d), which are seen as dark structures. There is also increased visibility of cortical fibers connecting adjacent sulci (arcuate fibers) (arrows in b–d), especially on phase (b) and processed magnitude (c) images. The thick (16-mm) minimum intensity projection SW image (d) shows dark vessels, with increased depiction of small veins.

Figure 10c

Axial SW MR images obtained at 3 T (20/30; flip angle, 15°; bandwidth, 100 Hz/pixel; voxel size, 0.5 × 1.0 × 2.0 mm; FOV, 256 × 256 mm). Original magnitude image (a), original phase image (b), processed single-section magnitude image (c), and minimum intensity projection through eight sections (d) show enhancement of the veins and globi pallidi (arrowheads in b–d), which are seen as dark structures. There is also increased visibility of cortical fibers connecting adjacent sulci (arcuate fibers) (arrows in b–d), especially on phase (b) and processed magnitude (c) images. The thick (16-mm) minimum intensity projection SW image (d) shows dark vessels, with increased depiction of small veins.

Figure 10d

Axial SW MR images obtained at 3 T (20/30; flip angle, 15°; bandwidth, 100 Hz/pixel; voxel size, 0.5 × 1.0 × 2.0 mm; FOV, 256 × 256 mm). Original magnitude image (a), original phase image (b), processed single-section magnitude image (c), and minimum intensity projection through eight sections (d) show enhancement of the veins and globi pallidi (arrowheads in b–d), which are seen as dark structures. There is also increased visibility of cortical fibers connecting adjacent sulci (arcuate fibers) (arrows in b–d), especially on phase (b) and processed magnitude (c) images. The thick (16-mm) minimum intensity projection SW image (d) shows dark vessels, with increased depiction of small veins.

Figure 11a

Perfusion MR imaging of a patient after resection of a left parieto-occipital glioma. (a, c, d) Dynamic contrast-enhanced SW GRE echo-planar perfusion images (GRE-EPI; Siemens) (1490/40; flip angle, 90°; bandwidth, 1502 Hz/pixel; voxel size, 1.8 × 1.8 × 5.0 mm; FOV, 230 × 230 mm; echo-planar imaging factor, 128; 60 measurements) were obtained during rapid intravenous injection of gadopentetate dimeglumine. (a) Axial source image shows that all of the areas having high concentrations of gadolinium, such as arteries, are dark. (b) Graph of signal intensity versus time represents changes in signal intensity during injection of gadolinium-based contrast agent. Decrease in signal intensity between BLe and FPe lines represents first pass of contrast agent through the circulation. BLe = baseline ends, BLs = baseline starts, FPe = end of first pass. (c, d) Tracer kinetics software is applied to the curve of signal intensity versus time to obtain various color maps, where red and yellow represent high value and blue represents low value of the particular parameter. (c) Color maps show cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), time to peak (TTP), corrected cerebral blood volume (CCBV), and permeability map (K2). (d) Separate map shows CCBV. In this case, blue (representing low flow) is seen in and around the resection cavity (arrow in c and d), which suggests that there is no viable tumor in this region. (Case courtesy of Meher Ursekar, MD, Jankharia-Piramal Imaging, Mumbai, India.)

Figure 11b

Perfusion MR imaging of a patient after resection of a left parieto-occipital glioma. (a, c, d) Dynamic contrast-enhanced SW GRE echo-planar perfusion images (GRE-EPI; Siemens) (1490/40; flip angle, 90°; bandwidth, 1502 Hz/pixel; voxel size, 1.8 × 1.8 × 5.0 mm; FOV, 230 × 230 mm; echo-planar imaging factor, 128; 60 measurements) were obtained during rapid intravenous injection of gadopentetate dimeglumine. (a) Axial source image shows that all of the areas having high concentrations of gadolinium, such as arteries, are dark. (b) Graph of signal intensity versus time represents changes in signal intensity during injection of gadolinium-based contrast agent. Decrease in signal intensity between BLe and FPe lines represents first pass of contrast agent through the circulation. BLe = baseline ends, BLs = baseline starts, FPe = end of first pass. (c, d) Tracer kinetics software is applied to the curve of signal intensity versus time to obtain various color maps, where red and yellow represent high value and blue represents low value of the particular parameter. (c) Color maps show cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), time to peak (TTP), corrected cerebral blood volume (CCBV), and permeability map (K2). (d) Separate map shows CCBV. In this case, blue (representing low flow) is seen in and around the resection cavity (arrow in c and d), which suggests that there is no viable tumor in this region. (Case courtesy of Meher Ursekar, MD, Jankharia-Piramal Imaging, Mumbai, India.)

Figure 11c

Perfusion MR imaging of a patient after resection of a left parieto-occipital glioma. (a, c, d) Dynamic contrast-enhanced SW GRE echo-planar perfusion images (GRE-EPI; Siemens) (1490/40; flip angle, 90°; bandwidth, 1502 Hz/pixel; voxel size, 1.8 × 1.8 × 5.0 mm; FOV, 230 × 230 mm; echo-planar imaging factor, 128; 60 measurements) were obtained during rapid intravenous injection of gadopentetate dimeglumine. (a) Axial source image shows that all of the areas having high concentrations of gadolinium, such as arteries, are dark. (b) Graph of signal intensity versus time represents changes in signal intensity during injection of gadolinium-based contrast agent. Decrease in signal intensity between BLe and FPe lines represents first pass of contrast agent through the circulation. BLe = baseline ends, BLs = baseline starts, FPe = end of first pass. (c, d) Tracer kinetics software is applied to the curve of signal intensity versus time to obtain various color maps, where red and yellow represent high value and blue represents low value of the particular parameter. (c) Color maps show cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), time to peak (TTP), corrected cerebral blood volume (CCBV), and permeability map (K2). (d) Separate map shows CCBV. In this case, blue (representing low flow) is seen in and around the resection cavity (arrow in c and d), which suggests that there is no viable tumor in this region. (Case courtesy of Meher Ursekar, MD, Jankharia-Piramal Imaging, Mumbai, India.)

Figure 11d

Perfusion MR imaging of a patient after resection of a left parieto-occipital glioma. (a, c, d) Dynamic contrast-enhanced SW GRE echo-planar perfusion images (GRE-EPI; Siemens) (1490/40; flip angle, 90°; bandwidth, 1502 Hz/pixel; voxel size, 1.8 × 1.8 × 5.0 mm; FOV, 230 × 230 mm; echo-planar imaging factor, 128; 60 measurements) were obtained during rapid intravenous injection of gadopentetate dimeglumine. (a) Axial source image shows that all of the areas having high concentrations of gadolinium, such as arteries, are dark. (b) Graph of signal intensity versus time represents changes in signal intensity during injection of gadolinium-based contrast agent. Decrease in signal intensity between BLe and FPe lines represents first pass of contrast agent through the circulation. BLe = baseline ends, BLs = baseline starts, FPe = end of first pass. (c, d) Tracer kinetics software is applied to the curve of signal intensity versus time to obtain various color maps, where red and yellow represent high value and blue represents low value of the particular parameter. (c) Color maps show cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), time to peak (TTP), corrected cerebral blood volume (CCBV), and permeability map (K2). (d) Separate map shows CCBV. In this case, blue (representing low flow) is seen in and around the resection cavity (arrow in c and d), which suggests that there is no viable tumor in this region. (Case courtesy of Meher Ursekar, MD, Jankharia-Piramal Imaging, Mumbai, India.)

Figure 12

Axial composite image in a healthy person. Functional MR imaging was performed at 3 T (Philips) (2000/30; flip angle, 90°; bandwidth, 49.2 Hz/pixel; echo-planar imaging factor, 47; voxel size, 3 × 3 × 4 mm; FOV, 230 × 230 mm). Signal change from increased local T2* caused by less deoxyhemoglobin during activation was converted into a color map by using statistical processing. This color map was overlaid onto the anatomic image to produce the composite image. This axial composite image shows blood oxygen level–dependent activation in the right hand’s motor area (yellow) and the left hand’s motor area (blue) from a boxcar experiment of alternating left and right finger tapping.

Figure 13a

Iron overload imaging of the upper portion of the abdomen. Axial T2*-weighted GRE MR images (Siemens) (500/2.4–30; flip angle, 60°; bandwidth, 380 Hz/pixel; voxel size, 1.4 × 1.0 × 6.0 mm; FOV, 400 × 400 mm) acquired with TEs of 2.4 msec (a), 5.2 msec (b), 8.0 msec (c), and 10.8 msec (d) show progressive darkening of the liver with increasing TE, which is suggestive of iron overload.

Figure 13b

Iron overload imaging of the upper portion of the abdomen. Axial T2*-weighted GRE MR images (Siemens) (500/2.4–30; flip angle, 60°; bandwidth, 380 Hz/pixel; voxel size, 1.4 × 1.0 × 6.0 mm; FOV, 400 × 400 mm) acquired with TEs of 2.4 msec (a), 5.2 msec (b), 8.0 msec (c), and 10.8 msec (d) show progressive darkening of the liver with increasing TE, which is suggestive of iron overload.

Figure 13c

Iron overload imaging of the upper portion of the abdomen. Axial T2*-weighted GRE MR images (Siemens) (500/2.4–30; flip angle, 60°; bandwidth, 380 Hz/pixel; voxel size, 1.4 × 1.0 × 6.0 mm; FOV, 400 × 400 mm) acquired with TEs of 2.4 msec (a), 5.2 msec (b), 8.0 msec (c), and 10.8 msec (d) show progressive darkening of the liver with increasing TE, which is suggestive of iron overload.

Figure 13d

Iron overload imaging of the upper portion of the abdomen. Axial T2*-weighted GRE MR images (Siemens) (500/2.4–30; flip angle, 60°; bandwidth, 380 Hz/pixel; voxel size, 1.4 × 1.0 × 6.0 mm; FOV, 400 × 400 mm) acquired with TEs of 2.4 msec (a), 5.2 msec (b), 8.0 msec (c), and 10.8 msec (d) show progressive darkening of the liver with increasing TE, which is suggestive of iron overload.