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495
Pictorial
Thymic
Masses
Essay
on MR Imaging
Paul L. Molina,1 Marilyn J. Siegel, and Harvey S. Glazer
MR imaging is an excellent technique for identifying and defining the extent of thymic tumors. Ti-weighted spin-echo MR images (e.g., 600/15 [TR/TE]) best demonstrate tumor extent, and T2-weighted images (e.g., 2500/90 [TRITE]) help differentiate “cystic” from solid thymic masses. Cyst formation and/or hemon’hage appear as areas of high signal intensity (greater than that of fat) on T2-weighted images. Focal areas of low signal intensity
(less
than
that
of muscle)
correspond
pathologically
Normal
a homogeneous, relatively low signal intensity, slightly greater than that of muscle (Fig. 1A). On T2-weighted images the thymus increases in relative signal intensity and approaches that of fat (Fig. 1 B) [1]. In younger children, the thymus often has a quadrilateral shape and convex lateral contours (Fig.
to
fibrous capsules and septa, air, or calcification. In this report, the MR appearance of the normal thymus is reviewed briefly, and the gamut of abnormal thymic masses on MR is illustrated.
Fig. 1.-Normal neonate. A, Ti-weighted
a homogeneous signal
thymus
in
axial MR image
quadrilateral
intensity
slightly
higher
a
Thymus
The normal thymus lies in the anterior mediastinum at a level between the horizontal portion of the left innominate vein superiorly and the horizontal course of the right pulmonary artery inferiorly. On Ti -weighted images the thymus has
2-month-old
(600/15)
shows
thymus (thy) with than
that of chest
wall musculature. B, T2-weighted axial MR mus has a signal IntensIty cutaneous fat. s = superior vena cava, of left brachiocephalic vein,
Received
February
All authors: requests
image (2500/90). Thysimilar to that of subby = horizontal portion A = aortic arch.
21, 1990;
Mallinckrodt
accepted
Institute
after revision
of Radiology,
April 6. 1990.
Washington
University
School
of Medicine,
510 S. Kingshighway
to P. L. Molina.
AJR 155:495-500, September
1990 0361 -803x/90/1
553-0495
© American
Roentgen
Ray Society
Blvd.,
St. Louis,
MO 63110.
Address
reprint
MOLINA
496
ET AL.
AJR:155,
September
1990
Downloaded from www.ajronline.org by 117.253.152.201 on 11/14/15 from IP address 117.253.152.201. Copyright ARRS. For personal use only; all rights reserved
Fig. 2.-Ti-weighted axial MR image (680/30) in a 2-year-old child shows a normal quadrilateral thymus with convex lateral margins. Signal intensity is homogeneous and slightly higher than that of chest wall musculature. v = superior vena cava, aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
Fig. 3.-Balanced axial MR image (2715/20) in a 24-year-old man. Thymus is triangular with a dominant left lobe (arrowhead). aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
Fig. 4.-Surgically proved posterior mediastinal thymus in a 16-month-old boy. A, Ti-weighted axial MR Image (1091/20) shows a normal homogeneous thymus in anterior medlastinum with posterior extension to chest wall between superior vena cava (v) and trachea (1). Signal intensity of thymus is greater than that of muscle and less than that of fat. B, On T2-weighted image (3200/80), signal intensity of thymus is slightly greater than that of fat. by = horizontal portion of left brachiocephalic vein, A = aortic arch.
2), whereas in older children and young adults it is triangular or bibbed with a more prominent left lobe (Fig. 3) [2]. Although the bulk of the thymus usually is positioned antenor to the great vessels, it may extend above the level of the left innominate vein or posteriorly between the superior
vena cava and trachea
(Fig. 4). The diagnosis
posterior
thymic extension is suggested nuity of the aberrant thymus with anterior
of superior
or
by the direct contimediastinal thymic
tissue, their identical signal intensities, and the absence narrowing or displacement of adjacent structures [2].
of
Thymic Cysts Congenital
thymic
cysts
are rare
lesions
resulting
from
a
persistent
Fig. 5.-Thymic cyst with surgically proved hemorrhage. Ti-weighted axial MR image (300/ 30) shows a well-circumscribed anterior mediasfinal mass (arrowheads) of very high signal intensity, reflecting Ti shortening due to paramagnetic effect of methemoglobin. aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
thymopharyngeal
tensity on Ti -weighted
duct. MR images
They are low in signal inand high in signal intensity
on T2-weighted images. If spontaneous hemorrhage into a thymic cyst occurs, the mass will show relatively higher signal intensity on Ti -weighted images, reflecting Ti shortening due
to the paramagnetic
effect of methemoglobin
(Fig. 5).
Thymic cysts also may occur in association with Hodgkin disease and are believed to be related to initial thymic involvement with the disease, rather than being a result of radiotherapy or chemotherapy. The cysts can persist despite regression of disease elsewhere in the chest, and they may simulate persistent or recurrent disease (Fig. 6) [3].
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AJR:155,
September
MR
1990
OF THYMIC
497
MASSES
Fig. 6.-Cystic lymphoma in a 14-year-old boy. A, Ti-weighted axial MR image (740/30) at level of left atrium shows a large, Intermediate-signal-intensity mass with central area of high signal Intensity in left lobe of thymus. High-signal-intensity central component is surrounded by thin rim of low signal Intensity. Supraclavicular lymph node biopsy revealed nodular sclerosing Hodgkin lymphoma. Artifact from overlying Infusaport device (asterisk). B, Ti-weighted MR image (700/30) after 4 months of chemotherapy shows diminution of left lobe lesion. Thick rim of low signal intensity (arrowheads) now surrounds central area of high signal intensity. Identical findings were noted on T2-welghted images. Patient subsequently underwent surgical excision of residual mass. C, Section of pathologic specimen shows cystic mass filled with brownish yellow material. Microscopic examination revealed a thymic cyst containing cellular keratinous material with numerous cholesterol clefts. Cyst was encircled by extracellular hemoslderin and fibrotic tissue (arrowheads). No residual tumor was identified. F = mediastinal fat. aa = ascending aorta, LA = left atrium, da = descending aorta.
A
B
C
Fig. 7.-il-year-old child with lymphoma. A, Ti-weighted axial MR image (810/30) shows diffuse thymic enlargement, particularly of right lobe. Signal intensity of infiltrated than that of muscle and less than that of fat. B, On T2-weighted image (3100/90), thymic mass has a signal intensity greater than that of fat. C, T2-weighted image (2000/90) after 1 month of chemotherapy. Mass has decreased in size and relative signal intensity. s = superior vena cava, A = aortic arch.
Neoplasms The MR diagnosis of thymic neoplasm is usually based on the presence of diffuse or focal enlargement of the gland. The signal intensity of the abnormal thymus is similar to that of the normal thymus, although the abnormal thymus is often inhomogeneous on T2-weighted MR images. Lymphomas, particularly the nodular sclerosing form of
thymus
is greater
Hodgkin disease, commonly infiltrate and enlarge the thymus (Fig. 7). Lymphomatous involvement usually has a relatively low signal intensity on Ti -weighted MR images and a high signal intensity on T2-weighted images. Rarely, the signal intensity remains low on T2-weighted images. This observation is important because tumors of relatively low signal intensity on 12-weighted images do not decrease in size with therapy as much as higher intensity masses do, possibly
MOLINA
498
El
AL.
AJR:155,
September
1990
Downloaded from www.ajronline.org by 117.253.152.201 on 11/14/15 from IP address 117.253.152.201. Copyright ARRS. For personal use only; all rights reserved
Fig. 8.-Leukemic A, Ti-weighted
infiltration of thymus. MR image (500/30) shows diffuse thymic enlargement, with signal intensity slightly greater than that of muscle. Focal area of high signal Intensity (arrowhead) is presumed to represent hemorrhage within mass. B, On relatively T2-weighted image (1500/90), thymus Is very heterogeneous in appearance, with mixed areas of low and high signal intensity. Peripheral blood smear and bone marrow aspiration biopsy confirmed diagnosis of leukemia. v = superior vena cava, aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
Fig. 9-33-year-old thymoma. A, Ti-weighted
woman MR
image
with
a
benign
(500/30).
Round
mass arising in left lobe of thymus has an inhomogeneous signal intensity Intermediate between that of muscle and fat. Low-signal-intensity rim (arrowheads) surrounds tumor mass. v = superior vena cava, aa = ascending aorta, da = descending aorta. B, Pathologic specImen. Bivalved thymoma nicely shows fibrous capsule (arrows) corresponding to low-signal-intensity rim seen in A.
reflecting
large
areas
mass [4]. Indeed,
of residual
fibrous
stroma
MR may help differentiate
within
the
between
poston Ti and
treatment fibrosis, which has a low signal intensity 12-weighted images, and lymphoma, which increases in relative signal intensity on T2-weighted images. However, persisting high signal on 12-weighted images after treatment is -
a nonspecific
finding
and also may represent
inflammation,
hemorrhage, or even posttreatment change. Leukemic infiltration of the thymus also results in diffuse thymic enlargement, and its MR signal characteristics are similar to those just described for lymphoma (i.e., low signal intensity on li-weighted images and high signal intensity on 12-weighted images) (Fig. 8). Thymomas typically appear on MR images as oval, round, or lobulated masses that distort the normally smooth outer
margin of the thymus (Fig. 9). Signal intensity is relatively on Ti -weighted images and increases on 12-weighted
low im-
ages.
Germ cell tumors of the thymus usually represent dermoid cysts or benign teratomas. On MA, these tumors are typically well circumscribed and inhomogeneous, containing a variable admixture of fat, water, soft tissue, and calcification (Fig. i 0). Thymolipoma
is a rare,
benign
thymic
tumor
occurring
in
children and young adults. It can grow exceedingly large and may displace mediastinal structures. On Ti -weighted MR images
this
neoplasm
signal intensity,
reflecting
ii). Linear strands of residual fibrous stroma
coursing
through
appears
as a mass
its predominantly lower and/or
the mass [5].
of relatively
high
fatty content
(Fig.
signal intensity thymic tissue
representing may be seen
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AJR:155,
September
1990
MR
OF
THYMIC
499
MASSES
Fig. 10.-Benign cystic teratoma. A, T2-weighted MR image (2100/90) shows low-signal-intensity band (arrowheads) separating two areas of very high signal intensity within thymic mass. v = superior vena cava, aa = ascending aorta, pa = pulmonary artery, da = descending aorta. B, Pathologic specimen. Low-signal-intensity band on MR Image corresponds to fibrous septum (arrow) separating two large cystic areas (asterisks) within teratoma. Calcification was present on other sections.
Fig. ii.-Thymolipoma. Ti-weighted axial MR image (TA gated, TE = 28 msec) at midthoracic level shows large mass filling entire left hemithorax, with displacement of heart to right. At higher levels, this mass appeared to originate from anterior mediastinum. Mass is mostly high in signal intensity, reflecting its predominantly fatty content. Low-signal-intensity strands course through mass, probably representing residual thymic tissue and/or fibrous stroma. (Courtesy of A. Shirkhoda, Royal Oak, Ml.)
1, -
‘
Fig. 12.-2-month-old neonate with chest wall and mediastinal lymphangioma. A, On Ti-weighted axial MR image (500/30), chest wall mass (M) is of homogeneous low signal intensity, less than that of muscle. Note defined medium-signal-intensity mass infiltrating axilla and right lobe of thymus (arrows). B, On T2-weighted MR image (1800/120), lymphangioma is of very high signal intensity and is easily differentiated from adjacent structures. lymphangioma had replaced entire right lobe of thymus. C, Histologic section shows typical lymphangioma with large lymphatic spaces (L) separated by septa containing fat and fibrous tissue. s = superior vena cava, T = thymus. (A and B reprinted with permission from Siegel [6].)
Lymphangioma Mediastinal lymphangiomas usually are extensions of cervical lesions, and although most are well defined, they may infiltrate adjacent structures and be difficult to remove surgically (Fig. 12). Typically they appear as thin-walled, multiloculated cystic masses of very high signal intensity on 12weighted MR images. Components of the mass also may be
of relatively high signal intensity fluid has a high lipid content. Ectopic
Parathyroid
poorly
At surgery,
images
if the
Adenoma
Rarely, intrathymic parathyroid parathyroid adenomas elsewhere hemorrhage
on Ti-weighted
also
into the adenoma
adenomas are seen. As with in the mediastinum or neck, can occur
(Fig. 13).
MOLINA
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500
El
AL.
AJR:1 55, September
1990
Fig. i3.-lntrathymic parathyroid adenoma in a 17-year-old girl with persistent hypercalcemia after surgical neck exploration. A and B, Ti-weighted (1077/15) (A) and T2weighted (2263/90) (B) MR images show oval Icsion (arrowheads) in left lobe of thymus. Lesion is of very high signal intensity on both images. Transcervical thymectomy revealed a hemorrhagic parathyroid adenoma within thymus. v = superior vena cava, aa = ascending aorta, da = descending aorta.
Fig. 14.-Ti-weighted axial MR images (500/ 30) in a 25-year-old woman with benign thymic hyperplasia associated with Graves disease. A, There is marked enlargement of both lobes of thyroid due to hyperthyroidism. Signal intensity is homogeneous and intermediate between that of muscle and fat. Jugular vein (arrow); carotid artery (arrowhead). t = trachea. B, Thymus also is diffusely enlarged and has a signal intensity slightly greater than that of muscle. Mediastinal biopsy of thymus revealed benign thymic hyperplasia. v = superior vena cava, aa = aortic arch, pa = pulmonary artery, da = descending aorta.
Thymic True
Hyperplasia thymic
cortex and medulla can be caused by hyperthyroidism (Fig. 14), red cell aplasia, or chemotherapy. On MR images, diffuse enlargement of the thymus is recognizable, especially in its thickness, with preservation of the normal shape. Its signal intensity is similar to that of the normal thymus. Rebound thymic hyperplasia after
chemotherapy
hyperplasia
REFERENCES
presents
involving
both
a diagnostic
problem
because
it may
simulate a primary neoplasm or recurrent disease. If the patient is doing well clinically, and no recurrent or residual disease is evident elsewhere in the body, the patient can be
followed corroborates
up with serial scanning. the diagnosis
A gradual
of benign
thymic
reduction hyperplasia.
in size
1 . DeGeer G, Webb WA, Gamsu G. Normal thymus: assessment with MR and CT. Radiology i986;158:313-317 2. Siegel MJ, Glazer HS, Wiener JI, Molina PL. Normal and abnormal thymus in childhood: MR imaging. Radiology i989;172:367-371 3. Lindfors KK, Meyer JE, Dedrick G, Hassell LA, Harris NL. Thymic cysts in mediastinal Hodgkin disease. Radiology i985;1 56:37-41 4. Nyman AS, Rehn SM, Glimelius BLG, Hagberg HE, Hemmingsson AL, Sundstrom CJ. Residual mediastinal masses in Hodgkin disease: prediction of size with MR imaging. Radiology 1989:170:435-440 5. Shirkhoda A, Chasen MH, Eftekhari F, Goldman AM, Decaro LF. MR imaging of mediastinal thymolipoma. J Comput Assist Tomogr i987;1 1: 364-365 6. Siegel MJ. Pediatric applications. In: Lee JKT, Sagel SS, Stanley RJ, eds. Computed body tomography with MRI correlation. New York: Raven, 1989:1063-1099
495
Pictorial
Thymic
Masses
Essay
on MR Imaging
Paul L. Molina,1 Marilyn J. Siegel, and Harvey S. Glazer
MR imaging is an excellent technique for identifying and defining the extent of thymic tumors. Ti-weighted spin-echo MR images (e.g., 600/15 [TR/TE]) best demonstrate tumor extent, and T2-weighted images (e.g., 2500/90 [TRITE]) help differentiate “cystic” from solid thymic masses. Cyst formation and/or hemon’hage appear as areas of high signal intensity (greater than that of fat) on T2-weighted images. Focal areas of low signal intensity
(less
than
that
of muscle)
correspond
pathologically
Normal
a homogeneous, relatively low signal intensity, slightly greater than that of muscle (Fig. 1A). On T2-weighted images the thymus increases in relative signal intensity and approaches that of fat (Fig. 1 B) [1]. In younger children, the thymus often has a quadrilateral shape and convex lateral contours (Fig.
to
fibrous capsules and septa, air, or calcification. In this report, the MR appearance of the normal thymus is reviewed briefly, and the gamut of abnormal thymic masses on MR is illustrated.
Fig. 1.-Normal neonate. A, Ti-weighted
a homogeneous signal
thymus
in
axial MR image
quadrilateral
intensity
slightly
higher
a
Thymus
The normal thymus lies in the anterior mediastinum at a level between the horizontal portion of the left innominate vein superiorly and the horizontal course of the right pulmonary artery inferiorly. On Ti -weighted images the thymus has
2-month-old
(600/15)
shows
thymus (thy) with than
that of chest
wall musculature. B, T2-weighted axial MR mus has a signal IntensIty cutaneous fat. s = superior vena cava, of left brachiocephalic vein,
Received
February
All authors: requests
image (2500/90). Thysimilar to that of subby = horizontal portion A = aortic arch.
21, 1990;
Mallinckrodt
accepted
Institute
after revision
of Radiology,
April 6. 1990.
Washington
University
School
of Medicine,
510 S. Kingshighway
to P. L. Molina.
AJR 155:495-500, September
1990 0361 -803x/90/1
553-0495
© American
Roentgen
Ray Society
Blvd.,
St. Louis,
MO 63110.
Address
reprint
MOLINA
496
ET AL.
AJR:155,
September
1990
Downloaded from www.ajronline.org by 117.253.152.201 on 11/14/15 from IP address 117.253.152.201. Copyright ARRS. For personal use only; all rights reserved
Fig. 2.-Ti-weighted axial MR image (680/30) in a 2-year-old child shows a normal quadrilateral thymus with convex lateral margins. Signal intensity is homogeneous and slightly higher than that of chest wall musculature. v = superior vena cava, aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
Fig. 3.-Balanced axial MR image (2715/20) in a 24-year-old man. Thymus is triangular with a dominant left lobe (arrowhead). aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
Fig. 4.-Surgically proved posterior mediastinal thymus in a 16-month-old boy. A, Ti-weighted axial MR Image (1091/20) shows a normal homogeneous thymus in anterior medlastinum with posterior extension to chest wall between superior vena cava (v) and trachea (1). Signal intensity of thymus is greater than that of muscle and less than that of fat. B, On T2-weighted image (3200/80), signal intensity of thymus is slightly greater than that of fat. by = horizontal portion of left brachiocephalic vein, A = aortic arch.
2), whereas in older children and young adults it is triangular or bibbed with a more prominent left lobe (Fig. 3) [2]. Although the bulk of the thymus usually is positioned antenor to the great vessels, it may extend above the level of the left innominate vein or posteriorly between the superior
vena cava and trachea
(Fig. 4). The diagnosis
posterior
thymic extension is suggested nuity of the aberrant thymus with anterior
of superior
or
by the direct contimediastinal thymic
tissue, their identical signal intensities, and the absence narrowing or displacement of adjacent structures [2].
of
Thymic Cysts Congenital
thymic
cysts
are rare
lesions
resulting
from
a
persistent
Fig. 5.-Thymic cyst with surgically proved hemorrhage. Ti-weighted axial MR image (300/ 30) shows a well-circumscribed anterior mediasfinal mass (arrowheads) of very high signal intensity, reflecting Ti shortening due to paramagnetic effect of methemoglobin. aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
thymopharyngeal
tensity on Ti -weighted
duct. MR images
They are low in signal inand high in signal intensity
on T2-weighted images. If spontaneous hemorrhage into a thymic cyst occurs, the mass will show relatively higher signal intensity on Ti -weighted images, reflecting Ti shortening due
to the paramagnetic
effect of methemoglobin
(Fig. 5).
Thymic cysts also may occur in association with Hodgkin disease and are believed to be related to initial thymic involvement with the disease, rather than being a result of radiotherapy or chemotherapy. The cysts can persist despite regression of disease elsewhere in the chest, and they may simulate persistent or recurrent disease (Fig. 6) [3].
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AJR:155,
September
MR
1990
OF THYMIC
497
MASSES
Fig. 6.-Cystic lymphoma in a 14-year-old boy. A, Ti-weighted axial MR image (740/30) at level of left atrium shows a large, Intermediate-signal-intensity mass with central area of high signal Intensity in left lobe of thymus. High-signal-intensity central component is surrounded by thin rim of low signal Intensity. Supraclavicular lymph node biopsy revealed nodular sclerosing Hodgkin lymphoma. Artifact from overlying Infusaport device (asterisk). B, Ti-weighted MR image (700/30) after 4 months of chemotherapy shows diminution of left lobe lesion. Thick rim of low signal intensity (arrowheads) now surrounds central area of high signal intensity. Identical findings were noted on T2-welghted images. Patient subsequently underwent surgical excision of residual mass. C, Section of pathologic specimen shows cystic mass filled with brownish yellow material. Microscopic examination revealed a thymic cyst containing cellular keratinous material with numerous cholesterol clefts. Cyst was encircled by extracellular hemoslderin and fibrotic tissue (arrowheads). No residual tumor was identified. F = mediastinal fat. aa = ascending aorta, LA = left atrium, da = descending aorta.
A
B
C
Fig. 7.-il-year-old child with lymphoma. A, Ti-weighted axial MR image (810/30) shows diffuse thymic enlargement, particularly of right lobe. Signal intensity of infiltrated than that of muscle and less than that of fat. B, On T2-weighted image (3100/90), thymic mass has a signal intensity greater than that of fat. C, T2-weighted image (2000/90) after 1 month of chemotherapy. Mass has decreased in size and relative signal intensity. s = superior vena cava, A = aortic arch.
Neoplasms The MR diagnosis of thymic neoplasm is usually based on the presence of diffuse or focal enlargement of the gland. The signal intensity of the abnormal thymus is similar to that of the normal thymus, although the abnormal thymus is often inhomogeneous on T2-weighted MR images. Lymphomas, particularly the nodular sclerosing form of
thymus
is greater
Hodgkin disease, commonly infiltrate and enlarge the thymus (Fig. 7). Lymphomatous involvement usually has a relatively low signal intensity on Ti -weighted MR images and a high signal intensity on T2-weighted images. Rarely, the signal intensity remains low on T2-weighted images. This observation is important because tumors of relatively low signal intensity on 12-weighted images do not decrease in size with therapy as much as higher intensity masses do, possibly
MOLINA
498
El
AL.
AJR:155,
September
1990
Downloaded from www.ajronline.org by 117.253.152.201 on 11/14/15 from IP address 117.253.152.201. Copyright ARRS. For personal use only; all rights reserved
Fig. 8.-Leukemic A, Ti-weighted
infiltration of thymus. MR image (500/30) shows diffuse thymic enlargement, with signal intensity slightly greater than that of muscle. Focal area of high signal Intensity (arrowhead) is presumed to represent hemorrhage within mass. B, On relatively T2-weighted image (1500/90), thymus Is very heterogeneous in appearance, with mixed areas of low and high signal intensity. Peripheral blood smear and bone marrow aspiration biopsy confirmed diagnosis of leukemia. v = superior vena cava, aa = ascending aorta, pa = pulmonary artery, da = descending aorta.
Fig. 9-33-year-old thymoma. A, Ti-weighted
woman MR
image
with
a
benign
(500/30).
Round
mass arising in left lobe of thymus has an inhomogeneous signal intensity Intermediate between that of muscle and fat. Low-signal-intensity rim (arrowheads) surrounds tumor mass. v = superior vena cava, aa = ascending aorta, da = descending aorta. B, Pathologic specImen. Bivalved thymoma nicely shows fibrous capsule (arrows) corresponding to low-signal-intensity rim seen in A.
reflecting
large
areas
mass [4]. Indeed,
of residual
fibrous
stroma
MR may help differentiate
within
the
between
poston Ti and
treatment fibrosis, which has a low signal intensity 12-weighted images, and lymphoma, which increases in relative signal intensity on T2-weighted images. However, persisting high signal on 12-weighted images after treatment is -
a nonspecific
finding
and also may represent
inflammation,
hemorrhage, or even posttreatment change. Leukemic infiltration of the thymus also results in diffuse thymic enlargement, and its MR signal characteristics are similar to those just described for lymphoma (i.e., low signal intensity on li-weighted images and high signal intensity on 12-weighted images) (Fig. 8). Thymomas typically appear on MR images as oval, round, or lobulated masses that distort the normally smooth outer
margin of the thymus (Fig. 9). Signal intensity is relatively on Ti -weighted images and increases on 12-weighted
low im-
ages.
Germ cell tumors of the thymus usually represent dermoid cysts or benign teratomas. On MA, these tumors are typically well circumscribed and inhomogeneous, containing a variable admixture of fat, water, soft tissue, and calcification (Fig. i 0). Thymolipoma
is a rare,
benign
thymic
tumor
occurring
in
children and young adults. It can grow exceedingly large and may displace mediastinal structures. On Ti -weighted MR images
this
neoplasm
signal intensity,
reflecting
ii). Linear strands of residual fibrous stroma
coursing
through
appears
as a mass
its predominantly lower and/or
the mass [5].
of relatively
high
fatty content
(Fig.
signal intensity thymic tissue
representing may be seen
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AJR:155,
September
1990
MR
OF
THYMIC
499
MASSES
Fig. 10.-Benign cystic teratoma. A, T2-weighted MR image (2100/90) shows low-signal-intensity band (arrowheads) separating two areas of very high signal intensity within thymic mass. v = superior vena cava, aa = ascending aorta, pa = pulmonary artery, da = descending aorta. B, Pathologic specimen. Low-signal-intensity band on MR Image corresponds to fibrous septum (arrow) separating two large cystic areas (asterisks) within teratoma. Calcification was present on other sections.
Fig. ii.-Thymolipoma. Ti-weighted axial MR image (TA gated, TE = 28 msec) at midthoracic level shows large mass filling entire left hemithorax, with displacement of heart to right. At higher levels, this mass appeared to originate from anterior mediastinum. Mass is mostly high in signal intensity, reflecting its predominantly fatty content. Low-signal-intensity strands course through mass, probably representing residual thymic tissue and/or fibrous stroma. (Courtesy of A. Shirkhoda, Royal Oak, Ml.)
1, -
‘
Fig. 12.-2-month-old neonate with chest wall and mediastinal lymphangioma. A, On Ti-weighted axial MR image (500/30), chest wall mass (M) is of homogeneous low signal intensity, less than that of muscle. Note defined medium-signal-intensity mass infiltrating axilla and right lobe of thymus (arrows). B, On T2-weighted MR image (1800/120), lymphangioma is of very high signal intensity and is easily differentiated from adjacent structures. lymphangioma had replaced entire right lobe of thymus. C, Histologic section shows typical lymphangioma with large lymphatic spaces (L) separated by septa containing fat and fibrous tissue. s = superior vena cava, T = thymus. (A and B reprinted with permission from Siegel [6].)
Lymphangioma Mediastinal lymphangiomas usually are extensions of cervical lesions, and although most are well defined, they may infiltrate adjacent structures and be difficult to remove surgically (Fig. 12). Typically they appear as thin-walled, multiloculated cystic masses of very high signal intensity on 12weighted MR images. Components of the mass also may be
of relatively high signal intensity fluid has a high lipid content. Ectopic
Parathyroid
poorly
At surgery,
images
if the
Adenoma
Rarely, intrathymic parathyroid parathyroid adenomas elsewhere hemorrhage
on Ti-weighted
also
into the adenoma
adenomas are seen. As with in the mediastinum or neck, can occur
(Fig. 13).
MOLINA
Downloaded from www.ajronline.org by 117.253.152.201 on 11/14/15 from IP address 117.253.152.201. Copyright ARRS. For personal use only; all rights reserved
500
El
AL.
AJR:1 55, September
1990
Fig. i3.-lntrathymic parathyroid adenoma in a 17-year-old girl with persistent hypercalcemia after surgical neck exploration. A and B, Ti-weighted (1077/15) (A) and T2weighted (2263/90) (B) MR images show oval Icsion (arrowheads) in left lobe of thymus. Lesion is of very high signal intensity on both images. Transcervical thymectomy revealed a hemorrhagic parathyroid adenoma within thymus. v = superior vena cava, aa = ascending aorta, da = descending aorta.
Fig. 14.-Ti-weighted axial MR images (500/ 30) in a 25-year-old woman with benign thymic hyperplasia associated with Graves disease. A, There is marked enlargement of both lobes of thyroid due to hyperthyroidism. Signal intensity is homogeneous and intermediate between that of muscle and fat. Jugular vein (arrow); carotid artery (arrowhead). t = trachea. B, Thymus also is diffusely enlarged and has a signal intensity slightly greater than that of muscle. Mediastinal biopsy of thymus revealed benign thymic hyperplasia. v = superior vena cava, aa = aortic arch, pa = pulmonary artery, da = descending aorta.
Thymic True
Hyperplasia thymic
cortex and medulla can be caused by hyperthyroidism (Fig. 14), red cell aplasia, or chemotherapy. On MR images, diffuse enlargement of the thymus is recognizable, especially in its thickness, with preservation of the normal shape. Its signal intensity is similar to that of the normal thymus. Rebound thymic hyperplasia after
chemotherapy
hyperplasia
REFERENCES
presents
involving
both
a diagnostic
problem
because
it may
simulate a primary neoplasm or recurrent disease. If the patient is doing well clinically, and no recurrent or residual disease is evident elsewhere in the body, the patient can be
followed corroborates
up with serial scanning. the diagnosis
A gradual
of benign
thymic
reduction hyperplasia.
in size
1 . DeGeer G, Webb WA, Gamsu G. Normal thymus: assessment with MR and CT. Radiology i986;158:313-317 2. Siegel MJ, Glazer HS, Wiener JI, Molina PL. Normal and abnormal thymus in childhood: MR imaging. Radiology i989;172:367-371 3. Lindfors KK, Meyer JE, Dedrick G, Hassell LA, Harris NL. Thymic cysts in mediastinal Hodgkin disease. Radiology i985;1 56:37-41 4. Nyman AS, Rehn SM, Glimelius BLG, Hagberg HE, Hemmingsson AL, Sundstrom CJ. Residual mediastinal masses in Hodgkin disease: prediction of size with MR imaging. Radiology 1989:170:435-440 5. Shirkhoda A, Chasen MH, Eftekhari F, Goldman AM, Decaro LF. MR imaging of mediastinal thymolipoma. J Comput Assist Tomogr i987;1 1: 364-365 6. Siegel MJ. Pediatric applications. In: Lee JKT, Sagel SS, Stanley RJ, eds. Computed body tomography with MRI correlation. New York: Raven, 1989:1063-1099
