AJR 2001; 176:1525-1530
© American Roentgen Ray Society
Factors Influencing Visualization of Vertebral Metastases on MR Imaging Versus Bone Scintigraphy
Toshiaki Taoka1,
Nina A. Mayr,
Han J. Lee,
William T. C. Yuh,
Teresa M. Simonson,
Karim Rezai and
Kevin S. Berbaum
1
All authors: Department of Radiology, Magnetic Resonance Imaging Center,
University of Iowa College of Medicine, 200 Hawkins Dr., Iowa City, IA
52242.
Received July 24, 2000;
accepted after revision December 8, 2000.
Address correspondence to W.T.C. Yuh.
Abstract
OBJECTIVE. The purpose of this study was to investigate whether the
location and size of vertebral body metastases influence the difference in
detection rates between MR imaging and bone scintigraphy.
MATERIALS AND METHODS. We retrospectively evaluated the vertebral
body lesions detected on MR imaging in 74 patients with known widely
disseminated metastatic disease. Three radiologists independently reviewed the
MR images and bone scintigraphs. MR imaging findings included lesion size and
its spatial relationship to the bony cortex (intramedullary, subcortical, and
transcortical) and results were correlated with those of planar technetium 99m
bone scintigraphy.
RESULTS. Findings on bone scans were negative for all intramedullary
lesions without cortical involvement shown on MR imaging, regardless of their
size. Findings on bone scans (71.3% for transcortical and 33.8% for
subcortical) were frequently positive for lesions with cortical involvement
(trans- or subcortical), and the probability of positive findings on bone
scans was also influenced by the lesion size. Statistical analysis showed a
positive correlation among cortical involvement, lesion size, and positive
findings on bone scintigraphy (p < 0.0001).
CONCLUSION. Location (the presence of cortical bone involvement on
MR imaging) and size of the vertebral body metastases appear to be important
contributing factors to the difference in detection rates between MR imaging
and bone scintigraphy. Cortical involvement is likely the cause of positive
findings on bone scans. Early vertebral metastases tend to be small and
located in the medullary cavity without cortical involvement, and therefore,
findings may be positive on MR images but negative on bone scans.
Introduction
Bone scintigraphy remains the method of choice in the evaluation of bone
metastases because of its accessibility, reasonable cost, and ability to show
the entire skeletal system. However, MR imaging has been advocated as a useful
method for the evaluation of bone marrow diseases, including bone metastases
[1,2,3,4,5,6,7,8].
Nonetheless, the detection rates of vertebral metastases can be different
between these two modalities and may cause a clinical dilemma in patient
treatment. Previous studies have reported that positive MR imaging findings
and negative findings on bone scans can occur in patients with vertebral body
metastases [4,
9,10,11,12,13,14,15,16].
To our knowledge, the cause of such a difference remains undetermined. Many
possible explanations have been proposed
[4,
7,
11,
15,16,17].
To our knowledge, none has been proven. The purpose of this study was to
investigate whether lesion size and, in particular, its spatial relationship
to the bony cortex can influence the differences in detection rates of
vertebral body metastases between MR imaging and bone scintigraphy.
Materials and Methods
From the clinical charts of 450 patients with a diagnosis of widely
disseminated spinal metastases in the past 10 years at our institution, 74
patients who had undergone both bone scintigraphy and unenhanced MR studies of
the spine within a 4-week period (mean, 6.2 days) were selected for
retrospective evaluation. There were 42 males and 32 females between the ages
of 9 and 88 years (mean age, 59 years). The primary tumors included carcinomas
of breast (n = 22), lung (n = 21), prostate (n =
13), genitourinary tract (n = 5), gastrointestinal tract (n
= 2), and other carcinomas (n = 11). Bone scintigraphy was performed
with technetium-99m methyl diphosphonate (Tc-99m MDP). The dose of Tc-99m MDP
was 22 mCi (814 MBq), and a high-resolution collimator was used. Because
single-photon emission computed tomography (SPECT) was not performed in all
patients, only planar imaging was evaluated. Metastases were primarily
examined on axial and sagittal T1-weighted MR images (spin echo; TR range/TE
range, 316-650/11-20), and T2-weighted images (spin echo; TR/TE range,
2000/90-100) were available for reference. The MR images and bone scintigrams
were reviewed independently by three radiologists. Lesions were included as
metastatic foci only if they were hypointense on T1-weighted imaging and
hyperintense on T2-weighted imaging. Schmorl's nodes, benign bone islands, and
hemangiomas were excluded on the basis of their characteristic radiologic
findings. Differences in interpretation were solved by consensus.
Histopathologic confirmation of each metastatic focus was not available
because most of the patients had end-stage disease.
The size of each lesion was measured at its greatest dimension on MR images
and was categorized as small (<2 cm) or large (
2 cm). Coalescent
lesions were included in the large-lesion group. We classified the
relationship between the lesion and cortical bone on the basis of the sagittal
and axial T1-weighted images by determining whether the lesion was distant
from (intramedullary), had contact with (subcortical), or had invaded the bony
cortex (transcortical). The three radiologists classified lesion-cortical bone
relationships without knowledge of the scintigraphic results. MR imaging
findings were compared with those of planar bone scintigraphy. The Pearson's
chisquare test and two-tailed t test were used for statistical
comparisons.
Results
The probability of positive findings on a bone scan was dependent on lesion
size (small versus large) and location (intramedullary, subcortical, and
transcortical). Both the Pearson's chi-square and the two-tailed t
tests confirmed a statistically significant correlation between cortical
involvement and the size of the lesion (p < 0.0001), cortical
involvement and bone scintigraphic results (p < 0.0001), and
lesion size and bone scintigraphic results (p < 0.0001).
Lesion Size as Measured on MR Imaging
Two hundred sixty-two small lesions and 281 large lesions were identified
in 281 vertebrae on MR imaging (Table
1). Positive bone scintigraphic findings were noted more
frequently in large lesions (55.5%) than in small lesions (3.4%)
(Table 1 and
Fig. 1). The mean size of
lesions with positive findings on bone scans was significantly greater
(n = 9; mean size, 11.2 mm) than that of lesions measured on MR
images with negative findings on bone scans (n = 253; mean size, 6.6
mm; p < 0.0001), even among the subgroup of transcortical,
subcortical, and intramedullary lesions.
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TABLE 1 Correlation of Percentage of Positive Bone Scan Findings of Vertebral
Involvement with Lesion Location and Size on MR Imaging
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Fig. 1. —Graph of correlation of positive bone scan with lesion
location and size. Z-axis indicates the percentage of positive
findings on bone scintigraphy, which were noted more frequently in large
lesions than in small lesions. Positive bone scintigraphy findings were also
frequently associated with MR evidence of cortical involvement, particularly
with transcortical lesions.
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Lesion Location
Positive findings on bone scans were frequently associated with MR evidence
of cortical involvement. Findings on bone scintigraphy were negative for all
intramedullary lesions, regardless of size
(Table 2 and Figs.
1 and
2A,2B).
Transcortical lesions (71.3%) had a higher incidence of positive bone scan
findings than subcortical lesions (22.3%)
(Table 1 and Figs.
1 and
3A,3B,3C).
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TABLE 2 Correlation of Number of Positive Bone Scan Findings of Vertebral
Involvement with Lesion Location and Size on MR Imaging
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Fig. 2A. —75-year-old man with biopsy-proven lung carcinoma who
complained of right flank pain. Parasagittal T1-weighted MR image of spine
shows large lesion without cortical involvement at T12 and diffuse lesion with
cortical extension at L1.
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Fig. 3A. —56-year-old man who complained of back pain during treatment
for biopsy-proven ileal carcinoid tumor. Sagittal T1-weighted MR image of
lumbar spine shows large transcortical lesion at anteroinferior L3
vertebra.
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Fig. 3B. —56-year-old man who complained of back pain during treatment
for biopsy-proven ileal carcinoid tumor. Axial T1-weighted MR image through
inferior L3 shows lesion (arrows) extending through richly innervated
periosteum of cortex. Note loss of adjacent fat plane compared with that of
opposite side.
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Fig. 3C. —56-year-old man who complained of back pain during treatment
for biopsy-proven ileal carcinoid tumor. Planar bone scan shows positive
correlation with B, reflecting large area of cortical involvement.
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Among the large lesions, the frequency of transcortical and subcortical
involvement (50% and 8.2%, respectively) was much higher than that of
transcortical and subcortical involvement among small lesions (0.4% and 3%,
respectively) and among most (62.2%) small intramedullary lesions
(Fig. 4). Bone scintigraphic
findings were negative in 90 (34.3%) of 262 small transcortical lesions and in
124 (44.1%) of 281 large transcortical lesions
(Table 1). Seventy-eight
(66.1%) of 118 large subcortical lesions and 89 (91.7%) of 97 small lesions
were associated with negative findings on bone scintigraphy
(Table 1). However, not all
findings of transcortical lesions were positive on bone scintigraphy (Fig.
5A,5B,5C,5D).
Findings on bone scintigraphy were positive in only 72% of the large lesions
and half of the small transcortical lesions
(Table 1 and Figs.
1 and
6A,6B,6C).
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Fig. 4. —Graph of direct relationship between cortical involvement and
lesion size. Incidence of transcortical and subcortical involvement was much
higher in large lesions than in small lesions and in most small intramedullary
lesions.
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Fig. 5B. —43-year-old woman with breast carcinoma who complained of
back pain. Parasagittal T1-weighted MR image shows multiple small lesions at
L3, L4, and S1 with cortical contact. L4 has two intramedullary lesions.
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Fig. 5C. —43-year-old woman with breast carcinoma who complained of
back pain. Axial T1-weighted MR image through inferior L4 shows small lesion
with cortical contact but no definite cortical break.
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Fig. 6A. —67-year-old woman with lung carcinoma who complained of newly
developed lower back pain. Parasagittal T1-weighted MR image of spine shows
large lesion involving right pedicle and lamina of T12 (arrow).
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Fig. 6C. —67-year-old woman with lung carcinoma who complained of newly
developed lower back pain. Planar bone scan shows no increased uptake on T12
despite MR imaging findings of large lesion with cortical destruction.
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Discussion
Bone metastases are by far the most common malignant bone tumors seen in
adults. The prevalence of bone metastases in patients with known primary
cancer is about 70% of the patients with metastases
[18]. Bone metastases may
occur with almost all malignancies, but they are most common in carcinomas of
the breast (47-85%), lung (32%), prostate (54-85%), kidney (33-40%), or
thyroid (28-60%) [18,
19].
The spine is the most common site of skeletal metastases (39%) because of
the abundant vascularization and red bone marrow
[5,
13,
15]. Most bone metastases are
hematogenous in origin [5,
20]. The initial seeding of
metastatic deposits via hematogenous spread is typically localized in the
hematopoietic (red) marrow. This location explains the predominance of
metastatic bone lesions in the axial skeleton (>90% of metastatic bone
lesions) [11].
Back pain is the most frequent initial complaint in patients with spinal
metastatic disease [9,
21,
22] because lesions invade the
richly innervated periosteum with or without detected invasion, possibly
through the cortex via the haversian canals. The imaging of spinal metastatic
disease may include conventional radiography, myelography, radionuclide bone
scintigraphy, CT, and MR imaging. Radiography, CT, and bone scans assess
mainly the bony abnormality, particularly the cortex, whereas MR imaging
examines bone marrow, in which the early metastatic deposits frequently occur
[20]. Findings of abnormal
bone on radiography, bone scintigraphy, and CT mainly result from tumor
invasion of the cortical bone rather than of the medullary bony matrix.
Although conventional radiography is useful for determining the integrity of
cortical bone [10], at least
50% of the cortical bone mass must be destroyed before a lesion can be clearly
seen on conventional radiography
[23]. CT is sensitive in
detecting subtle cortical invasion but is less sensitive for medullary bone or
bone marrow involvement [10,
13]. Bone destruction can be
difficult to detect on CT in the presence of osteoporosis or degenerative
changes, which are common in elderly adults with cancer
[9].
The mechanism of abnormal Tc-99m MDP uptake shown in bone scanning is
complex. Abnormal radionuclide uptake is generally believed to increase with
regional bone-blood flow, bone remodeling, formation of new bone, and enhanced
bone matrix turnover [12,
24,25,26,27].
Bone scintigraphy is sensitive for detecting areas of bone remodeling,
particularly cortex, associated with metastatic deposits as little as 5-10% in
the lesion-to-normal bone ratio, for abnormal focus to be appreciated on
planar scintigraphy [4,
11]. Scintigraphy may reveal
bone metastases up to 18 months before radiography shows them and has a 50-80%
greater sensitivity [28].
However, abnormal increase of uptake in the medullary cavity because of tumor
destruction of the medullary bony matrix may not be as obvious because of the
relatively small amount of medullary bone and relatively high uptake of the
normal cortical bone.
MR imaging is a sensitive method of detecting intramedullary metastases to
those bones with large marrow cavities such as vertebral bodies
[10,
20]. With increased capability
for scanning the whole body with fast imaging techniques, screening the bone
marrow cavities can be cost-effective
[29,
30]. Early results suggest
that the skeletal system of the entire body can be scanned within 30-45 min on
turbo short tau inversion recovery (STIR) imaging and within 6 min with total
body echo-planar imaging, and the detectability of metastatic tumor with these
imaging techniques seems to be better than that with bone scintigraphy.
However, turbo STIR imaging and total body echo-planar imaging are not widely
used methods and are suboptimal for the metastatic workup of the entire
skeletal system on classic MR sequences. MR imaging is not cost-effective in
examining bones with small cavities such as ribs because it cannot globally
examine the entire skeletal system as bone scintigraphy can
[10]. Therefore, in general,
the role of MR imaging in the examination of suspected bone metastases is
limited to those bones with large bone marrow cavities such as the spine or
proximal extremities and is complementary to other imaging methods
[1,
8,
12].
Because the vertebral body has a relatively large marrow cavity, early or
small metastases tend to be intramedullary lesions without cortical
involvement [20] and may not
cause sufficient bony remodeling to be detected on bone scans. In our study,
positive findings on bone scans were always associated with MR evidence of
cortical involvement. Findings on bone scans were always negative for
intramedullary lesions (without cortical involvement), although only one large
lesion did not have cortical involvement
[11,
17]. Small lesions or lesions
localized away from the cortex are, therefore, likely to be undiagnosed on
bone scintigraphy, despite destruction of trabecular bone. Even if most of the
bone marrow has been infiltrated with metastases, the uptake of radioactive
tracers caused by the destruction of the relatively small amount of medullary
bony matrix remains low and, therefore, may not be easily appreciated when the
uptake is contrasted with that of the normal cortex.
There are limitations in our study. Our purpose was to investigate possible
causes for the difference in detection rates of bone metastases on MR imaging
and bone scintigraphy, and we did not attempt to compare the sensitivity and
specificity of these two methods. Our results did not prove or disprove that
cortical involvement is the cause of the difference in the detection rates. In
addition, only planar bone scans were used because not all patients underwent
SPECT bone scintigraphy. The patients in our study were limited to those with
widely disseminated metastatic disease detected on MR imaging, and pathologic
confirmation was not available in this group of patients.
In conclusion, our findings suggest that cortical involvement is likely a
key factor contributing to the difference in detection rates of vertebral body
metastases on MR imaging and bone scintigraphy. Findings of early and small
vertebral metastatic deposits in the marrow cavity may be negative on bone
scans. Although MR imaging is useful in the detection of early metastases that
are localized completely in the bone marrow cavity, bone scintigraphy remains
the most cost-effective method for examination of the entire skeleton. MR
imaging can be useful in cases in which bone scan findings remain negative and
vertebral involvement is suspected on the basis of clinical findings. However,
MR imaging is inadequate in assessing cortical involvement
[31,
32].
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Abstract
Introduction
