MRI_of_the_Bone_Marrow by cuiliqing


									                                                                                                                         MRI of Bone Marrow

MRI of Bone Marrow
John F. Feller
MRI is the imaging modality of choice for the   hands and feet. There is then a “distal-to-          The clinical advantages of STIR are due to
investigation of bone marrow disorders.         proximal“ trend of conversion within the             the following characteristics: (1) additive T1
Accurate interpretation of MR examinations      bones of the extremities. Conversion also            and T2 contrast; (2) marked fat suppression;
of bone marrow requires an understanding        occurs at different rates within the same            (3) a two-fold increase in the magnetization
of the anatomy, physiology, distribution, and   bone. Within long bones, marrow converts             range of spin-echo sequences. As a result,
conversion patterns of bone marrow. Tech-       first in the diaphysis, then in the distal           STIR images demonstrate extraordinarily
nical factors of the MR examination are also    metaphysis, and finally in the proximal              high contrast, conspicuousness, and
important. Common pathophysiologic path-        metaphysis.                                          sensitivity for the depiction of most types of
ways allow a useful classification of bone      By age 25 years, the adult distribution of           bone marrow pathology. The obvious
marrow disorders.                               bone marrow is attained which is                     drawbacks of this pulse sequence, however,
                                                characterized by red marrow persisting in            include relatively long imaging times, and a
BONE MARROW ANATOMY                             the axial skeleton, proximal humeri, and             low signal-to-noise ratio.
                                                proximal femora. With advancing age there            Conventional intermediate weighted and T2
AND PHYSIOLOGY                                  is further replacement of red marrow by              weighted spin-echo sequences demonstrate
The bone marrow is the 5th largest organ of     yellow marrow, with older individuals                relatively low contrast between red marrow
the human body. Its chief function is           commonly having a spine and pelvis                   and yellow marrow. In addition there is
hematopoietic, providing the optimal supply     dominated by yellow marrow. Residual                 decreased sensitivity and conspicuousness
of circulating platelets, white and red blood   islands of hematopoietic marrow can persist          for the depiction of most types of bone
cells to meet the body´s requirements for       in the long bones. The most common sites             marrow pathology. These problems are
coagulation, immunity, and oxygenation.         are the proximal and distal femora, and              corrected by utilizing the very long TR and TE
The histology of normal bone marrow             proximal humeri. This pattern should not be          times of heavily T2 weighted fast spin-echo
consists of a number of components              mistaken for pathology. Another common               images used in conjunction with fat-
including: (1) an osseous component; (2) a      normal variation in distribution of marrow is        saturation. The sensitivity of this sequence
cellular component; (3) a supporting system.    the presence of focal fatty marrow within the        for detecting bone marrow pathology is
The osseous component consists of               spine. The distal appendicular skeleton              similar to that of STIR imaging. Several
cancellous bone composed of primary and         usually has a uniform distribution of yellow         practical advantages compared with STIR
secondary trabeculae. The cellular              marrow in adults.                                    include: (1) significantly decreased imaging
component includes hematopoietic, fat, and                                                           time; (2) improved signal-to-noise ratio. The
reticulum cells. The bone marrow supporting     MR TECHNIQUE                                         major disadvantage of T2 weighted fast
system consists of vascular, neural, and                                                             spin-echo with fat saturation is its
                                                Pulse sequence selection determines the MR
lymphatic elements.                                                                                  dependence on excellent magnetic field
                                                appearance of normal bone marrow as well
                                                                                                     homogeneity for adequate fat suppression.
Hematopoietically active bone marrow is         as the sensitivity and specificity for
                                                                                                     Optimal results with fat saturation usually
referred to as hematopoietic marrow or red      evaluating bone marrow disorders. A highly
                                                                                                     require high-field strength systems, whereas
marrow. Red marrow contains approximately       effective combination of pulse sequences for
                                                                                                     STIR images can be obtained on low-or high
40% water, 40% fat, and 20% protein.            the evaluation of bone marrow pathology
                                                                                                     field strength systems.
Hematopoietically inactive marrow is            includes: (1) T1 weighted spin-echo; and
referred to as yellow marrow or fatty marrow.   either (2) fat-saturation T2 weighted fast           The fast inversion recovery techniques
It contains approximately 15% water, 80%        spin-echo; or (3) STIR / Fast STIR.                  significantly reduce the imaging time
fat, and 5% protein. These differences in                                                            required for STIR-like images. The role of
                                                There is superb differentiation between red
chemical composition account for the                                                                 these techniques in the evaluation of bone
                                                and yellow bone marrow on T1 weighted
appearance of red and yellow marrow on                                                               marrow pathology is currently evolving.
                                                spin-echo images. On T1 weighted images
various MRI pulse sequences. There is also a                                                         Recent studies suggest an emerging role for
                                                the yellow marrow is hyperintense in signal
structural difference between red and yellow                                                         this pulse sequence for performing whole-
                                                intensity as contrasted with the relatively
marrow. In particular, the vascular network                                                          body bone marrow MRI for the evaluation of
                                                decreased signal intensity of red marrow.
of red marrow can be characterized as                                                                patients with suspected skeletal metastasis or
                                                These differences in signal intensity are a
being rich, while that of yellow marrow is                                                           multiple myeloma.
                                                direct reflection of the differences in
more sparse.                                    fat/water content within red and yellow              Opposed-phase GRE sequences with a long
At birth, red marrow is present throughout      marrow. Specifically, increased fat content          repitition time have recently been shown to
the entire skeleton. Epiphyses and              within yellow marrow contributes to                  be sensitive for demonstrating red bone
apophyses are cartilaginous at birth,           significant shortening of the T1 relaxation          marrow pathology. This type of sequence
however, they later contain yellow marrow       time compared with red marrow. Both                  results in low signal intensity of intact red
throughout life. Epiphyseal red marrow can      benign and malignant disorders of bone               bone marrow and high signal intensity
be a normal variant in adults, however, in      marrow have long T1 values which result in           positive contrast imaging of pathology.
the humeral head and femoral head.              marrow signal intensity that is significantly
Normal physiological conversion of red-to-      decreased. The signal intensity of these             CLASSIFICATION OF BONE
yellow marrow occurs in a predictable and       lesions on T1 weighted spin-echo images is
orderly fashion with completion by the age      usually less than that of intervertebral discs
                                                                                                     MARROW DISORDERS
of 25 years when the adult pattern is           in the spine and less than that of muscle in         MRI is ideally suited for evaluation of both
reached. Conversion occurs first in the         the extremities.                                     diffuse and focal bone marrow disease. The

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MRI of Bone Marrow

bone marrow can be affected by a wide            susceptibility “blooming“ of trabeculae. The     should not be confused for more significant
variety of pathologic processes, such as         finding of bone marrow edema is non-             pathology.
myeloproliferative diseases, osteomyelitis,      specific, and can be seen as a result of         One of the most common clinical problems
and hemochromatosis, but metastatic              trauma, infection, ischemia, reaction to         is the distinction of benign osteoporotic
disease and multiple myeloma are the most        adjacent neoplasia, or it may be idiopathic.     compression fracture of the spine from
common causes of bone marrow disease.            For example, the bone marrow edema seen          pathologic compression fracture due to
Vogler et. al., have nicely grouped the bone     on MR images of the hip may be secondary         malignant processes. A vertebral body
marrow disease conditions according to           to transient or migratory osteoporosis, early    wedge       compression       fracture     that
common pathophysiological patterns.              osteonecrosis, or the bone marrow edema          demonstrates complete preservation of
                                                 syndrome.                                        normal bone marrow signal intensity is
Reconversion                                                                                      consistent with a chronic benign
The first group includes diseases where there
                                                 Marrow Ischemia                                  compression fracture. Acute or subacute
                                                                                                  benign vertebral body compression fractures
is an increased need for hematopoietic           This category of bone marrow disease
                                                                                                  demonstrate marrow signal alteration in a
marrow. Failure of conversion from red           encompasses both avascular necrosis of
                                                                                                  regular pattern with smooth margins.
marrow to yellow marrow occurs when the          subchondral bone and medullary bone
                                                                                                  Frequently this is in a band-like pattern
disorder       develops     in    childhood.     infarcts. Bone marrow ischemia favours fatty
                                                                                                  paralleling the involved vertebral body end
Reconversion occurs when the adult bone          marrow over hematopoietic marrow. This is
                                                                                                  plate. An underlying fracture line is
marrow is stressed. Reconversion occurs in       most likely due to the limited vascular supply
                                                 of yellow marrow relative to red marrow.         frequently visible. The posterior height of the
the reverse order of normal conversion,
                                                                                                  vertebral body is also frequently preserved.
progressing from proximal-to-distal. This is
                                                                                                  The marrow signal returns to normal in one-
seen in entities such as anemia of chronic       POTENTIAL PROBLEMS                               to-three months. Malignant vertebral body
illness, thalassemia, and sickle cell disease.
                                                 There are a number of potential problems         compression         fractures    demonstrate
                                                 encountered in the interpretation of bone        complete replacement of normal marrow
Marrow Infiltration                              marrow MR imaging. The most common               signal intensity in 88% of cases. If
Marrow infiltration can be either focal or       include recognition of normal variants.          incomplete, it is generally in an irregular
diffuse and is most commonly due to              These include: (1) residual islands of           distribution. Focal metastasis elsewhere,
neoplastic disease. There is replacement of      hematopoietic marrow, especially in the          posterior element involvement, cortical
the normal fatty marrow cells by neoplasia       proximal femora and humeri; (2) residual         disruption, and a ballooned morphology of
or other pathologic tissue. Examples of          red marrow within the proximal humeral           the vertebral body favour a neoplastic
neoplastic disease include metastatic            epiphysis and femoral capital epiphysis; (3)     etiology. Neither the presence nor the
disease, lymphoma/leukemia, and multiple         focal fatty marrow, especially within the        absence of enhancement following
myeloma. Examples of non-neoplastic              spine.                                           intravenous gadolinium appears specific
disorders include eosinophilic granuloma,        Other problems are purely technical in           enough to preclude serial MRI or biopsy. The
Gaucher´s         disease,     and      the      nature. For example, there is a significant      presence of a paraspinal mass can also be
mucopolysaccharidoses.                           decrease in sensitivity for bone marrow          present with either a benign or malignant
                                                 pathology when intermediate weighted             etiology.
Myeloid Depletion                                sequences are utilized in place of true T1       It should be noted that vertebral
                                                 weighted spin-echo images. Similar               compression fractures in patients with
These disorders are characterized by
                                                 problems arise if fast spin-echo imaging is      multiple myeloma can appear benign at MR
replacement of hematopoietic elements by
                                                 utilized without adequate fat saturation.        imaging, and their distribution is similar to
fat cells. These processes include aplastic
                                                 Similarly, it is important to obtain pre-        that observed in osteoporotic fractures. The
anemia, radiation therapy, and chemo-
                                                 contrast T1 weighted spin-echo images as         possibility of multiple myeloma, therefore,
therapy.     Following     bone     marrow
                                                 post-gadolinium images alone, without the        should not be excluded in patients with
transplantation, a marrow “band“ pattern is
                                                 use of fat saturation, can cause yellow          benign-appearing vertebral compression
seen reflecting repopulation of the bone
                                                 marrow to appear homogeneously high              fractures at MR imaging.
marrow in the peripheral sinusoids of the
                                                 signal, completely obscuring lesions which
vertebral body. This finding consists of
                                                 are easily seen on pre-gadolinium images.        References
central fatty marrow with a peripheral
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