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Spectral Adiabatic Inversion Recovery _SPAIR_ MR imaging of the

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					Clinical Abdomen




Spectral Adiabatic Inversion Recovery
(SPAIR) MR imaging of the Abdomen
Thomas C. Lauenstein

Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Germany



Background
Magnetic resonance imaging (MRI) has           form in abdominal imaging is the use of       fat spins have zero longitudinal magneti-
become a major imaging tool for the            a 180° excitation pre-pulse, which sup-       zation at this point they will not contrib-
depiction and characterization of abdom-       presses the signal specific tissue de-        ute to the MR signal.
inal disease. Standard abdominal MRI           pending on the inversion time applied.
protocols encompass different forms of         The inversion time (TI) is set according      Clinical applications
T1-weighted (T1w) and T2-weighted              to the T1 of fat in order to selectively      Homogeneity and
(T2w) data acquisition. These sequences        null the fat signal (TI = 150-170 ms). In     degree of fat suppression
can be collected in less than 20 seconds,      the most common implementation, the           The implementation of SPAIR fat sup-
which typically is within the patients’        inversion pulse is applied with a wide        pression techniques will result in a more
ability to suspend respiration. Hence, ar-     frequency bandwidth to include both fat       profound and homogenous fat satura-
tifacts due to physiological motion in-        and water spins. A potential drawback to      tion compared to conventional fat sup-
cluding respiration and bowel motion can       this approach is that the water signal        pression techniques. In a recent study,
be reduced, if not avoided. While most         will not be fully recovered during data       SNR of mesenteric and retroperitoneal
T1-weighted imaging techniques of the          acquisition, and the overall water signal-    fat was measured for both IR and SPAIR
abdomen include gradient echo (GRE)            to-noise ratio (SNR) will be diminished.      fat suppression in conjunction with
sequences, T2-weighted imaging is based        This can negatively impact the contrast-      T2-weighted SSFSE imaging in order to
on the collection of single shot fast spin     to-noise ratio (CNR) of lesions surrounded    determine the degree of fat suppression
echo (SSFSE) data. The latter sequences        by tissue, such as tumors within the liver.   [13]. The study showed that improved
in conjunction with fat saturation play                                                      fat suppression was found when SPAIR-
a key role for the interpretation of differ-   Technical considerations                      SSFSE was applied (Fig. 1).
ent abdominal processes as liver lesions       for SPAIR
can be most accurately delineated and          The inversion recovery (IR) technique         Depiction of anatomical structures
specified [1]. Furthermore, T2-weighted        can be modified by using chemical selec-      An advantage of SPAIR compared to con-
imaging with fat saturation is crucial         tive or spectral pre-saturation attenuat-     ventional IR techniques is demonstrated
for the depiction of edema and/or free         ed inversion-recovery pre-pulses. SPAIR       by the improvement in CNR of the he-
fluid. This is particularly helpful for the    (Spectral Adiabatic Inversion Recovery)       patic lesions. The better liver lesion con-
depiction of inflammatory processes            is a powerful technique for fat suppres-      trast on SPAIR-SSFSE images is consis-
of the bowel, e.g. in patients with            sion which offers different advantages        tent with the predicted benefits of
Crohn’s disease [2, 3], appendicitis [4, 5]    over conventional fat suppression tech-       applying a frequency-sensitive inversion
or diverticulitis [6–8]. Finally, T2-weight-   niques. The technique is insensitive to       pulse. This leaves the maximum possible
ed data may be particularly useful in          B1 inhomogenities and only fat spins are      water signal intact as only the fat spins
the setting of pregnant patients*. As the      suppressed/inverted. SPAIR uses a spec-       are inverted. Two types of focal liver
intravenous administration of gadolini-        trally selective adiabatic inversion pulse    lesions have been evaluated [13]: heman-
um based contrast agents is contraindi-        to invert the fat spins in the imaging vol-   giomas with a relatively high CNR and
cated in this patient group, T1-weighted       ume. After the adiabatic pulse a large        metastases with a relatively low CNR.
imaging is restricted and only provides        spoiler is utilized in order to destroy any   The CNR was found significantly increased
limited information. Hence, T2-weighted        transverse magnetization. The fat spins       for both families of lesions when using
imaging with fat saturation has been           will now decay according to the T1 re-        SPAIR compared to IR SSFSE (Figs. 2
found to be the key sequence in preg-          laxation rate and after a certain charac-     and 3).
nant* women with suspected abdominal           teristic time (TI null) the longitudinal      Furthermore, delineation of bowel
inflammation or tumor disease [9–12].          magnetization will be zero. At this time      wall structures is markedly improved on
Different techniques for fat saturation        point the excitation pulse of the SSFSE       SPAIR SSFSE (Fig. 4). This improvement
in MRI can be used. The most common            T2-weighted module is applied. As the         is due to two different factors that dif-


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                                                                                                                        Abdomen Clinical




1                                                                  2




    1 Homogeneous fat saturation in the retroperitoneum (dashed        2 Patient with liver metastases (arrow) of colorectal cancer.
    arrow) and the mesenteries (arrow) with the SPAIR technique.       The lesion is evident and provides high CNR values on SPAIR
                                                                       T2-weighted imaging.


3                                                                  4




    3 Patient with several hemangiomas (arrows).                       4 Conspicuous bowel loops (arrow) using the SPAIR technique.
    SPAIR T2-weighted MRI.




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Clinical Abdomen




5A                                            5B
                                                               5 Patient with active colitis. There
                                                              is increased contrast enhancement
                                                              after iv gadolinium administration
                                                              shown on T1-weighted GRE imag-
                                                              ing (5A; arrow). A high T2 signal of
                                                              the bowel wall can be depicted on
                                                              T2-weighted SPAIR images (5B;
                                                              arrow), which is consistent with ac-
                                                              tive inflammatory disease due to
                                                              edematous changes.




6A                                            6B
                                                               6 Patient with mildly active in-
                                                              flammatory changes of the as-
                                                              cending colon (arrow). T1-weight-
                                                              ed contrast-enhanced MRI reveals
                                                              increased contrast uptake of the
                                                              inflamed bowel segment and
                                                              thickening of the bowel wall (6A).
                                                              The T2 signal on the SPAIR image
                                                              is only slightly elevated (6B).




7A                                            7B
                                                               7 Patient with non-active / fibrot-
                                                              ic inflammation of the sigmoid co-
                                                              lon (arrow). Similar to the active
                                                              forms of inflammatory bowel dis-
                                                              ease (IBD) there is increased con-
                                                              trast enhancement on T1-weight-
                                                              ed MRI (7A). However, there is
                                                              lack of edema, and thus the T2
                                                              signal is not elevated on the SPAIR
                                                              image (7B).




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                                                                                                                             Abdomen Clinical




8A                                                                           8B




   8 SPAIR T2-weighted SSFSE MRI can be used as a stand-alone sequence for therapeutic monitoring. This patient presented with sign of active
  inflammation in the terminal ileum and highly elevated T2 signal on SPAIR imaging (8A). One week after the initiation of anti-inflammatory
  medication the T2 signal dropped as a correlation of therapeutic response (8B).




ferentiate SPAIR SSFSE: one factor is the              were either time-consuming (e.g. MR           flammation show an increased contrast
relatively greater sensitivity to motion               based perfusion analyses), invasive           enhancement [15]. Hyperintensity on
of standard IR SSFSE. In addition, bowel               (colonoscopy / biopsy) or inaccurate          T2-weighted images, however, is related
wall visualization should benefit from                 (CDAI). Hence, a relatively fast, simple      to increased edema and inflammatory
the increased SNR of water-containing                  and non-invasive technique is desired         fluid components within or adjacent to
structures on SPAIR SSFSE.                             in appraising the level of inflammatory       the bowel wall, whereas T1-weighted
                                                       activity and also in following up these       hyperintensity may be attributed to a
Inflammatory abdominal processes                       patients for treatment response.              hypervascularity (in active disease) or a
Evaluation of disease activity in patients             SPAIR T2-weighted SSFSE sequences and         delayed wash-out (in fibrotic /chronic
with inflammatory bowel disease (IBD)                  gadolinium enhanced T1-weighted se-           disease).
is often a challenging clinical situation.             quences are complementary techniques          Examples of contrast-enhanced
While active inflammation is treated                   in patients with IBD [14]. Gadolinium-        T1-weighted GRE images and SPAIR
with systemic corticosteroids or other                 enhanced T1-weighted data is helpful          T2-weighted SSFSE images are shown in
immuno-modulator drugs, surgical ther-                 to detect IBD independent of its activity     figures 5–7 for highly active, intermedi-
apeutic options are chosen for chronic                 state with a high sensitivity. However,       ate active and non-active IBD. Once the
disease. This discrepancy in therapy                   accuracy of T1-weighted imaging to            diagnosis of IBD is established, SPAIR
strategies underlines the need for an                  differentiate between active and non-         T2-weighted SSFSE imaging can be used
accurate categorization and differentia-               active disease is only moderate. En-          as a stand-alone sequence for therapy
tion between active and chronic disease.               hancement patterns of T1-weighted im-         monitoring (Fig. 8). Furthermore, this
Attempts of classifying IBD in the past                aging are unspecific: both bowel              method is also very helpful not only for
were based on different variables that                 segments with active and chronic in-          the assessment of IBD including Crohn’s

* The safety of imaging fetuses/infants has not been established.




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Clinical Abdomen




disease and Ulcerative colitis, but also               9
                                                                                                                                           9 Patient with
for diverticulitis (Fig. 9) and the depic-
                                                                                                                                          active diverticulitis.
tion of fistulae (Fig. 10).                                                                                                               There is increased
                                                                                                                                          T2-weighted signal
Conclusion                                                                                                                                in and adjacent to
                                                                                                                                          the wall of the sig-
There are overall benefits of SPAIR SSFSE
                                                                                                                                          moid colon (arrow)
that can be measured on clinical abdom-                                                                                                   in keeping due to
inal MR images regarding fat saturation,                                                                                                  edema.
particularly in fat adjacent to bowel and
for improving overall image contrast
even between non-fatty soft tissues, such
as can be demonstrated with liver mass-
es. Furthermore, SPAIR SSFSE is a crucial
tool for the depiction of inflammatory
processes in the abdomen, particularly                 10
IBD. By means of SPAIR T2-weighted                                                                                                         10 T2-weighted
                                                                                                                                          SPAIR imaging can
SSFSE a differentiation between active
                                                                                                                                          easily display not
and non-active inflammatory processes                                                                                                     only an inflamma-
can be easily established.                                                                                                                tory process itself,
                                                                                                                                          but also complica-
                                                                                                                                          tions such as a fluid-
   Contact                                                                                                                                filled fistula – be-
   Thomas C. Lauenstein, M.D.                                                                                                             tween bowel and
   University Hospital Essen                                                                                                              cutis – (arrow).
   Dept. of Diagnostic and Interventional
     Radiology and Neuroradiology
   Hufelandstr. 55
     45122 Essen, Germany
   thomas.lauenstein@uni-due.de




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