BOLD contrast MRI was successfully accomplished in 9 of 10 patients with cervical cancer and three normal volunteers. The oxygen breathing paradigm was well tolerated and the only imaging failure was due to metal seeds implanted in the cervix for purposes of treatment planning.
We acquired images in the transaxial plane for the initial volunteer and patients, since this has been traditionally favored as the primary imaging plane for crosssectional diagnostic imaging of cervical cancer. Transaxial images generally allow for high resolution imaging to reveal parametrial tumor extension, and they also allow concurrent assessment of nodal involvement. For the BOLD response to oxygen challenge, we imaged a single plane through a central part of the tumor, as identified by the radiologist during the scanning procedure, so that both central and peripheral regions could be observed. Comparison of the high resolution diagnostic scans with BOLD provided anatomical identification, and muscle tissue was chosen as a reference standard. The iliacus muscle signal was quite stable both during baseline air breathing and in response to oxygen challenge (e.g
., ). This coincides with previous observations in humans at 1.5 T, where oxygen breathing generated no response in R2
* in muscle (or liver and spleen), while carbogen elicited a significant response in all three organs (44
). Likewise Winter et al
. reported a lack of response in rabbit paraspinal muscle at 1.5 T with respect to oxygen challenge, though again carbogen induced a change (45
). The differential response to these two hyperoxic gases is often attributed to vasoactivity of carbogen, and has been explored extensively with respect to tumorand wound-induced skin angiogenesis (25
). The mature vasculature of well perfused muscle may show little response to oxygen. Meanwhile a large response has been reported accompanying the hyperemia in human muscle following constriction and ischemia (46
Tumor showed distinct heterogeneity, but a significant mean increase in all four patients (). Three patients showed quite similar signal response (2.7 to 5.4%), whereas patient #1 showed a much larger change. It became clear that the tumor in patient #1 had become displaced from the original imaging planning location. The tumor showed negligible displacement during the oxygen challenge, but the image plane now coincided with the tumor periphery instead of center due to bladder filling. The larger increase in BOLD response in this patient compared to the other three in this group is likely attributable to more extensive vasculature in the periphery as compared to the center. This artifact prompted us to alter the imaging acquisition plane for the later patients.
To date relatively few R2
* tissue rates have been reported at 3 T. We found cervical tumor and normal uterus to have quite similar rates around R2
* = 24 s-1
* = 42 ms) (). By comparison human kidney is reported to have T2
* = 47 ms with a significant response to oxygen breathing (ΔT2
* ~ 1-2 ms (47
)), as seen in the tissues here. Human skeletal muscle has been reported to have T2
* around 27 ms at 1.5 T when subjects breathed air (unchanged with oxygen) (44
). A range of rates has been reported previously at 3 T, e.g.
* = 20 - 30 ms in human tibialis anterior muscle and soleus muscle (48
), which is very similar to our measurement in the paraspinal muscle (T2
*=24.8 ms), though a value of T2
* = 19 ms was reported for calf muscle (46
Imaging in the sagittal plane avoids issues of displacement due to bladder filling and clearly shows the relationship of normal cervix or cervical tumor to the uterus and vagina. The well-vascularized endometrium provides a useful positive control, which is highly responsive to oxygen challenge (). Echo planar imaging was less satisfactory in the sagittal plane due to signal losses from extensive susceptibility variations in the bowel regions.
For the later patients (#5-10), we implemented a multi-echo gradient echo sequence, which provides R2*, as opposed to sampling signal intensity changes alone. This should provide further insurance against artifacts, since R2* distribution may be compared irrespective of motion artifacts, which otherwise could compromise effective signal subtraction. R2* maps show heterogeneity in both the uterus and normal cervix (e.g., in ) with significant decrease in R2* upon breathing oxygen.
Two patients presented with surgical metal clips. Artifact from the metal clips obscured most of the tumor in patient 7, which made it hard to analyze (data excluded). Patient 5 also had a metal clip artifact that obscured part of the tumor. Nonetheless, a significant drop in R2* was observed upon breathing oxygen.
The motivation for developing oxygen sensitive MRI of human cervical cancer is provided by strong evidence that hypoxia influences tumor aggressiveness, notably angiogenesis and metastasis, as well as poor response to therapy and shorter recurrence free intervals. Specifically, in cancer of the cervix, several studies based on the Eppendorf Histograph polarographic electrode system, indicated that patients with hypoxic tumors (variously defined as HF5
>50% or median pO2
less than the population mean) had a poorer clinical outcome. In 1998, Fyles, et al.
, reported that cervical cancer patients with larger tumors (>5 cm diameter) had significantly poorer disease-free survival (DFS), if the fraction of pO2
measurements less than 5 torr (~ 670Pa) was greater than 50%, in a study of 74 patients (DFS 12% vs. 65% at 2 years, P = 0.0001) (49
). An extended study of 106 patients published in 2002 noted that the predictive value applied only to node negative patients (7
) and a follow-up report indicates that stratification based on hypoxia is less relevant after 10 years (50
). Nonetheless, hypoxia was clearly associated with short-term disease free survival and this measurement could become a common and useful clinical tool, if accomplished with a simple noninvasive method.
In addition to assessing tumor hypoxia the dynamic response to an intervention may be important. Electrodes are highly invasive and do not conveniently allow repeat dynamic maps, although Aquino-Parsons et al.
did examine a group of women with respect to hyperoxic gas breathing challenge, comparing the influence of breathing oxygen or carbogen on cervical tumor pO2
). Results indicated that carbogen was more effective at eliminating hypoxia than oxygen, but carbogen is noted to be quite stressful and thus we opted to apply oxygen breathing challenge here. It has been reported that carbogen-light (98%O2
) causes lower stress, while retaining the hemodynamic attributes of carbogen (52
), and this appears worthy of future investigation for BOLD studies. Warming and humidifying the inhaled gas may also be helpful.
BOLD MRI indicates changes in vascular oxygenation, but may be further influenced by flow, vascular volume, pH, R1
changes, and hematocrit (21
). We tested two pulse sequences to acquire BOLD images. Semi-quantitative approaches based on simple changes in T2
-weighted signal intensity are particularly sensitive to flow (21
), although this has been applied to many pre-clinical and clinical investigations (13
). Use of MGRE to assess R2
* is relatively insensitive to inflow (21
) and has been favored in more recent studies (27
). It is noteworthy that we observed strong correlation between changes in SI and R2
* () suggesting that inflow and R1
changes are not a major factor in response. Nonetheless, significant changes in S0
were observed for two tumors and two uteri with respect to oxygen challenge. The S0
changes may have been caused by changes in R1
or vascular volume. A correlation was also found between ΔR2
* and baseline R2
* (), in line with a previous report for chemically induced spontaneous rat breast tumors (36
Rates of tissue response may also provide useful insight into tumor perfusion and oxygenation. Tumors in Patients #3 and #8, shown in and respectively, showed considerable increase, albeit with minor hiccups, in signal over the whole 8-minute oxygen challenge. Other tumors reached a plateau or maximum at an earlier time.
The EPI sequence is sensitive to susceptibility differences, which result in signal loss, limited spatial resolution, and image distortion. Indeed, we were unable to achieve useful EPI in volunteer # 3 and subsequent studies were performed using MGRE. This has the added bonus of providing both relative signal intensity changes and R2*. Relative SI changes were larger on the EPI sequence compared to the MGRE. Since different MRI parameters were in both sequences, a direct comparison between relative SI is not feasible.
BOLD response is sensitive to tumor vascular oxygenation as well as the extent of vasculature (21
). Measurements of tissue R1
therefore may be relevant, since they are directly sensitive to changes in pO2
). While there is a small R1
response to deoxyhemoglobin (57
), we believe it will ultimately be useful to implement and evaluate interleaved BOLD and TOLD (Tissue Oxygenation Level Dependent) measurements. Indeed, a preliminary report of well defined rat prostate tumors indicated differential temporal response to T1
- and T2
*-weighted signal, presumably reflecting alteration in vascular oxygenation followed by diffusion of oxygen into the tumor tissues (59
). A preliminary report did show significant response in T1
of cervical squamous carcinoma in response to oxygen breathing at 1.5 T (44
). Further development of oxygen sensitive MRI of the cervix may usefully compare different gases, e.g.
, carbogen (25
), implementation of alternative oxygen delivery approaches and masks (60
), and various pulse sequences (61
). Dynamic contrast enhanced (DCE) MRI following infusion of paramagnetic contrast agents has also been reported to provide insight into tumor hypoxia (10
). Likewise, histological correlates could provide further insight into the nature of tumor hypoxia and perfusion (63
). In our initial protocol described here, emphasis was on optimizing the BOLD study, but including additional methods should be straightforward in the future.
Despite several decades of significant treatment advances for cervical cancer, it remains a prevalent life-threatening disease. As such development of accurate prognostic biomarkers will almost certainly improve and eventually optimize and personalize therapy. This preliminary study demonstrates that BOLD MRI at 3-tesla is feasible for examining the potentially valuable biomarker of oxygenation seen in cervical cancer. It remains to be seen whether baseline R2*, signal response to hyperoxic gas breathing or a multi parametric comparison including additional parameters such as tumor size, and stage will be most useful. Further parameters such as vascular perfusion and permeability based on DCE, cellularity based on diffusion, and TOLD response to oxygen challenge may also be readily incorporated into a dynamic evaluation. More patients must be evaluated and followed clinically before the prognostic value of this non-invasive technique is determined.