Pulse Sequence Timing

The optimal timing for breast MRI sequences is dependent on the goal of imaging. This is because high-resolution and short imaging-time requirements represent competing examination strategies on current equipment. A single high sensitivity, high-resolution, fat suppression 3D technique (with voxel sizes less than 1 mm) is sometimes used. This can be performed within approximately 2-4 min which is in the time window where the differential enhancement between malignant and benign lesions is greatest; image interpretation has to be based on morphological characteristics alone. Several investigators have reported on the architectural features that can be used for lesion diagnosis using this technique (Harms et al. 1993; Orel et al. 1994; Stomper et al. 1995; Orel et al. 1997).

200 300 400 Tim? (seconds)

Fig. 10.4a-h. Normal versus double dose of contrast medium. 54 year old woman with infiltrating lobular carcinoma of the breasts. An 8 cm mass was palpable in the right breast. Mammograms were suspicious of tumour in the right breast only. a-c and d-e represent coronal Tl-weighted 3D GRE images before and after 0.1 and 0.2 mmol/kg body weight of Gd-DPTA contrast medium and corresponding subtraction images respectively. The image datasets are acquired 1 month apart. Subjective, a greater intensity of enhancement is seen in the breast parenchyma of both breasts with the higher dose of contrast medium which is diffusely infiltrated with tumour. In a more localised lesion (arrow), shows a greater intensity of enhancement with the higher dose of contrast medium which is confirmed on the relative signal intensity (%) time curve shown in image g. Image h is a T2-weighted spin-echo image of the right breast showing a mass with distortion of the parenchyma in the region of the axillary tail (arrow). Pathological examination following bilateral mastectomy showed an 8 cm invasive lobular carcinoma of the right breast and a 1.6 cm tumour on the left of the same histology. We are grateful to Dr. Kausar Raza (Hillingdon Hospital, Hillingdon, UK) for providing this illustration.

200 300 400 Tim? (seconds)

Fig. 10.4a-h. Normal versus double dose of contrast medium. 54 year old woman with infiltrating lobular carcinoma of the breasts. An 8 cm mass was palpable in the right breast. Mammograms were suspicious of tumour in the right breast only. a-c and d-e represent coronal Tl-weighted 3D GRE images before and after 0.1 and 0.2 mmol/kg body weight of Gd-DPTA contrast medium and corresponding subtraction images respectively. The image datasets are acquired 1 month apart. Subjective, a greater intensity of enhancement is seen in the breast parenchyma of both breasts with the higher dose of contrast medium which is diffusely infiltrated with tumour. In a more localised lesion (arrow), shows a greater intensity of enhancement with the higher dose of contrast medium which is confirmed on the relative signal intensity (%) time curve shown in image g. Image h is a T2-weighted spin-echo image of the right breast showing a mass with distortion of the parenchyma in the region of the axillary tail (arrow). Pathological examination following bilateral mastectomy showed an 8 cm invasive lobular carcinoma of the right breast and a 1.6 cm tumour on the left of the same histology. We are grateful to Dr. Kausar Raza (Hillingdon Hospital, Hillingdon, UK) for providing this illustration.

If the goal is to maximise specificity, then repeated data acquisitions are recommended. On current MRI systems, higher temporal resolution necessitates reduced spatial resolution, decreased coverage or a combination thereof. The highest specificity MRI breast studies have used 2D dynamic techniques with data acquisition rates of 1-12 s. Higher temporal resolution techniques appear to improve the specificity of examinations because of better characterisation of the signal intensity time curve; one study has suggested that characterisation of breast lesions is optimal at 1-2 s image acquisition (Boetes et al. 1994). It should be recognised that the temporal requirements of fast dynamic imaging limits the spatial resolution and coverage and therefore multi-focal disease may be overlooked and such sequences are not suitable for breast cancer screening. In the latter cases, volume coverage at the slower data acquisition rates is often used.

It is now clear that both kinetic and morphologic information is required to achieve optimal discrimination between benign and malignant disease. Therefore, as a compromise, a dynamic high-resolution 3D technique (slower dynamic 3D technique) is used by many with each time point being acquired every 60-90 s. This approach does not require prior knowledge of lesion location. Both Kuhl et al. (1999) and Liu et al. (1998) have reported on the value of such techniques where an integration of contrast agent kinetics and architecture evaluation is performed. Both groups make the very important point that there must be concordance between the kinetic information and the morphologic features. There are some malignant lesions including invasive ductal and lobular carcinomas or certain ductal carcinoma in-situ lesions, that do not enhance rapidly but in which lesion morphology suggests the presence of malignancy e.g. architectural distortion or mass with spiculated borders. Thus, in general, if the goal is to determine the likelihood that a previously identified lesion is malignant, then the best strategy is to use the minimal number of slices necessary to cover the lesions and obtain a series of acquisitions with high temporal resolution. However, if the goal is to detect multicentric or multi-focal disease, search for a primary tumour or demonstrate the extent of a known cancer, then good spatial resolution and coverage are essential and is best achieved with a 3D MR technique.

0 0

Post a comment